Kidneys

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).

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.

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.

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).

Ureters

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.

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.

Fixation

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

Scrotum

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.

Testes

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.

Epididymis

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.

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).