The Urogenital System

The Urogenital System

Urinary Organs

The urogenital system (apparatus urogenitalis) is so named because of similar embryologic origins of several component parts and some of the same functional structures in the adults of both sexes. Urinary organs are the first to develop, and several of the early kidney ducts are appropriated by the male reproductive system. There are remnant structures in each sex that are functional components of the other. Because of this commonality in development and the fact that genetic anomalies or hormonal influences in early development can alter the morphologic characteristics of the system, there are frequent instances of malformation. Hollow remnants in either sex are prone to forming fluid-filled cysts.

Intersexes and disorders of sexual development in the dog are common (Hare, 1976; Meyers-Wallen & Patterson, 1986). A dog may be a genetic female with a bicornuate uterus and have abdominal testes (Bodner, 1987 in a Doberman; Stewart et al., 1972 in a Pug). For a comprehensive review of intersexes and freemartins and a well-referenced general treatment of the reproductive system in vertebrates see van Tienhoven (1983) and Lamming (1990). A perceptive book by Willis (1962) provides a background for an understanding of pathologic problems resulting from the duality in development of the urogenital system, as does the embryology text by Noden and de Lahunta (1985) and McGeady et al (2009). Pathologic conditions of the reproductive system have been discussed by McEntee (1990), and congenital malformations by Szabo (1989).

Urinary organs (organa urinaria) include the kidneys (renes), ureters, bladder (vesica urinaria), and urethra (urethra masculina, urethra feminina).


The kidney (ren), nephros in Greek (Figs. 9-1 to 9-5), is a reddish-brown, paired structure lying against the lumbar hypaxial muscles on either side of the vertebral column. Each kidney has a cranial and a caudal pole, a medial and a lateral border, and a dorsal and a ventral surface. The cranial and caudal extremities are joined by a convex lateral border. The medial border has an indentation, the hilus, that defines a space, the renal sinus. The sinus contains the ureter, renal artery and vein, lymph vessels, and nerves. Of these structures, the renal artery is the most dorsal, and the renal vein the most ventral. Commonly the renal vein is paired on one or both sides, and sometimes the renal artery may also be paired. The nerves and lymphatics lie in close relationship to the renal vein (Bulger et al 1979).

FIGURE 9-1 Female urogenital system in situ, ventral aspect.

FIGURE 9-2 Dorsoventral contrast radiograph of the left kidney and ureter.

FIGURE 9-3 Right kidney, and vessels of hilus. A, Medial aspect. B, Dorsal aspect.

FIGURE 9-4 Left to right lateral contrast abdominal radiograph of the kidneys. The right kidney is more firmly attached to the dorsal wall and is cranial to the left.

FIGURE 9-5 Details of structure of left kidney. A, Dorsal aspect, dissected in dorsal plane. B, Dorsal aspect, internal surface middorsal plane. C, Cross section. D, Cast of renal pelvis, dorsal aspect. E, Cast of renal pelvis, medial aspect.

Both kidneys are retroperitoneal. The dorsal surface is in contact with lumbar hypaxial muscles and often surrounded by fat; the ventral surface is covered by transparent parietal peritoneum. Each lies lateral to the aorta and caudal vena cava. The dorsal surface of each kidney is less convex than the ventral surface. The cranial pole of each kidney is covered with peritoneum on both the dorsal and the ventral surfaces, whereas only the ventral surface of the caudal pole is covered.

The kidneys lie in an oblique position, tilted cranioventrally. The right kidney is more firmly attached to the dorsal wall than is the left and has a correspondingly larger retroperitoneal area. Both kidneys are invested with a fibrous capsule surrounded by adipose tissue and are held in position by transversalis fascia. They are not rigidly fixed and may move during respiration or may be displaced by a full stomach. In some lean animals it is possible to palpate the kidneys, especially the left kidney. The right kidney lies more cranially than the left (see Fig. 9-1) and is in contact with the liver. Grandage (1975) has considered some effects of posture on the radiographic appearance of the kidney.

The kidney of an average-sized dog measures 6 to 9 cm in length, 4 to 5 cm in width, and 3 to 4 cm in thickness. The weight of the freshly excised kidney averages 25 to 35 g. Finco et al. (1971) made kidney measurements on radiographs of 27 normal male dogs prior to direct measurement at necropsy to establish a basis for estimating kidney size radiographically. Kidney weight and volume were highly correlated; kidney length was best correlated with kidney weight. Several tables of normal values and variations were presented.

Position and Relations

The craniolateral surface of the left kidney is in contact with the dorsal end of the medial surface of the spleen, the greater omentum, and the greater curvature of the stomach. Cranially, it is in contact with the left lobe of the pancreas and left adrenal gland. Dorsally, the kidney, with its adipose capsule, is related to the quadratus lumborum, transversus abdominis, and psoas muscles, as well as to the deep layer of the thoracolumbar fascia underlying the retroperitoneal or pararenal fat. Caudally, the left kidney of the female is in contact with the descending colon and the mesovarium. The peritoneum on the ventral surface of the kidney blends with the peritoneum suspending the ovary. In the male, the renal peritoneum is reflected onto the dorsal body wall as parietal peritoneum. Medially, the left kidney of the male is related to the left adrenal gland descending colon, mesocolon, and ascending duodenum. The descending colon is also related to the ventral surface of the kidney. The medial edge of the left kidney is located approximately 1 cm from the middorsal line in an average-sized dog; the cranial pole lies approximately 5 cm caudal to the dorsal third of the last rib.

The right kidney has its cranial pole embedded in the fossa of the caudate process of the caudate lobe of the liver. This extremity is located at the level of the thirteenth rib. It may be a few centimeters craniad or caudad, depending on the degree of gastric or, in the female, uterine distention. It may be in contact with the diaphragm and retractor costae muscle. The right adrenal gland is also related to the cranial pole of the right kidney. Medially, the right kidney is in close proximity to the caudal vena cava, and, ventrally, it is in contact with the right lobe of the pancreas and the ascending colon.

The kidney has an indentation or cavity on its medial border referred to as the renal hilus (hilus renalis). The space defined by the walls of the hilus is the renal sinus (sinus renalis) (see Figs. 9-3 and 9-5). The sinus contains the renal pelvis, a variable amount of adipose tissue, and branches of the renal artery, vein, lymphatics, and nerves. After they pass through the sinus, the vessels and nerves enter the parenchyma of the kidney.

The renal pelvis (pelvis renalis) (see Fig. 9-5) is a funnel-shaped structure that receives urine from the papillary ducts of the kidney and passes it into the ureter. The pelvis of the kidney is elongated in a craniocaudal direction, and is curved to conform with the lateral border of the kidney. It extends into the renal parenchyma both dorsally and ventrally by means of curved diverticula, the recesses of the renal pelvis (recessus pelvis). There are generally five or six recesses curving peripherally from each border of the pelvis.


The parenchyma of the kidney is made up of an internal medulla (medulla renis) and an external cortex (cortex renis) (see Fig. 9-5). When cut transversely (see Fig. 9-5C) the peripheral portion of the renal parenchyma or cortex appears granular, owing to the presence of numerous renal corpuscles and convoluted tubules (nephrons). When the kidney is cut in a dorsal plane, numerous cut ends of arcuate arteries and veins are apparent at the corticomedullary junction. The thickness of the renal cortex is approximately the same as the transverse diameter of the renal medulla. The peripheral surface of the cortex is covered by the fibrous capsule.

A median plane longitudinal section of the kidney shows the medulla as a continuous striated structure with its free edge facing the renal pelvis. This is the renal crest (crista renalis). Similar dorsal plane longitudinal sections on either side of the renal crest show the medulla separated into cone-shaped renal papillae (papilla renalis) with interlobar vessels between them. These renal papillae are the apices of the renal pyramids (pyramides renales), the base of which is at the level of the renal cortex. These pyramids extend from the cortex on the dorsal and ventral surfaces of the kidney into the center where they fuse into the renal crest. A variable number papillary foraminae (foramina papillaria) open on the border of the renal crest that faces the renal pelvis. These are the openings of the papillary ducts (ductus papillares) that pass urine into the renal pelvis, which leads to the ureter. These foraminae compose the area cribrosa of the renal crest. The renal papillae of the pyramids are surrounded by extensions of the renal pelvis called pelvic recesses (recessus pelvis) but there are no papillary foraminae on these papillae.

The Nephron

The nephron (nephronum) (Fig. 9-6) is a continuous contorted tube that serves for urine production and for the regulation of the volume and composition of the extracellular fluid (Reese, 1991). There are approximately 500,000 nephrons in the dog kidney. Each nephron begins at the double-layered glomerular capsule (capsula glomeruli), which is invaginated by a spherical rete of blood capillaries, the glomerulus. Vimtrup (1982) commented on the number, shape, and structure of glomeruli in several mammals. The glomerulus and capsule together form the renal corpuscle (corpusculum renale) (see Fig. 9-6). Renal corpuscles are present in the renal cortex, but not in the medulla. The following components compose the tubular nephron in order from the glomerular capsule to the collecting tubules and papillary ducts in the renal medulla: proximal convoluted tubule, proximal straight tubule, attenuated (thin) tubule that forms a loop, distal straight tubule and distal convoluted tubule.

FIGURE 9-6 Schema of vessels around the nephron or renal tubule.

Eisenbrant and Phemister (1979) investigated normal postnatal development of the dog kidney in puppies between 2 and 200 days of age. They found a subcapsular nephrogenic zone was present until approximately 8 days of age. This zone produced new nephrons and interstitial tissues. Deep to the nephrogenic zone there were renal corpuscles of increasing maturity at successively deeper levels. They estimated the total number of nephrons to be 445,000 per kidney and this did not vary significantly during subsequent growth. The corpuscular volume per nephron increased 249% between day 14 and day 200, whereas the increase in the tubular volume per nephron was 303%.

Books on renal morphology and function include Smith (1951); The Kidney, in four volumes, by Rouiller and Muller (1969-1971); and The Kidney, in two volumes, by Brenner and Rector (1991).

Vessels and Nerves

The kidney is a highly vascular organ, as would be expected (Fuller & Heulke, 1973; Morison, 1926). Briefly, blood enters the renal artery from the aorta, goes through end arteries, interlobar vessels, arcuate vessels, interlobular arteries, and finally to glomeruli via afferent arterioles. Efferent arterioles leave the glomeruli and course directly into the outer layer of the medulla, giving rise to long capillary nets that extend to the apical end of the pyramid, or they branch directly into intertubular capillary networks. The intrarenal vascular system of the puppy kidney was described by Evan et al. (1979).

The renal artery (arteria renis) bifurcates into dorsal and ventral branches. The site of bifurcation is extremely variable (Christensen, 1952). Variations in the renal artery are common, ranging from a single vessel to one with numerous branches or to completely doubled renal arteries. The two primary branches of the renal artery, end branches, divide into two to four interlobar arteries (aa. interlobares renis). These branch into arcuate arteries at the corticomedullary junction. The arcuate arteries (aa. arcuatae) radiate toward the periphery of the cortex, where they redivide into numerous interlobular arteries (aa. interlobulares). Afferent arterioles (arteriola glomerularis afferens) leave these to supply the glomeruli and thence the efferent arterioles (arteriola glomerularis efferens). The mean glomerular diameter in a 35-pound dog is 170 µm. According to Rytand (1938), there are 408,100 glomeruli in one canine kidney, with a total glomerular volume of 1247 mm3. Finco and Duncan (1972) found a correlation between kidney and nephron size and the body size of the dog.

Venous drainage of the kidney stems from the numerous stellate veins (venulae stellatae) in the fibrous capsule. These connect with veins of the adipose capsule and empty into interlobular (vv. interlobulares), arcuate (vv. arcuatae), and interlobar veins (vv. interlobares) before entering the main trunk of the renal vein (vena renis), which joins the caudal vena cava. Venous arcuate vessels, unlike their arterial counterparts, unite to form elaborate arches. Arcuate veins span the medulla to join the dorsal and ventral parts of the kidney. Evan et al. (1979) investigated the vascular system of the puppy kidney between 1 and 21 days after birth. They found the vascular system of the puppy kidney to be strikingly different from that of the adult. The most obvious difference was the lack of peritubular capillaries throughout the cortex. In their place were large sinusoidal vessels directly continuous with the venous system. The vascular arrangement of the efferent arterioles and the sinusoidal vessels appeared to function as a postglomerular shunt.

Bentley et al. (1988) examined the architecture and vasculature of the dog kidney using a dynamic spatial reconstructor. This device is a high-speed, volume-scanning, computed, radiotomographic imaging system, which, when used in conjunction with radiopaque methacrylate injections, allowed them to compare casts with the reconstructed images. Interlobar arteries and occasionally arcuate arteries could be clearly detected. Analysis of artery-to-vein transit times showed some to be as short as 3 seconds.

Capsular and parenchymal lymphatics are connected to interlobular plexuses that pass into trunks that leave the kidney at the hilus. They terminate in the lumbar lymph nodes. According to Peirce (1944) lymphatics in the kidney accompany the interlobular, arcuate, and interlobar vessels, surrounding them in an irregular network. The periarterial rete is thicker than the perivenous network. Cortical and perirenal lymphatics anastomose (O’Morchoe & Albertine, 1980).

A renal plexus (plexus renalis) surrounds the renal arteries where they enter the renal sinus. This plexus consists of postganglionic sympathetic axons with their cell bodies in aorticorenal ganglia (ganglia aorticorenalia). The plexus also contains preganglionic parasympathetic axons from the vagus nerve. Innervation is provided to the nephrons, blood vessels, and muscle in the renal pelvis.


The ureters (Figs. 9-1, 9-2, 9-7, and 9-8) carry urine from the kidneys to the bladder. The diameter of a single ureter measures 0.6 to 0.9 cm when it is distended. The length of the ureter depends on the size of the animal, averaging between 12 and 16 cm in a 35-pound dog. The right ureter is slightly longer than the left because of the more cranial position of the right kidney.

FIGURE 9-7 Bladder and prostate. A, Dorsal aspect. B, Ventral aspect, partially opened on midline.

FIGURE 9-8 Dorsolateral contrast radiograph of the bladder and ureters.

The abdominal part of the ureter begins at the renal pelvis, which receives urine from the renal crest. Running caudoventrally and mesially toward the urinary bladder, it is retroperitoneal being bound dorsally by the psoas muscles and ventrally by the peritoneum (see Fig. 9-1). The ureters lie dorsal to the testicular vessels in the male and to the ovarian artery and vein in the female. The right ureter lies in close association with the caudal vena cava and is 1 to 2 cm lateral to the aorta. The ureters pass ventral to the deep circumflex iliac and external iliac arteries and veins. In the male, the ureter crosses dorsal to the ductus deferens, 2 cm from the junction of the ductus deferens with the pelvic urethra. The pelvic part of the ureter enters between the two layers of peritoneum forming the lateral ligament of the bladder and reaches the dorsolateral surface of the bladder just cranial to its neck. In the female, it reaches the lateral ligament of the bladder after being associated with the broad ligament of the uterus. The ureters enter the bladder obliquely and after a short intramural course open by means of two slitlike orifices.

Woodburne and Lapides (1972) studied the size and shape of the dog ureter during peristaltic enlargement. They found it to enlarge 17 times during diuresis by thinning of the muscle coats. The collapsed ureter has a stellate lumen with the epithelial surfaces in contact.

Urinary Bladder

The urinary bladder (vesica urinaria) (see Fig. 9-7) is a hollow, musculomembranous organ that varies in form, size, and position, depending on the amount of urine it contains. The bladder in a 25-pound dog is capable of holding 100 to 120 mL of urine without being overly distended. When relaxed, the bladder in a 25-pound dog measures 17.5 cm in diameter and 18 cm in length. When contracted, it measures 2 cm in diameter and 3.2 cm in length. The bladder may arbitrarily be divided into a neck (cervix vesicae) connecting with the urethra, a body (corpus vesicae), and a blind cranial part, the apex (apex vesicae). The visceral peritoneum of the ventral surface of the bladder is separated from the parietal peritoneum of the abdominal wall, just cranial to the pubis. The greater omentum frequently occupies the space between these peritoneal layers. The median ligament of the bladder is the peritoneal fold that attaches the ventral surface of the bladder to the linea alba and symphysis pubis. Dorsally, the bladder is in contact with the small intestine (jejunum and frequently ileum) and with the descending colon cranial to the divergence of the uterine horns from the body of the uterus. In the male the deferent ducts and their genital fold lie dorsal to the neck of the bladder, whereas in the female the cervix and body of the uterus are in contact with the dorsal surface of the bladder. When empty, the bladder lies entirely, or almost entirely, within the pelvic cavity. The space on each side of the bladder is occupied by the small intestine.


There are three layers of muscle in the wall of the urinary bladder, similar to the arrangement of muscle fibers in the ureter: outer and inner longitudinal layers, and a relatively thick middle circular layer. The muscle fibers all take on an oblique appearance at the urethral-bladder junction. This bladder muscle is often referred to as the detrusor muscle. The tunica mucosa of the urinary bladder, like that of the ureter and renal pelvis, is made up of transitional epithelium. It is irregularly folded when the bladder is empty but the mucosal folds disappear during distention. A loose tela submucosa lies between the mucosa and the muscular layer. Internally, a triangular area near the neck of the bladder is termed the trigone of the bladder (trigonum vesicae). The apex of the trigone is at the urethral orifice, and the base is indicated by a line connecting the ureteral openings. This area is free from the characteristic mucosal folds, but poorly developed ridges, converging toward the urethral crest, denote the boundaries of the trigone.


The reflection of the peritoneum from the lateral and ventral surfaces of the urinary bladder to the lateral walls of the pelvis and to the ventral abdominal wall are known as ligaments of the bladder. These are made up of double layers of peritoneum separated by intercalated blood vessels, nerves, lymphatics, and adipose tissue, as well as by the ureters, deferent ducts, and vestiges of embryonic structures. The largest peritoneal fold, the median ligament of the bladder (lig. vesicae medianum) is reflected from the ventral surface of the bladder to the symphysis pelvis and the linea alba of the abdominal wall as far cranially as the umbilicus. It is median in position and triangular in shape. In the fetus the median ligament contains the urachus (stalk of the embryonic allantois) and the umbilical arteries. These normally disappear shortly after birth, leaving only the peritoneal fold. A vestigial fibrous urachus may sometimes be found in the free edge of the ligament. In an average-sized dog this median ligament has its greatest height caudally (6 cm) and narrows cranially to form an acute angle with the abdominal wall at the umbilicus. Caudally, the ligament ends approximately at the level of the vaginovestibular junction in the female, and at the level of a transverse plane through the middle of the prostate gland in the male.

The lateral ligaments of the bladder (lig. vesicae laterales) (Fig. 9-9) connect the lateral surfaces of the bladder to the lateral pelvic walls. They are also triangular in shape. The lateral ligaments of the bladder contain the round ligament of the bladder and the ureter. In the fetus each lateral ligament contains a large umbilical artery that extends cranially along the rudimentary bladder and courses in the median ligament of the bladder with the urachus to the umbilicus. The urachus is the stalk of the allantois, which connects from the apex of the rudimentary bladder to the allantoic portion of the fetal membranes. Before birth, the bilateral umbilical arteries (branches of the internal iliac arteries) carry blood from the fetus to the placenta and are components of the umbilical cord. When the umbilical cord is severed at birth, the arteries retract and become fibrous cords between the bladder and the umbilicus that disappear in the young dog and are rarely visible in adult dogs. The narrowed lumen of each umbilical artery remains patent between the internal iliac artery and the bladder, where the relatively minute cranial vesical artery leaves the umbilical artery to vascularize the apex and body of the bladder. The remnants of these arteries in the lateral ligaments of the bladder are referred to as the round ligaments of the bladder (lig. teres vesicae). The ureter and round ligament of the bladder cross at nearly right angles to each other at the junction of the broad and lateral ligaments. The ureter is the more mesial of the two structures. The lateral ligaments of the bladder, in the female, blend laterally with the broad ligament of the uterus (mesometrium) as well as with the lateral pelvic wall. In the male, the ureter and ductus deferens cross each other a few centimeters from the entrance of the ureters into the bladder (see Fig. 9-7). The ductus deferens is suspended by the mesoductus deferens, a fold of peritoneum, which at the vaginal ring separates from the mesorchium, which is the peritoneal fold containing the testicular vessels and nerves. The ductus deferens and its mesoductus deferens course dorsocaudally, dorsal to the ureters in the lateral ligaments of the bladder to terminate in the prostatic urethra. Dorsal to the bladder a short fold of peritoneum connects between each ductus deferens. This is the genital fold (plica genitalis). In the female this genital fold connects between the two uterine horns. The peritoneal pocket between the rectum and the genital fold and the two ductus deferens or the initial part of the two uterine horns, the uterus and cranial vagina is the rectogenital pouch (excavatio rectogenitalis). A small pubovesical pouch (excavatio pubovesicalis) is present between the bladder and its lateral ligaments and the pubis

FIGURE 9-9 Urogenital ligaments of the male, ventral aspect.

Vessels and Nerves

The urinary bladder receives its major blood supply through the caudal vesical arteries. These are branches of the vaginal or prostatic arteries that are branches of the internal pudendals from the internal iliac arteries. The small cranial vesical artery from the umbilical supplies the bladder apex. The venous plexus on the urinary bladder drains primarily into the internal pudendal veins. The lymphatics of the bladder drain into the hypogastric and lumbar lymph nodes (Baum, 1918).

Bladder innervation is complex from an anatomic as well as a physiologic understanding. The bladder receives autonomic innervation from both the sympathetic and parasympathetic general visceral efferent neurons. The sympathetic innervation primarily functions in urine storage and the parasympathetic innervation primarily functions in evacuation of urine. Sympathetic preganglionic cell bodies are located in the lateral horn of the first four lumbar spinal cord segments. Preganglionic axons enter the abdomen through the lumbar sympathetic trunk and splanchnic nerves to synapse on cell bodies of postganglionic axons in the caudal mesenteric ganglion or in the bladder wall. For the former, postganglionic axons leave the caudal mesenteric ganglion in the hypogastric nerves and course to the pelvic plexus, which is associated with the vaginal or prostatic artery. These axons follow the branches of the artery to the bladder to innervate the detrusor muscle with inhibitory synapses and the bladder neck sphincter muscle with excitatory synapses. The sympathetic preganglionic axons that did not synapse in the caudal mesenteric ganglion course to the bladder wall through the hypogastric nerves, the pelvic plexus and its branches that follow the arteries to the bladder. Synapse occurs on cell bodies of short postganglionic axons within the wall of the bladder. Parasympathetic cell bodies of preganglionic axons are located in the lateral horn of the sacral spinal cord segments. The preganglionic axons course into the pelvic nerve through the ventral branches of the sacral spinal nerves. The pelvic nerve courses to the pelvic plexus and synapses on cell bodies of postganglionic axons in pelvic plexus ganglia or on cell bodies in the bladder wall. These axons of postganglionic cell bodies are excitatory to the detrusor muscle causing contraction and bladder evacuation. The striated urethralis muscle which functions to store urine is innervated by general somatic efferent neurons with their cell bodies in the ventral horn of sacral spinal cord segments. Their axons are distributed to this muscle through the sacral plexus and the branches of the pudendal nerve. The axons of visceral afferent neurons that innervate the bladder and urethra reverse the pathway of the efferent neurons. Their cell bodies are in cranial lumbar and sacral spinal ganglia. This complex reflex pathway of innervation is under voluntary control by cranially projecting sensory spinal cord pathways, centers in the caudal brainstem and caudally projecting upper motor neuronal pathways.

Petras and Cummings (1978) describe the location of the cells of origin for the sympathetic and parasympathetic innervation of the urinary bladder and urethra in the dog. Their study, using horseradish peroxidase injected into the bladder and urethra and counterstained with cresyl violet, demonstrates the presence of both sympathetic and parasympathetic intramural ganglia and axons. Thus there is a direct preganglionic sympathetic pathway to the urinary bladder and urethra in addition to the postganglionic sympathetic innervation.

Reproductive Organs

The reproductive system in vertebrates is a most varied assemblage of primary and accessory organs and parts, which begin developmentally in a similar fashion but result in strikingly different forms in the adult. In recent years the study of reproduction in animals (theriogenology) has made great strides, and as a result of our new understanding we are now able to manipulate the system in many ways such as to facilitate artificial insemination, egg or embryo transfer, freezing and storage of eggs and embryos and cloning of various species. For explanations of developmental processes in domestic animals see Noden and de Lahunta (1985). For an overall view of the reproductive system from fish to humans there is nothing better than Marshall’s Physiology of Reproduction. The most recent revision of Marshall’s by Lamming (1990-1992) devotes one volume to a consideration of reproductive cycles and female anatomy, a second to reproductive structures and functions of the male, and a third to pregnancy and lactation. Other books on the reproductive system include Austin and Short (1982), Segal et al. (1973), and Cupps (1991). For information on the reproductive habits, cycles, and gestations of mammals of the world, including canids, reference should be made to Asdell’s Patterns of Mammalian Reproduction: A Compendium of Species-Specific Data by Hayssen and van Tienhoven (1993).

Male Genital Organs

The canine male genital organs (organa genitalia masculina) (Figs. 9-10 to 9-34) consist of the scrotum, the testes, the epididymides, the deferent ducts, the spermatic cord, the prostate gland, the penis, and the urethra.

FIGURE 9-10 Topographic relations of the penis and other pelvic structures. (The right ischium is removed.) (From Christensen GC: Angioarchitecture of the canine penis and its role in the process of erection, Ph.D. Thesis, Ithaca, NY, 1953, Cornell University.)

FIGURE 9-11 Ventral view of the abdomen. The left inguinal mammary gland has been removed to expose the superficial inguinal ring with the vaginal process of the female extending through it. (Figure 9-13 is a transection of the process to show that it is collapsed and wrapped around fat and the round ligament of the uterus.)

FIGURE 9-12 Schema of the vaginal tunic in the male with an inset of a transection. (From Evans HE, de Lahunta A: Miller’s guide to the dissection of the dog, 3rd ed, Philadelphia, 1988, Saunders, p. 181.)

FIGURE 9-13 Diagram of transected vaginal process in male and female. (Dotted lines indicate spermatic fascia. In the male the contents of the vaginal tunic are not shown. See Figure 9-12.)

FIGURE 9-14 Structures of testes and scrotum. A, Right testis, lateral aspect. B, Left testis, medial aspect. C, Schematic cross section through scrotum and testes.

FIGURE 9-15 Male genitalia, ventral view. As the vaginal tunic with spermatic cord leaves the superficial inguinal ring, it is joined by muscle fibers of the internal abdominal oblique that form the cremaster muscle.

FIGURE 9-16 Diagram of peritoneal reflections and the male genitalia.

FIGURE 9-17 Lateral contrast radiograph of the bladder and urethra in the male.

FIGURE 9-18 Male perineum. A, Superficial muscles, caudal aspect. B, Dorsal section through pelvic cavity. The bilobed bulb of the penis is transected, and the proximal portion removed.

FIGURE 9-19 Anal region and root of the penis with superficial muscles, right lateral aspect.

FIGURE 9-20 Schematic left lateral aspect of pelvic structures and a median section of the penis. (Drawn by L. Buchholz, DVM Class of 1994.)

FIGURE 9-21 Internal morphologic characteristics of the penis. Upper drawing, a parasagittal section. A to E, Cross-sections at five levels indicated by letters on upper drawing. (From Christensen GC: Angioarchitecture of the canine penis and its role in the process of erection, Ph.D. Thesis, Ithaca, NY, 1953, Cornell University.)

FIGURE 9-22 Corrosion preparation of proximal half of the penis. (From Christensen GC: Angioarchitecture of the canine penis and its role in the process of erection, Ph.D. Thesis, Ithaca, NY, 1953, Cornell University.)

FIGURE 9-23 Semidiagrammatic view of penis. The pars longa glandis and the muscles of the root are illustrated as if transparent. The vessels of only one side are shown. (From Christensen GC: Angioarchitecture of the canine penis and its role in the process of erection, Ph.D. Thesis, Ithaca, NY, 1953, Cornell University.)

FIGURE 9-24 Internal morphologic characteristics of the glans penis. (The pars longa glandis has been slit and partially reflected.) (From Christensen GC: Angioarchitecture of the canine penis and its role in the process of erection, Ph.D. Thesis, Ithaca, NY, 1953, Cornell University.)

FIGURE 9-25 Os penis, lateral aspect with two transections.

FIGURE 9-26 Drawings of corrosion specimen of bulbus glandis and part of corpus spongiosum. (The os penis has been removed.) A, Superficial view showing distribution of branches of dorsal artery of the penis. B, The near half of the bulbus glandis is cut away, showing the route of the deep branches of the dorsal arteries. (From Christensen GC: Angioarchitecture of the canine penis and its role in the process of erection, Ph.D. Thesis, Ithaca, NY, 1953, Cornell University.)

FIGURE 9-27 Development of the os penis in littermate Beagles 35 days after birth. The first stages of ossification as seen after clearing in glycerine and staining with alizarine red. Dorsal view. A, There is no indication of cartilage or bone in the penis of this pup. Note that the proximal part of each corpus cavernosum consists of fibrous trabeculae with cavernous spaces. The distal right and left fibrocartilaginous portions of the corpora cavernosa fuse dorsal to the urethra. B and C, Paired cartilaginous nodules with ossification plaques form in the noncavernous portion of each corpus cavernosum in the region of the future base of the os penis. The entire distal portion of the corpora cavernosa will eventually fuse completely and ossify except for the distal tip, which remains cartilaginous. (Figures 9-27 to 9-30 drawn by M. Simmons from preparations of H. Evans.)

FIGURE 9-28 A lateral view shows the relationship of the developing os to the urethra.

FIGURE 9-29 Right and left bony plaques, surrounded by a cartilaginous capsule, are beginning to fuse on the middorsal line. The distal fibrocartilage is destined to ossify except for the terminal end, which will remain cartilaginous in the adult (see Fig. 9-33). The vascular connections to the bulbus glandis, seen leaving the corpus spongiosum surrounding the urethra, are shown coursing over the ventrolateral margin of each bony plaque. A, Ventral view. B, Dorsal view.

FIGURE 9-30 The middle portion of the developing penis in a Beagle pup 65 days after birth. A, Ventral view. By this stage the right and left ossifications in the corpora cavernosa have elongated and fused completely except for a slight notch at the proximal end. Vascular connections from the corpus spongiosum surrounding the urethra are shown forming the bulbus glandis. B, Lateral view of the os penis to show that the urethra is hidden within the urethral groove.

FIGURE 9-31 Diagrams of circulatory pathways of the penis. A, In nonerection. B, In erection. The upper arrows on each side of the dorsal vein of the penis represent the effect of contraction of the ischiourethral muscles. The lower arrows, caudal to the bulbus glandis, represent the effect of the constrictor vestibulae muscles of the female. (From Christensen GC: Angioarchitecture of the canine penis and its role in the process of erection, Ph.D. Thesis, Ithaca, NY, 1953, Cornell University.)

FIGURE 9-32 Diagram of venous pathways in the bulbus glandis connecting the deep vein of the glans and the dorsal vein of the penis. The ventral shunt (B to A) is the principal route when the penis is relaxed; the dorsal shunt (C to D) is used during erection. (From Christensen GC: Angioarchitecture of the canine penis and its role in the process of erection, Ph.D. Thesis, Ithaca, NY, 1953, Cornell University.)

FIGURE 9-33 A schematic interpretation of changes in shape of the glans penis during erection and copulation. A, Resting state. B, Erection as blood fills the cavernous spaces of the bulbus glandis and pars longa glandis. C, Intromission and engorgement during copulation when contraction of the ischiourethral muscle in the male and the constrictor vestibulae of the female results in venous occlusion of the veins draining the penis. When the penis is in the fornix of the vagina, the ligament of the cartilage of the os deforms the distal end of the glans to form a corona glandis. When this shape is attained, the opening of the urethra faces dorsally in close proximity to the cervix.

FIGURE 9-36 Longitudinal section of left and right ovaries of a Beagle, early in gestation. There are seven corpora lutea in the left ovary and one in the right. The uterus contained four embryos in the left horn and three in the right, indicating that at least two blastocysts migrated from the left uterine horn to the right. (From Evans HE: Gaines symposium, Ithaca, NY, 1974.)


The scrotum (see Figs. 9-14 and 9-15) is a pouch of skin divided by a median septum into two components, each of which is occupied by a testis, an epididymis, and the distal part of the spermatic cord. The scrotal septum (septum scroti) is a median partition that is made up of all the layers of the scrotum except the skin. In the dog, the scrotum is located approximately two thirds of the distance from the preputial opening to the anus. It lies between the thighs and has a spherical shape, indented in an oblique craniocaudal direction by an indistinct raphe scroti. The left testis is usually farther caudad than the right, allowing the surfaces of the testes to glide on each other more easily and with less pressure.

The scrotal integument is pigmented and covered with fine scattered hairs. Sebaceous and tubular (sudoriparous) glands are well developed. Deep to the outer integument of the scrotum is a poorly developed layer of smooth muscle mixed with collagenous and elastic fibers that is sometimes spoken of as the tunica dartos. Dorsally, the tissue forming the septum blends with the abdominal fascia. Contraction of the dartos causes the integument of the scrotum to retract and draw the testes close to the body.

Extending into each scrotal sac is an evaginated pouch of peritoneum, the vaginal tunic (tunica vaginalis) (see Figs. 9-12 and 9-14), covered by spermatic fascia of the abdominal wall. The vaginal tunic and fascia wrap the descended testis and spermatic cord in such a way as to result in a double-walled extension of abdominal peritoneum. This was a vaginal process before the descent of the testis with its duct system, vessels, and nerves (see Figs. 9-12 and 9-15) Zietzsehman (1928). The outer wall, or parietal layer of the vaginal tunic, is separated by a space, the vaginal canal (canalis vaginalis) or vaginal cavity (cavum vaginale), from the visceral layer of the vaginal tunic. The vaginal canal surrounds the spermatic cord and the vaginal cavity surrounds the testis. The vaginal canal is continuous with the peritoneal cavity at the vaginal ring.

The development of the vaginal tunic in the male and the vaginal process in the female is similar. As the evaginating peritoneum passes through the deep inguinal ring, it is invested by the transversalis fascia; as it emerges from the superficial inguinal ring it is joined by the superficial and deep abdominal fascia. The combined fascias form the spermatic fascia, which covers the parietal layer of the vaginal tunic (see Fig. 9-12 ).

The cremaster muscle (see Fig. 9-14) arises from the caudal free border of the internal abdominal oblique (or occasionally from the transversus abdominis) and inserts on the spermatic fascia and parietal layer of the vaginal tunic. The action of the muscle is protective in that it reflexly pulls the testis closer to the body in response to cold.

The scrotum, because of its thin, hairless skin, its lack of subcutaneous fat, and its ability to contract toward the body, functions as a temperature regulator for the tail of the epididymis. Evidence indicates that the epididymis, as the site of sperm storage, is the most heat-sensitive region of the male reproductive tract (Bedford, 1978). When the question is raised as to why a scrotum exists in some animals and not in others (there are approximately 1500 ascrotal species) we still do not have a satisfactory answer. Freeman (1990) has reviewed the question and came to the conclusion that the scrotum evolved to provide a cool environment for sperm storage, and testicular descent evolved because it improves sperm quality so that fewer are needed. He provides tables that show the proportional size of the testes in many species of animals. There are six mammalian orders that have species with internal testes as well as species with external testes.

Vessels and Nerves

The principal blood vessel to the scrotum is the ventral scrotal branch of the external pudendal artery. The cremasteric artery arises from the deep femoral artery. The scrotal arteries run along the cranioventral surface of the testis, superficial to the parietal layer of the vaginal tunic. The perineal branches of the internal pudendal artery supply dorsal scrotal arteries. The draining veins follow the same course in reverse.

The genital rami, branches of the genitofemoral nerve from the ventral branches of the third and fourth lumbar nerves, innervate the skin of the prepuce. The superficial perineal nerve, a branch of the pudendal from sacral nerves 1, 2, and 3, supplies all of the scrotum, according to Spurgeon and Kitchell (1982b). Postganglionic sympathetic axons supplying the tunica dartos enter via the sacral plexus and the pudendal and superficial perineal nerves.


The testis, or male gonad (see Fig. 9-14), is oval in shape and located within the scrotum. The length of the testis in a 25-pound dog averages 3 cm and the width 2 cm. The fresh organ weighs approximately 8 g. In normal position, the testis of the dog is situated obliquely, with the long axis running dorsocaudally. The epididymis is adherent to the dorsolateral surface of the organ, with its head located at the cranial end and its tail at the caudal extremity of the testis.

The surface of the testis is invested by the tunica albuginea, a dense, white fibrous capsule. Covering the testis most immediately is the visceral vaginal tunic, a serous membrane continuous with the peritoneum of the spermatic cord and the abdominal cavity. The tunica albuginea joins the centrally located mediastinum testis by means of interlobular connective tissue lamellae (septula testis), which converge centrally. The mediastinum testis is a cord of connective tissue running lengthwise through the middle of the testis. The lobuli testes (wedge-shaped portions of testicular parenchyma) are bounded by the septula. The lobuli contain the convoluted seminiferous tubules (tubuli seminiferi contorti), a large collection of twisted canals. Spermatozoa are formed within the epithelial lining of the tubules, which contains spermatogenic cells and sustentacular (Sertoli) cells. The organization, motility, and structure of sperm cells have been reviewed by André (1982). The longevity of spermatozoa in the reproductive tract of the bitch can be several days (Doak et al., 1967), and at least 6 days as shown by Concannon, Whaley, and Lein (1983). HE

Straight seminiferous tubules (tubuli seminiferi recti) are formed by the union of the convoluted seminiferous tubules of a lobule. The mediastinum testis contains a network of confluent spaces and ducts called the rete testis. These connect the straight tubules with the efferent ductules (ductuli efferentes testis). Testicular blood vessels and lymphatics enter and leave through the mediastinum. The lobuli testis also contains interstitial cells (of Leydig) between tubular elements. Johnson et al. (1970) and Setchell (1978) consider the anatomy, physiology, biochemistry, and other parameters of the testis.

Vessels and Nerves

The testicular artery and the artery of the ductus deferens supply the testis and epididymis. The testicular artery (homologue of the ovarian artery of the female) arises from the ventral surface of the aorta at the level of a transverse plane through the fourth lumbar vertebra. The right artery originates cranial to the left, corresponding to the embryonic positions of the testes. The artery of the ductus deferens, a branch of the prostatic artery from the internal pudendal, follows the ductus deferens into the spermatic cord to the level of the epididymis. It sends branches to the epididymis and anastomoses with the testicular artery. The testicular vein follows the arterial pattern but forms an extensive pampiniform plexus (plexus pampiniformis) in the spermatic cord, surrounding the testicular artery lymphatics, and nerves. The right testicular vein empties into the caudal vena cava at the level of the origin of its arterial counterpart. The left drains into the left renal vein. Harrison (1949) made a detailed comparative study of the vascularization of the mammalian testis.

The testicular and epididymal lymphatics anastomose into a variable number of trunks that drain into the lumbar lymph nodes (see Chapter 13).

The nerve supply to the testis is derived from the sympathetic division of the general visceral efferent component of the autonomic nervous system. The nerves of the testicular plexus accompany the testicular arteries distally and enter the testis with either the blood vessels or the efferent ducts. Indirectly, they are derived from the fourth, fifth, and sixth lumbar sympathetic trunk ganglia. The testicular plexus is derived from the abdominal aortic plexus at the level of the origin of the testicular arteries. These testicular vessels, lymphatics and nerves are suspended in the spermatic cord by the mesorchium, which is a fold of the visceral layer of the vaginal tunic. This is continued in the abdomen as a fold of the parietal layer of the peritoneum. The blood vessels and smooth muscle fibers in the testis receive a sympathetic nerve supply, but the seminal epithelium and the interstitial secretory tissue do not. Elimination of the sympathetic nerve supply to the testis is followed by degeneration of the seminal epithelium and hypertrophy of the interstitial secretory tissue (Kuntz, 1919b). The degenerative changes are considered to be the result of paralysis of the blood vessels in the spermatic cord and testis.


Cryptorchidism, or failure of the testis to descend, is the most important congenital anomaly of the testis. This condition is comparatively frequent and is believed to be hereditary in some instances. Cox et al. (1978) investigated 12 cases of cryptorchidism in Miniature Schnauzers. Five were unilateral and seven were bilateral. All of the unilateral cases had retained testes on the right side. When retained testes were bilateral the right testis was always smaller. Their observations suggested a multigene defect. In a cryptorchid animal one or both testes are retained either in the abdominal cavity (in the region of the inguinal canal) or between the superficial inguinal ring and the scrotum. Sterile, cryptorchid dogs usually possess normal sexual desire. For a discussion of cryptorchidism see Wensing and van Straten (1980). Hayes et al. (1985) studied 1.8 million documented medical records and identified 2912 dogs (in 104 different breeds) that had cryptorchid testes. There were 14 breeds with significantly high risk.

According to Runnells (1954) testicular tumors of dogs have been reported to cause anatomic alterations, such as atrophy of the opposite testis and enlargement of the prepuce and prostate gland. Hayes et al. (1985) reported that testicular tumors were found in 5.7% of the 2912 cryptorchid dogs whose records they reviewed. Half had Sertoli cell tumors, and one-third had seminomas.

Male pseudohermaphroditism and true hermaphroditism have been reported in the dog by Lee and Allam (1952), Brodey et al. (1954), and Bodner (1987). Female pseudohermaphroditism, considered rare (Meyers-Wallen & Patterson, 1986), was reported by Olson et al. (1989) in three sibling Greyhounds.

Descent of the Testes

In the majority of mammalian species the testes migrate from their developmental position within the abdomen near the kidneys to a location outside of the body wall, usually in a scrotum (see Fig. 9-15). For these species, including the dog, if neither testis descends (bilateral cryptorchid) spermatogenesis is eliminated and the animal is infertile. If only one testis descends (monorchid or unilateral cryptorchid) fertility is lessened. (Many rodents have testes that descend periodically coincident with breeding. In such species the testes can be gently squeezed back into the body cavity at any time because of the large inguinal canal.)

Much important research on testicular descent in domestic animals has been conducted at the Institute of Veterinary Anatomy of the State University at Utrecht, the Netherlands, by Wensing and co-workers: Wensing (1968, 1973a, 1973b, 1980); Baumans et al. (1981, 1982, 1983); Baumans (1982); Wensing and Colenbrander (1986).

Whereas in most mammals testicular descent occurs in fetal life, in the dog it occurs at approximately the time of birth and this makes the dog a good subject for the study of the mechanics and hormonal control of descent. A descriptive and illustrated experimental study of normal development and the factors responsible for the descent of the testes in the dog was published as a thesis in the Netherlands by Baumans (1982). (Included in the thesis are six papers with co-workers, each with a bibliography.) Baumans found that the major factors essential for the descent were first an outgrowth and then a regression of the gubernaculum testis. The gubernaculum is a mesenchymal mass enclosed in a fold of peritoneum that extends from the testis across the mesonephros (kidney ridge) to the inguinal area. It is attached to the caudal pole of the testis and epididymal part of the mesonephric duct. Its distal end continues through the abdominal wall where the inguinal canal forms around it. Here, the gubernaculum is invaded by an outgrowth of parietal peritoneum, the vaginal process. During fetal growth the testis moves caudally to the level of the inguinal canal as the trunk elongates. There are two phases of the migration of the testis through the inguinal canal and into the scrotum. In the first phase the extra-abdominal part of the gubernaculum increases enormously in length and volume expanding beyond the inguinal canal and dilating it. When this expansion exceeds the size of the testis and passive resistance is reduced, the testis passes through the canal adjacent to the vaginal process. Here it rests in a mass of swollen gubernaculum, which is still covered by vaginal process peritoneum. An important feature of this testicular migration is that the testis brings with it the visceral peritoneum that formed around it at its site of development in the dorsal abdominal wall. Completion of this first phase of testicular descent initiates the second phase in which the gubernaculum is transformed by tissue degeneration from an expanded mucoid mass into a small fibrous structure in the scrotal sac, thus making room for the testis. This converts the gubernaculum into the proper ligament of the testis, which attaches the tail of the epididymis to the testis, and the ligament of the tail of the epididymis, which attaches the tail to the distal portion of the vaginal tunic and spermatic fascia. The latter is derived from the remnants of the gubernaculum that surrounds the vaginal tunic. The degeneration of the gubernaculum results in the testis being located at the distal extent of the vaginal tunic.

Baumans found that on the day of birth the testis was located halfway between the kidney and the deep inguinal ring. The gubernaculum had reached its maximum development and was beginning to show signs of regression histologically and histochemically. By day 3 or 4 after birth the testis passes through the inguinal canal and the gubernaculum regresses. By 35 to 40 days after birth the testis reaches its definitive position in the scrotum.

To test the factors responsible for gubernacular growth and regression, Baumans removed the testes in fetal dogs and found that gubernacular outgrowth ceased and there was no epididymal descent. Removal of the testis at birth resulted in retarded gubernacular regression and a delayed epididymal descent. After further investigation it was concluded that the testis induces gubernacular outgrowth and regression and thereby regulates its own descent. It was found that testicular hormones, particularly testosterone, are synthesized in the testis. In the first phase of descent an unidentified testicular factor stimulated gubernacular proliferation and swelling. Sustentacular cells are thought to be the source of this factor. In the second phase of descent testosterone induced gubernacular regression. In regard to the mechanical role of the gubernaculum, it was found that a connection between the testis and the gubernaculum (see Fig. 9-15) was essential for normal testicular descent. By the end of the second phase of descent the testis plays a mechanical role in distending the scrotum.

Wensing and Colenbrander (1986), in discussing normal and abnormal descent of the testis, make a distinction between the morphologic characteristics of testicular descent in mammals with a striplike cremaster muscle (ungulates, dog, and humans) and those with a saclike cremaster muscle (rodent, lagomorph). They believe that the bilaminar saclike cremaster muscle of rodents and lagomorphs continues to grow and increase in size even after the regression of the gubernaculum and thus plays a role in descent of the testis. Regardless of the morphologic characteristics of the cremaster muscle, they stress the importance of a normal gubernacular outgrowth and regression for affecting testicular descent. Their experimental results showed that the first phase in the descent process (gubernacular outgrowth) does not depend on the presence of active interstitial cells, testosterone, testosterone receptors, or gonadotropins, although the presence of the testis is essential.


The epididymis (see Fig. 9-14) is where spermatozoa are stored before ejaculation. It is comparatively large in the dog and consists of an elongated convoluted tube, the coils of which are held together by collagenous connective tissue. The epididymis lies along the dorsolateral border of the testis. The head (caput) begins on the cranial medial surface of the testis but immediately twists around the cranial extremity to attain the lateral side. It is slightly larger than the remainder of the epididymis. It continues as the body (corpus), which runs along the dorsolateral surface of the testis, and then as the tail (cauda epididymidis), which is attached to the caudal extremity of the testis by the proper ligament of the testis. Beyond the tail this duct is continued craniodorsally as the ductus deferens which becomes part of the spermatic cord along with the testicular vessels and nerves. The epididymis has its concave surface in juxtaposition with the testis. Its medial edge is attached to the testis by visceral vaginal tunic, the distal mesorchium (mesorchium distale). The latter extends medially between the lateral edge of the body of the epididymis and the testis, forming a potential space, the testicular bursa (bursa testicularis). The bursa is limited cranially and caudally by the epididymal head and tail, which adhere tightly to the testis.

Ductus Deferens

The deferent duct (ductus deferens) (see Figs. 9-16 to 9-18) is the continuation of the duct of the epididymis. Beginning at the tail of the epididymis, it passes cranially along the dorsomedial border of the testis, continues dorsally in the spermatic cord, and enters the abdominal cavity through the inguinal canal. Running in a fold of peritoneum, the mesoductus deferens, it crosses ventral to the ureter at the lateral ligament of the bladder and penetrates the prostate to open into the pelvic urethra, lateral to the colliculus seminalis.

In a 25-pound dog, the ductus deferens averages 17 to 18 cm in length and 1.6 to 3 mm in diameter. The epididymal end of the duct is slightly tortuous, but it straightens out in its course along the medial surface of the testis. It is attached to the testis, along with the artery and vein of the ductus deferens, by a special fold of the visceral vaginal tunic called the mesoductus deferens. At the cranial extremity of the testis the mesoductus deferens blends with the peritoneal fold containing the testicular vessels, nerves, and lymphatics, the proximal mesorchium (mesorchium proximale) (see Fig. 9-12). In the abdominal cavity at the vaginal ring the deferential fold of peritoneum leaves the vaginal ring, to which it is attached at one edge, and courses caudodorsally to reach the dorsal surface of the bladder. The ductus deferens lies 3.4 cm from the body wall when the deferential fold of peritoneum is stretched out. Dorsal to the bladder, the right and left deferent ducts come into close apposition approximately 2 cm before they penetrate the prostate gland. For approximately 1.5 cm before they contact each other, the ducts are joined by a fold of peritoneum, the genital fold (plica genitalis). Peritoneum covering the pelvic portion of the ductus is reflected ventrally onto the prostate, bladder, and ureters. Dorsally, it is reflected over the prostate and then onto the ventral surface of the rectum at the rectogenital fossa.

Vessels and Nerves

The artery of the ductus deferens is a branch of the prostatic artery, which, in turn, arises from the internal pudendal. It accompanies the ductus deferens to the epididymis, which it also supplies with blood. The artery of the ductus deferens anastomoses with the testicular artery in the spermatic cord. The vein of the ductus deferens runs in the spermatic cord with the deferent duct, and empties into the internal iliac vein. The lymphatics drain into the hypogastric and medial iliac lymph nodes. Nerves to the ductus deferens are autonomic, arising from the pelvic plexus. The hypogastric nerves (sympathetic), via the pelvic plexuses, supply the pelvic part of the ductus deferens. Parasympathetic axons are thought to be distributed via the pelvic plexus only to the epididymis and musculature of the ductus deferens.

Spermatic Cord

Each spermatic cord (funiculus spermaticus) (see Figs. 9-12 and 9-16) is composed of the ductus deferens and its vessels, and the testicular vessels and nerves, along with their serous membrane coverings, the mesoductus deferens and the mesorchium. These structures pass through the inguinal canal during the descent of the testis. The spermatic cord begins at the vaginal ring, the point at which its component parts converge to leave the abdominal cavity via the inguinal canal. The ductus deferens, which arises from the tail of the epididymis, leaves the vaginal ring, runs caudomedially in the deferential fold of peritoneum, and enters the prostate gland before opening into the prostatic part of the pelvic urethra. The ductus deferens is accompanied by the small artery of the ductus deferens, which arises from the prostatic artery and the vein of the ductus deferens, which drains into the internal iliac vein. The testicular artery originates from the ventral surface of the aorta. The testicular arteries arise cranial to the origin of the caudal mesenteric artery. The testicular artery runs laterally and caudally, crossing the ventral surface of the ureter, at which point it is joined by the testicular vein and nerve. The left testicular vein empties into the left renal vein and the right into the caudal vena cava. The peritoneal fold, the proximal mesorchium, enclosing the testicular vessels is attached to the abdominal wall in a line slightly lateral to the junction of the transversus abdominis and psoas muscles. The plexus of the testicular nerves arises from the area of the sympathetic trunk between the third and the sixth lumbar sympathetic trunk ganglia. The testicular lymph vessels pass to the lumbar lymph nodes.

The components of the spermatic cord are joined together by loose connective tissue and are surrounded by the visceral layer of the vaginal tunic. The peritoneal ring formed by the vaginal tunic passing through the deep inguinal ring is termed the vaginal ring (see Fig. 9-12). There is usually an irregular mass of fat at the vaginal ring, covered by peritoneum. It overlaps the cranial border of the ring and probably acts as a valve to decrease the possibility of intestinal or omental herniation. The fat mass may be in two separate parts.

The ductus deferens, with its vessels, is enveloped by one fold of peritoneum at the vaginal ring, the mesoductus deferens and the testicular vessels and nerves are covered by another, the mesorchium. The double layer of peritoneum uniting these two folds to each other and to the edge of the vaginal ring is termed the mesofuniculus. It may be compared to the mesentery, which attaches the intestines to the abdominal wall.

Along the path of the spermatic cord from the deep inguinal ring to the testis, the relationship of the vaginal tunic and the enclosed structures remains constant. The tunic also reflects over the testis as its visceral peritoneum which joins the distal mesorchium along the dorsomedial border of the organ. A small, circumscribed area on the tail of the epididymis is free of tunic, allowing the ligament of the tail of the epididymis (embryonic gubernaculum testis) to attach the epididymis to the spermatic fascia.

The inguinal canal (see Fig. 9-12) is a fissure through the abdominal muscles that connects the deep and the superficial inguinal ring (Ashdown, 1963; McCarthy, 1976). It is located approximately 1 cm craniomedial to the femoral ring. The femoral ring affords passage for the femoral vessels. The inguinal canal is bounded medially by the rectus abdominis muscle, cranially by the internal oblique muscle, and both laterally and caudally by the aponeurosis of the external abdominal oblique muscle. The superficial ring, located 2 to 4 cm lateral to the linea alba, is merely a slit in the aponeurosis of the external abdominal oblique muscle. It represents where the abdominal wall formed around the gubernaculum in the fetus. The cranial wall of the inguinal canal is made up of the transversus abdominis and internal abdominal oblique muscles, as well as the aponeurosis of the external abdominal oblique muscle. Only the latter forms the caudal wall of the canal.

As the spermatic cord and testis pass through the inguinal canal surrounded by the peritoneum of the vaginal tunic, transversalis fascia (underlying parietal peritoneum) is reflected onto them, and is here known as internal spermatic fascia. The combined superficial and deep abdominal fascia, from the external surface of the external abdominal oblique muscle, is reflected onto the vaginal tunic as it emerges from the inguinal canal. It then lies superficial to the internal spermatic fascia and is known as the external spermatic fascia. The cremaster muscle, a caudal fasciculus of the internal abdominal oblique muscle, lies adjacent to the vaginal tunic between the internal and the external spermatic fascia.

Both scrotal and inguinal hernias may occur in male dogs. In both of these hernias abdominal organs (greater omentum or a loop of jejunum) enter the canal of the vaginal tunic. Inguinal hernias remain in the inguinal canal. The hernia may be bilateral or unilateral. For a description of surface palpation of the superficial inguinal ring, see McCarthy (1976). Inguinal hernia (intestines or omentum pushing into the inguinal canal within the vaginal canal) is recognizable as a soft, fluctuating enlargement to one side of the penis.

Prostate Gland

The prostate gland (prostata) (see Figs. 9-7, 9-16, 9-18 and 9-20) completely envelops the proximal portion of the male pelvic urethra at the neck of the bladder. It is the only accessory sex gland present in the male dog. The prostate develops from a series of symmetric buds of the pelvic urethra that appear at approximately the sixth week of gestation (Price, 1963). The size and weight of the prostate varies, depending on the age, breed, and body weight of the dog (Berg, 1958a; O’Shea, 1962). In most dogs, progressive enlargement occurs with age (Schlotthauer & Bollman, 1936). The latter authors found that in all dogs in which there was more than 0.7 g of prostate per kg of body weight, the prostate was abnormal on histologic examination. They suggested 0.7 g prostate per kg of body weight as the upper limit of normality for the dog. O’Shea (1962) divided prostatic growth into three phases: normal growth in the young adult, hyperplasia during the middle of adult life, and senile involution.

The prostate is bounded dorsally by the rectum and ventrally by the symphysis pubis and ventral abdominal wall. Its craniocaudal position is age-dependent, as discussed by Gordon (1961). The prostate lies entirely within the abdominal cavity until the urachal remnant breaks down at approximately 2 months of age. From that time until sexual maturity the gland is confined to the pelvic cavity. With sexual maturity it increases in size and extends cranially. By 4 years of age more than half of the gland is abdominal, and by 10 years of age the entire gland is in the abdomen. The degree of bladder distention was not found to alter these relationships to any significant degree. The prostate is androgen-dependent, and castration at any age results in a marked reduction in size (Hansel & McEntee, 1977).

The dorsal surface of the prostate is separated from the ventral surface of the genital fold and the two ductus deferentia by the two layers of the fold of peritoneum that bounds the vesicogenital space. The ventral surface of the prostate is retroperitoneal; the ventral sheet of the lateral ligament of the bladder is not continued onto the prostate. A layer of fat usually covers the ventral surface. In mature dogs, the caudal third of the dorsal surface of the prostate is attached to the rectum by a fibrous band (Gordon, 1960).

The prostate is semioval in transverse section; the dorsal surface is flattened. A middorsal sulcus is usually palpable per rectum. The prostatic part of the pelvic urethra passes through the gland somewhat dorsal to its center. A prominent median septum divides the gland into right and left lobes. Each lobe is further divided into lobules by capsular trabeculae. The lobules consist of numerous compound tubuloalveolar glands lined by columnar epithelium. Ducts from these glands enter the urethra throughout its circumference. The capsule of the prostate (capsula prostatae) is comparatively thick. Smooth muscle fibers are found throughout the capsule, and muscle fibers from the wall of the urinary bladder extend onto the dorsal surface of the capsule.

The two deferent ducts enter the craniodorsal surface of the prostate. They lie adjacent to each other, one on either side of the median plane. They run caudoventrally through the dorsal part of the gland to open into the urethra by two slits on each side of the seminal colliculus (colliculus seminalis) (see Fig. 9-7). The latter is a small round eminence in the center of the urethral crest (crista urethralis), which is a short longitudinal fold on the dorsal wall of the prostatic part of the pelvic urethra. The distal portion of the ductus deferens that enters the urethra is referred to as the ampulla (ampulla ductus deferentis) because of its enlargement resulting from the presence of mucosal glands. In the dog this ampulla is very small and difficult to recognize.

The function of the prostate is not entirely understood. Prostatic secretion contains citrate, lactate, cholesterol, and a number of enzymes. It is believed to be essential to provide an optimum environment for sperm survival and motility. Dog semen is unique in its absence of reducing sugars supplied by the accessory glands in other species; the source of readily metabolizable energy for the spermatozoa is unknown (Hansel & McEntee, 1977).

Vessels and Nerves

The blood and nerve supply of the prostate have been studied by Gordon (1960) and Hodson (1968). The prostatic artery (a. prostatica) arises from the internal pudendal at the level of the second or third sacral vertebrae, although it may arise from the umbilical (Hodson, 1968) near its origin. The prostatic artery gives rise to the artery of the ductus deferens (a. ductus deferentis), which is homologous with the uterine artery in the female. The artery of the ductus deferens gives rise to the caudal vesicle artery (a. vesicalis caudalis). The caudal vesicle artery gives branches to the ureter (ramus uretericus) and urethra (ramus urethralis) and then ramifies on the surface of the bladder, anastomosing with the contralateral caudal vesicle and cranial vesicle arteries. When the cranial vesicle artery is not present, the caudal vesicle supplies the entire bladder.

The prostatic artery continues caudoventrally and gives rise to the small middle rectal artery (a. rectalis media) before ramifying on the surface of the prostate. These branches penetrate the capsule on the dorsolateral surface of the gland to become subcapsular arteries (Hodson, 1968). Radial tributaries pass along the capsular septae toward the urethra to supply the glandular tissue. Cavernous tissue, continuous with the corpus spongiosum, surrounds the pelvic urethra. It is supplied by the artery of the bulb of the penis.

Anastomoses occur between the prostatic vessels and the urethral artery (internal pudendal), the cranial rectal artery (caudal mesenteric) and the caudal rectal artery (internal pudendal). These anastomoses complicate prostatectomy. The venous network of the gland drains by way of the prostatic and urethral veins into the internal iliac vein. The prostatic lymph vessels empty into the iliac lymph nodes. The nerve supply to the prostate is closely allied to the vasculature. The hypogastric nerve, which supplies sympathetic innervation to the prostate, follows the prostatic artery from the pelvic plexus. The pelvic nerve, which may be single or double (Gordon, 1960), accompanies the prostatic artery as far as the lateral surface of the rectum. Here it forms the pelvic plexus, together with branches of the hypogastric nerve. The middle portion of the pelvic plexus forms the prostatic plexus, which innervates the gland. Parasympathetic stimulation increases the rate of glandular secretion.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on The Urogenital System

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