All animals need a supply of nutrients and oxygen that are obtained via the digestive system and respiratory system, respectively. The digestive system consists of the digestive tract, also called the gastrointestinal (GI) or alimentary tract, and its accessory organs. The accessory organs include the teeth, tongue, salivary glands, liver, pancreas, and gallbladder. The digestive tract is a muscular tube running through the body extending from the mouth to the anus. Contents within the digestive tract are considered outside the body, so in addition to digesting and absorbing nutrients, the digestive tract must act as a barrier blocking the entry of pathogenic organisms. The digestive tract has eight functions: The peritoneal cavity is formed from a serous membrane, called the peritoneum, which lines the abdominopelvic cavity, forming the largest serous membrane in the body (Fig. 17.2). It consists of a layer of simple squamous epithelium, the mesothelium, with an underlying connective tissue layer. The peritoneal membrane has a serosa, or visceral layer, that covers the organs in the peritoneal cavity, and a parietal peritoneum that lines the inner surface of the body wall. Fig. 17.1 Peristalsis and segmentation. (A) During peristalsis, the circular smooth muscle layer behind the bolus contracts while that in front of the bolus relaxes. Conversely, the longitudinal smooth muscle layer behind the bolus relaxes while that in front of the bolus contracts. This increases the diameter of the lumen in front of the bolus while constricting the diameter of the lumen behind the bolus. This results in propulsion of the bolus down the digestive tract. (B) During segmentation, nonadjacent sections of the digestive tract contract and relax, resulting in mixing of the contents. Fig. 17.2 Peritoneum and peritoneal cavity. The cross section through the abdominal cavity of the horse shows the visceral and parietal peritoneum, and the dorsal and ventral mesentery. Although the peritoneal cavity is not shown, it is filled with organs. Note that some organs, such as the pancreas, are in a retroperitoneal position or outside the peritoneal cavity. The peritoneal membrane produces peritoneal fluid, providing lubrication between the serosa and parietal layers, called the peritoneal cavity, thus reducing friction and irritation. Diseases of the liver, kidney, and heart can cause increases in this fluid production, producing an abdominal swelling called ascites. Accumulation of this fluid can distort the internal organs, causing pain and discomfort. The mesentery consists of two layers of serous membranes fused back‐to‐back and suspends portions of the digestive tract from the body wall. The mesenteries have three functions: (1) to provide a route for blood vessels, lymphatic vessels, and nerves to travel to the digestive system; (2) to hold organs in place; and (3) to store lipids. During embryonic development, the digestive organs are suspended from the body wall by the dorsal and ventral mesentery. The ventral mesentery largely disappears except on the ventral surface of the stomach, between the stomach and liver, and between the liver and the ventral abdominal wall. The mesenteries are named for the organs they supply (e.g., mesoduodenum, mesoileum, and mesocolon). The omentum refers to those portions of the mesentery connecting the stomach to the abdominal organs or abdominal wall. In animals with simple stomachs, such as carnivores, pigs, and horses, the greater omentum connects the greater curvature of the stomach to the dorsal abdominal wall (Fig. 17.3). It folds over itself forming deep and superficial layers (i.e., four layers). It normally contains considerable adipose tissue. The lesser omentum connects the lesser curvature of the stomach and the initial segment of the duodenum with the liver. The falciform ligament attaches the liver to the ventral midline while the hepatoduodenal ligament connects the liver to the proximal duodenum. In ruminants, the superficial and deep portions of the greater omentum attach to the left side of the rumen and the right side of the rumen, respectively. They tract toward the right side of the animal, attaching to the intestine and then to the right abdominal wall. Although most abdominal organs are located within the peritoneum, some are located between the posterior parietal peritoneum and the posterior abdominal wall and, thus, are outside this cavity and said to be retroperitoneal (retro = behind). These organs include the kidneys, adrenal glands, ureters, duodenum, ascending colon, descending colon, and pancreas. Those organs whose mesenteries remain inside the peritoneal cavity are called intraperitoneal or peritoneal organs (Gheorghe, et al., 2004). Fig. 17.3 Arrangement of mesenteries in ruminants. (A) In this view of the left side of a large ruminant, the greater omentum is visible. (B) A cross section of the flank of a large ruminant showing greater omentum and peritoneum. Reprinted from Constantinescu and Constantinescu (2004). Used by permission of the publisher. The splanchnic circulation serves the digestive organs and hepatic portal system. The arteries of this system include the hepatic, splenic, and left gastric branches of the celiac trunk, serving the spleen, liver, and stomach, respectively; and the mesenteric arteries serving the small and large intestines. At rest, the splanchnic circulation receives approximately 25% of the cardiac output. The digestive tract includes four major layers (Fig. 17.4). Listed from the lumen outward, they are (1) the mucosa, (2) the submucosa, (3) the muscularis externa, and (4) the serosa. These layers vary somewhat according to the region of the digestive tract, so the following description applies to the small intestine. The mucosa layer is a mucous membrane lining the inside of the digestive tract. It consists of three sublayers including a layer of epithelial tissue in direct contact with the contents of the digestive tract, the lamina propria, and the muscularis mucosae. Within the oral cavity, pharynx, esophagus, and anal canal, the epithelial tissue is stratified squamous epithelium that performs a protective function. The remainder of the tract is mostly simple columnar epithelium and mucus‐producing goblet cells. Shortly after birth, the simple columnar epithelial cells develop tight junctions, thus forming a barrier between the lumen contents and the body. Prior to the formation of these tight junctions, an animal can absorb antibodies found in the colostrum until the epithelial cells undergo closure, or until the development of tight junctions. Closure generally occurs within a couple days after birth. Also found scattered among the epithelial cells are endocrine cells, collectively called enteroendocrine cells, which secrete hormones coordinating digestive functions. The epithelial cells have a life span of approximately 2–3 days in the esophagus and up to 6 days in the large intestine. These cells are continually being sloughed off, and progressively replaced. The lamina propria, consisting of areolar connective tissue, binds the epithelial cells to the muscularis mucosae. This layer also contains blood vessels, sensory neurons, lymphatic vessels, smooth muscle cells, and lymphatic nodules that are part of the mucosa‐associated lymphatic system (MALT). The MALT is present along the digestive tract and contains cells of the immune system. The appendix and tonsils are part of the MALT. The muscularis mucosa is a thin layer of smooth muscle fibers. This layer helps create folds in the stomach and small intestine, thus increasing their surface area. Fig. 17.4 Basic structure of the digestive tract. The four basic layers of the digestive tract, from the lumen outward, are the mucosa, submucosa, muscularis externa, and serosa. The submucosa consists of dense irregular connective tissue and contains large blood vessels, lymphatic vessels, and—in some regions—exocrine glands secreting buffers and enzymes into the lumen. Within the oral cavity, pharynx, and parts of the esophagus, depending on the species, this layer contains skeletal muscle that controls swallowing. The external anal sphincter also generally contains skeletal muscle permitting voluntary control of defecation. Along the remainder of the digestive tract, the muscularis externa generally consists of two layers of smooth muscle: an inner layer of circular smooth muscle fibers and an outer layer of longitudinal smooth muscle fibers. These layers control peristalsis and segmental contractions. Most portions of the digestive tract lie within the peritoneal cavity. The outermost portions, or superficial layer, of the digestive tract is lined with the adventitia. As the digestive tract passes through the diaphragm and enters the peritoneal cavity, this layer is composed of the visceral portion of the peritoneum, also called the serosa. There is no serosa around the oral cavity, pharynx, esophagus, or rectum. Instead, there is a layer of collagen fibers attaching the digestive tract to surrounding structures. The digestive tract has its own nervous system, called the enteric (enteric = gut) nervous system, sometimes called the “brain of the gut.” It is composed mostly of two large plexuses: the submucosal plexus and myenteric plexus. If the digestive tract is deinnervated, it will begin to function near normally due to the sensory and motor neurons in the enteric nervous system. The submucosal plexus, or Meissner’s plexus, is located within the submucosal layer. It includes sensory and motor neurons, and postganglionic fibers of both the sympathetic and parasympathetic nervous system. It regulates the activity of glands and smooth muscle in the mucosa. The myenteric plexus, or plexus of Auerbach, is located between the two layers of smooth muscle fibers in the muscularis externa. These neurons coordinate the frequency and strength of digestive tract motility. Therefore, this plexus controls patterns of peristalsis and segmentation through automatic local reflex arcs. The enteric nervous system also communicates with the central nervous system via afferent visceral fibers of the sympathetic and parasympathetic branches of the autonomic nervous system. The autonomic nervous system also exerts extrinsic control over the functions of the digestive tract. Parasympathetic input generally enhances digestive functions, whereas sympathetic input inhibits these functions. The digestive system shows great variation among species (Table 17.1). These variations in structure are necessary depending on whether the animal is a carnivore (meat‐eating), herbivore (plant‐eating), or omnivore (meat‐ and plant‐eating). A rabbit is a typical nonruminant example; its stomach and small intestine are relatively small, whereas the cecum is well developed to allow for microbial digestion. Nonruminant herbivores typically have a well‐developed cecum because this is the primary site of cellulose digestion. Ruminants have a complex stomach that accommodates microbial digestion, a proportionately long small intestine, and a large colon. Carnivores, such as dogs and cats, have a short and small intestines, poorly developed cecum, and average colon. The pig, which is an omnivore, has an intermediate‐sized colon because this is a major site of microbial digestion. Further evolutionary adaptations have occurred in the stomach and GI tract of animals to accommodate differing methods of digesting carbohydrates. As a result, animals can be classified into four digestive groups. The first group includes animals with a simple stomach, such as humans, pigs, dogs, and cats. The second group, the foregut fermenters, includes cattle, sheep, and goats. These animals have a ruminant stomach in which they can ferment nondigestible carbohydrates. The third group, the hindgut fermenters, includes horses, rabbits, and guinea pigs. These animals rely on fermentation that occurs in the cecum. The final group consists of birds in which various adaptations have occurred to both store and grind various foodstuffs. Table 17.1 Length and capacity of selected parts of the digestive tract. The mouth is the space extending from the lips or beak to the pharynx, and it is bounded laterally by the cheeks. It is also called the oral cavity, or buccal cavity, and is where food first enters the digestive tract. The mouth is lined with stratified squamous epithelium, which protects against friction. For further protection, the epithelium of the gums, hard palate, and dorsum of the tongue are slightly keratinized. The lips and cheeks contain skeletal muscle covered by skin. The orbicularis oris muscle forms the lips. The lips possess long, tactile hair, and regular hair. The median cleft of the upper lip in carnivores and small ruminants is called the philtrum. Fig. 17.5 Oral cavity. The hard and soft palate of the mouth is visible in the median section through the head of a dog. Reprinted from Constantinescu (2002). Used by permission of the publisher. The cheeks form the caudolateral wall of the oral cavity. The gums, or gingivae, enclose the necks of the teeth. The oral cavity is divided into the vestibule and oral cavity proper. The vestibule (porch) is the recess bounded internally by the gums and teeth and externally by the lips and cheeks. The oral cavity proper lies within the teeth and gums. The palate is the roof of the oral cavity and oropharynx, and it separates the respiratory and digestive passages within the head (Fig. 17.5). It consists of a rostral bony part called the hard palate and a caudal musculomembranous portion called the soft palate. The horse is unable to voluntarily raise its soft palate, and therefore breathes through its nose. The hard palate is formed by the palatine, maxillary, and incisive bones. It forms a hard surface against which the tongue can press food. The soft palate divides the rostral region of the pharynx into the oral and nasal portions. Projecting downward from the soft palate is the fingerlike uvula. The soft palate closes the nasopharynx as the animal swallows. Birds, unlike mammals, lack a soft palate. The oral and pharyngeal cavities are combined and referred to as the oropharynx. The tongue is the muscular organ filling most of the oral cavity. It is composed of interlacing bundles of skeletal muscle fibers, and it is involved in gripping, repositioning food, mixing food with saliva, and forming the compact mass of food called a bolus. The tongue has intrinsic and extrinsic muscles. The intrinsic muscles, confined to the tongue and not attached to bone, run in several directions, allowing the tongue to change shape as necessary for prehension, moving food, and making sounds. The extrinsic muscles attach the tongue to bones of the skull and the soft palate. They allow the tongue to protrude, retract, and move side to side. The lingual frenulum attaches the tongue to the floor of the mouth. The superior surface of the tongue has many papillae that are named for their shape. Filiform papillae are thorn‐shaped, giving the tongue roughness and thus aiding in licking and manipulating food. They have a mechanical function. In the ox and cat, they are heavily cornified. Fungiform papillae are mushroom‐shaped, scattered among the more numerous filiform papillae, have taste buds, and are thus mechanical and gustatory (Fig. 17.6). Foliate papillae have a series of leaf‐shaped ridges, are located on the lateral borders of the tongue, and have a gustatory function. They are absent in the ox. Vallate, or circumvallate, papillae are the largest and least numerous. They are in a V‐shaped row near the back of the tongue. They resemble the fungiform papillae but are circled by a cleft containing taste buds. Marginal taste buds are found along the edge of the rostral portion of the tongue of newborn dogs, but they disappear when puppies switch to solid food. Salivary glands are extramural glands (glands outside the wall of the digestive system) that are associated with the oral cavity. The secretions of the salivary glands can be serous, mucous, or mixed. Serous cells produce a watery secretion containing enzymes, ions, and a small amount of mucin, whereas mucous cells produce a viscous, stringy secretion called mucus. Minor salivary glands are located within the wall of the oral cavity and oral pharynx and have short ducts. They are named for their location (labial, buccal, and palatal). They are mixed glands, meaning they have mucous and serous secretions. Fig. 17.6 Tongue of the dog. The vallate and fungiform papillae are shown on the tongue of the dog. Reprinted from Constantinescu (2002). Used by permission of the publisher. The major salivary glands are located some distance from the oral cavity and require ducts to carry their secretions. The parotid salivary gland is located below the ear (auricular) cartilage, between the masseter muscle and skin (Fig. 17.7). The parotid duct parallels the zygomatic arch and opens into the buccal vestibule. It produces a predominantly serous secretion. The mandibular (submandibular, submaxillary) salivary gland is located caudal to the angle of the jaw and is a mixed gland. The mandibular duct runs rostrally along with the sublingual duct, medial to the mandible, and opens near the sublingual caruncle. The sublingual salivary gland is under the tongue and secretes mostly mucus. Saliva consists of water (97–99.5%) and is therefore hypoosmotic. Electrolytes in the saliva include sodium, potassium, chloride, bicarbonate, and phosphate. It tends to be slightly acidic (pH 6.75–7.00). Saliva has several functions: Fig. 17.7 Salivary glands. The major salivary glands are shown in the dog. Drawings reprinted from Constantinescu (2002) and Constantinescu and Constantinescu (2004). In nonruminants, as the secretion of saliva increases, the concentration of Na+, bicarbonate, and Cl− increases, while K+ decreases. In ruminants, as saliva production increases, the levels of Na+ and PO4− in the saliva decrease, while those of bicarbonate, Cl−, and K+ increase. Salivary glands continuously secrete saliva, thus keeping the oral cavity moist. However, presence of food increases salivation due to parasympathetic nervous system stimulation. Chemoreceptors and mechanoreceptors send signals to the superior and inferior salivatory nuclei in the brain stem. Parasympathetic impulses travel via the facial nerve (cranial nerve VII) and glossopharyngeal nerve (cranial nerve IX) to stimulate salivation. The sight, smell, sound, or thought of food can also stimulate saliva production. This was evident when Pavlov trained dogs to salivate at the sound of a bell. Such salivation helps initiate digestion as soon as food enters the oral cavity. The saliva in ruminants is isotonic, containing high concentrations of bicarbonate and phosphate, and a high pH. This saliva acts to buffer the acids produced during fermentation in the rumen. An adult cow can produce as much as 100–200 L of saliva daily. The teeth, or dentes, are accessory digestive organs. They are in the sockets of the alveolar processes of the mandible and maxillae. Domestic animals have two types of teeth: low‐crowned (brachydont) and high‐crowned (hypsodont). All domestic species have two sets of teeth, deciduous and permanent. Deciduous teeth are smaller and fewer in number. Low‐crowned teeth are simple teeth, as found in man, carnivores, pigs, ruminant incisors, and horse deciduous incisors. They consist of a crown, neck, and root. The crown is the part projecting above the gum line and is covered with enamel. The neck is the constriction between the crown and root, and it is located at the gum line. The root is the part below the gum line. High‐crowned teeth, which have no distinct neck, are found in all permanent horse teeth, ruminant cheek teeth (i.e., premolars and molars), and the tusks of pigs. Teeth are composed of three layers: cementum, enamel, and dentin (Fig. 17.8). Cementum, a thin, bonelike covering, is found on the entire tooth of high‐crowned teeth, but only on the root of low‐crowned teeth. It attaches the root to the periodontal ligament. Enamel, the hardest substance in the body (consisting of 95% calcium salts by dry weight), covers the crown of low‐crowned teeth and the body (the portion of the tooth below the crown in high‐crowned teeth) and crown of high‐crowned teeth. The enamel protects the dentin from acids. Dentin, which makes up the bulk of the tooth, is like bone only harder because it has a higher content of calcium salts. The dentin surrounds a cavity. Within the crown, this cavity is the pulp cavity, and it is filled with pulp, a connective tissue containing blood vessels, nerves, and lymphatic vessels. Narrow extensions of the pulp cavity project into the roots and are called the root canals. Teeth are divided into groups according to their location and function. Incisors are in the rostral portion of the mouth (Fig. 17.9). The upper incisors are embedded in the incisive bone and the lower incisors are in the incisive part of the mandible. The canine is the large tooth between the incisors and cheek teeth. Cheek teeth are those teeth caudal to the canine and incisors in the maxillary. They include the premolars located in the rostral cheek area and molars located caudal to the premolars. Cheek teeth function in grinding, while incisors are merely for shearing and biting. Fig. 17.8 Teeth of the horse. Vertical section of a permanent incisor (A). Vestibular surface (B) left side (C), and lingual surface (D) of the first right‐lower deciduous incisor. Vestibular surface (E) left side (F), and lingual surface (G) of the first right‐lower permanent incisor. Third right‐lower permanent premolar (H) and second left‐lower permanent molar (I). Reprinted from Constantinescu and Constantinescu (2004). Used by permission of the publisher. Fig. 17.9 Teeth of the dog. Reprinted from Constantinescu (2002). Used by permission of the publisher. Table 17.2 Number of teeth in various species. I, incisor; C, canine; P, premolar; M, molar. Pigs have 44 teeth; other domestic species have fewer due to fewer cheek teeth (Table 17.2). Ruminants lack upper incisors and canines, which are replaced by a dental pad. They also lack the first upper and lower premolars, giving them 32 permanent teeth. Dogs are missing the upper third molars and therefore have 42 teeth. Brachiocephalic breeds (those dogs that have shortened noses and very prominent eyes due to shallow orbit) may be missing additional teeth. Horses usually miss the first upper premolar and are always missing the lower first premolar. Mares often have smaller canines that may not erupt. As a result, horses may possess 36–42 permanent teeth. Various terms used when discussing teeth are shown in Table 17.3. The eruption and wear of the lower incisors can be used to estimate a horse’s age (Fig. 17.10). If no permanent incisors are present, the horse is probably under 2–1/2 years old. Deciduous teeth are characterized by a distinct neck and are smaller and usually lack longitudinal ridges seen in permanent teeth. The incisors erupt at the following times: I1, 2–1/2 years; I2, 3–1/2 years; and I3, 4–1/2 years. If all incisors have erupted, and I3 is worn such that a little dentin is seen, the horse is approximately 5 years old. Disappearance of the cup from the respective lower incisors can indicate age as follows: I1, 6 years old; I2, 7 years old; and I3, 8 years old. Disappearance of the cup from the upper incisors can indicate age as follows: I1, 9 years old; I2, 10 years old; and I3, 11 years old (Budras et al., 2003). Table 17.3 Nomenclature for teeth. Fig. 17.10 Aging horses using the lower incisors. (A) 4–7 days old. (B) 3–5 weeks old. (C) 7 months old. (D) 3 years old. (E) 4 years old. (F) 5 years old. (G) 10 years old. (H) 15 years old. (I) 20 years old. Reprinted from Constantinescu and Constantinescu (2004). Used by permission of the publisher. The pharynx is the common passageway for food and air. As food first passes from the mouth, it enters the pharynx. It extends from the internal nares to the esophagus. It connects the nasal and oral cavities with the trachea and esophagus, respectively. The soft palate divides the rostral portion of the pharynx into the oropharynx and nasopharynx, and the caudal‐most portion of the pharynx is called the laryngopharynx (Fig. 17.11). The nasopharynx is located dorsal to the soft palate extending from the caudal nares to the laryngopharynx with the caudal edge of the soft palate and palatopharyngeal arches separating it from the laryngopharynx. The oropharynx lies ventral to the soft palate. The laryngopharynx is where air crosses to the larynx and food and water crosses to the esophagus. It is located between the base of the epiglottis and the esophageal entrance. Fig. 17.11 The upper digestive tract of the horse. (A) A median section of the head of the horse showing the nasopharynx, oropharynx, and laryngopharynx. (B) An expanded drawing of the pharynx and larynx. Reprinted from Constantinescu and Constantinescu (2004). Used by permission of the publisher. The tonsils are an aggregation of lymphatic tissue in the mucosa of the pharynx. They are named for their location (i.e., palatine, pharyngeal, or tubal, which are found around the auditory tube). They help protect the pharyngeal opening against microorganisms and toxic substances. In birds, there is no sharp distinction between the pharynx and mouth. The act of swallowing, or deglutition, moves food from the mouth, through the pharynx to the esophagus, so that it can be transported to the stomach. Saliva and mucus facilitate this movement. Swallowing involves three stages: The tongue, after forming a bolus, propels it from the oral cavity to the oropharynx (Fig. 17.12). This is carried out by skeletal muscle fibers and is thus the voluntary stage. The presence of a bolus in the oropharynx initiates the pharyngeal stage in which impulses are carried to the deglutition center in the medullar oblongata and lower pons of the brain stem. Motor signals from these centers close off the nasopharynx and cause the larynx to move forward and upward, allowing the epiglottis to move backward and downward sealing off the rima glottide, the opening in the larynx. After the bolus travels from the laryngopharynx to the esophagus, the respiratory passageways reopen. The esophagus is a collapsible muscular tube lying behind the trachea. It extends from the laryngopharynx, passes through the mediastinum, pierces the diaphragm at the esophageal hiatus, and ends at the superior portion of the stomach. A hiatal hernia occurs when a part of the stomach protrudes through the diaphragm at the esophageal hiatus. In many species of birds, the upper portion of the esophagus is expanded to form the crop. The crop stores food and, in some species (i.e., pigeons), produces a secretion called crop milk that is used to feed the young. The esophagus has four layers, as described previously in this chapter. The outermost layer is the adventitia rather than the serosa because the areolar connective tissue is not covered by mesothelium, and the connective tissue merges with structures in the mediastinum, thus attaching the esophagus to surrounding structures. The muscularis externa layer varies in the proportion of skeletal and smooth muscle, depending on the species. The esophagus of birds consists entirely of smooth muscle; that of cats, dogs, pigs, and ruminants consists mostly of smooth muscle, with a small portion of skeletal muscle just as the esophagus nears the stomach. Fig. 17.12 Swallowing. During swallowing, the tongue forces the bolus into the oropharynx, thus raising the soft palate and closing off the nasopharynx. Then, the larynx rises, allowing the epiglottis to cover the glottis, directing the bolus to the esophagus while preventing its entry into the larynx. Once in the esophagus, the bolus is moved to the stomach by peristaltic waves. Figure modified from Pasquini et al. (1995). The stomach is located at the inferior end of the esophagus and cranial portion of the abdominal cavity. It is found left of the median plane. It has four functions: (1) storage of ingested food, (2) mechanical breakdown of ingested food, (3) disruption of chemical bonds of food through the action of acids and enzymes, and (4) production of the intrinsic factor required for vitamin B12 absorption from the small intestine. Monogastric animals have a single, simple stomach, and ruminants have a complex stomach consisting of four chambers. The true stomach is the area that produces hydrochloric acid. In monogastric animals, the stomach appears as a J‐shaped structure. Its concave lateral surface is the greater curvature, and the smaller concave medial surface is the lesser curvature. The greater and lesser omenta attach to the greater and lesser curvature, respectively. In addition to the circular and longitudinal smooth muscle layers found along the remainder of the digestive tract, the muscularis externa of the stomach has an additional inner oblique or transverse layer. This extra layer of muscle helps strengthen the stomach wall and assists with mixing the chyme, the partially digested food, with enzymes and acid. As food is ingested, the muscles of the stomach relax to accommodate the increased volume of food. While relaxed, prominent folds called rugae are visible on the stomach mucosa. As the stomach expands, the rugae spread or flatten out. The stomach is typically divided into four regions (Fig. 17.13): The stomach is innervated by fibers from the autonomic nervous system. Sympathetic fibers originate from the thoracic splanchnic nerves and carry signals to the celiac plexus. Parasympathetic fibers are supplied by the vagus nerve (cranial nerve X). The arterial supply comes from branches of the gastric and splenic arteries; the veins are part of the hepatic portal system. Fig. 17.13 The stomach of the horse. Adapted from Constantinescu and Constantinescu (2004). The stomach is lined with simple columnar epithelium with goblet cells. The many goblet cells produce a layer of mucus protecting the mucosal surface of the stomach. Gastric pits, or shallow depressions, are visible on the mucosal surface. The walls of the gastric pits are formed mainly from goblet cells. Gastric pits open into gastric glands (Fig. 17.14). The gastric glands are composed of different cells depending on the stomach region. The cardia and pyloric regions contain primarily mucus‐secreting cells, and the glands in the pyloric antrum produce mucus and hormones including gastrin. In the fundus, the glands produce most of the stomach secretions, including acid. There are four cell types in gastric glands. The mucosa of the stomach must provide a substantial barrier against the harsh environment found in the stomach. The concentration of H+ is 10,000 times that found in blood. Pepsin, if not contained, can digest the lining of the stomach. The mucosal barrier of the stomach contains a thick, bicarbonate‐rich mucus lining. In addition, the epithelial cells are connected by tight junctions to prevent the leakage of luminal contents to deeper gastric layers. Finally, the epithelial cells are replaced every 3–6 days by division of undifferentiated stem cells found in the gastric pits. Fig. 17.14 Gastric glands. The histological section of a gastric gland shows the gastric pit, the entrance to the gastric gland. Within the gland are neck cells that secrete mucus, parietal cells that secrete HCl, chief cells that secrete pepsinogen, and enteroendocrine cells that secrete hormones. G cells, which secrete gastrin, are an example of enteroendocrine cells. Once released into the stomach lumen, pepsinogen is converted to pepsin by the action of HCl and pepsin. Pepsin is not produced within the chief cells because this would cause the self‐digestion of the cells. Instead, chief cells produce the zymogen (i.e., the precursor form of an enzyme) pepsinogen, which is activated when it enters the stomach lumen and encounters HCl (Fig. 17.14). Once activated, pepsin can activate other pepsinogen molecules. Protein digestion is initiated in the stomach via the action of pepsin, the only enzyme found in the stomach of adult animals. Dietary proteins are denatured by HCl secreted by the parietal cells. However, HCl is not produced within the parietal cells because it would destroy the cell. Both H+ and Cl− are independently transported from the parietal cells into the stomach lumen (Fig. 17.15). Hydrogen ions are generated from the dissociation of carbonic acid that is produced by the enzyme carbonic anhydrase acting upon CO2 and H2O. Hydrogen ions are then transported into the stomach lumen in exchange for K+. Chloride ions enter the parietal cell in exchange for bicarbonate ions. The chloride ions then travel down their concentration gradient and enter the stomach lumen. Once in the lumen, hydrogen and chloride ions combine, producing HCl. Since the pH of the lumen can be as low as 1.5–2.0, this can represent nearly a millionfold (6 log units) increase in hydrogen ion concentration. When parietal cells are producing considerable HCl, a significant amount of bicarbonate enters the blood, thus increasing the pH in what is called the alkaline tide. Parietal cells respond to many signals. Located on their surface are receptors for histamine, acetylcholine (ACh), and gastrin (Fig. 17.16). Histamine comes from mast cells located in the lamina propria, ACh from postganglionic parasympathetic fibers, and gastrin from G cells. Histamine acts at H2 receptors, whereas ACh acts at muscarinic receptors. Stimulation of these receptors results in stimulation of protein kinase, which then stimulates the H+/K+ ATPase, thus increasing HCl secretion by parietal cells. Fig. 17.15 Secretion of HCl by the stomach. Carbonic acid is produced within the parietal cells by the action of carbonic anhydrase. After dissociation, the bicarbonate ions are transported into the intestinal fluid, whereas the hydrogen ion is actively transported into the stomach. Chloride ions enter the cell in exchange for HCO3−, and then move down their concentration gradient and into the stomach lumen where they combine with hydrogen ions forming HCl. Fig. 17.16 Regulation of HCl secretion from parietal cells. Parietal cells have receptor sites for histamine, gastrin, and acetylcholine (ACh). Histamine binds to H2 receptors, while ACh acts at muscarinic receptors. Stimulation of these receptors by the appropriate ligand initiates a second‐messenger system that results in increased HCl secretion into the stomach lumen. Endocrine cells located in pyloric glands produce at least seven hormones. The major hormone, gastrin, is secreted by G cells found most abundantly in the gastric pits of the pyloric antrum. Gastrin stimulates secretion of both parietal and chief cells and causes contractions of the gastric wall, thus mixing luminal contents. Pyloric glands also contain D cells that secrete somatostatin. Somatostatin, which is released into the interstitial fluid bathing the G cells, inhibits gastrin release. This inhibition can be overridden by other neural and hormonal stimuli such as ACh and histamine. With the arrival of food, the stomach can stretch to accommodate this increased volume without an increase in luminal pressure. This reflexive relaxation is mediated by the vagus nerve. In addition, the stomach can actively dilate in a process called adaptive relaxation, which appears mediated by the release of nitric oxide released by local neurons. In addition to the propulsion of food into the duodenum, the stomach churns and mixes food within its lumen. Peristalsis in the stomach begins near the cardiac sphincter with gentle ripple‐like movements toward the pyloric sphincter. The peristaltic waves strengthen as they move toward the pylorus. The pyloric sphincter, acting sort of like a dam, allows only liquids and small particles to pass over its opening. Heavier particles settle below the level of the sphincter and thus do not pass through. As the peristaltic wave nears the pyloric sphincter, a small amount of chyme is squirted through the sphincter before the peristaltic wave closes the sphincter, causing the remainder of the material to be propelled backward into the pylorus and further churned. Such an action further breaks down the particle size of the ingesta. The peristaltic rhythm is controlled by the spontaneous activity of pacemaker cells located in the longitudinal smooth muscle layer. These noncontractile cells, called interstitial cells of Cajal, are located near the cardiac sphincter depolarize and repolarize approximately three times per minute producing slow waves, or the basic electrical rhythm. These slow waves migrate throughout the stomach via the gap junctions that electrically couple smooth muscle cells. Slow waves establish the maximum rate of smooth muscle contraction by producing subthreshold depolarizations on which depolarizations resulting in contractions are superimposed (Fig. 17.17). The presence of irritants or toxins in the stomach can stimulate vomiting or emesis. Sensory impulses sent to the emetic center in the medulla oblongata initiate a motor response that causes the diaphragm and abdominal wall muscle to contract, increasing intra‐abdominal pressure. As the pressure increases, the cardiac sphincter relaxes; the soft palate rises to close off the nasopharynx; and the stomach contents are forced upward through the esophagus, pharynx, and mouth. Excessive vomiting can cause metabolic alkalosis, dehydration, and electrolyte imbalances. Egestion is a process unique to birds. During egestion, nondigestible materials such as bone, fur, or feathers are orally eliminated from the digestive tract. Approximately 12 minutes prior to egestion, gizzard contractions increase, resulting in the compaction of this undigestible material into a pellet. The pellet can contain exoskeletons of insects and indigestible plant material. Seconds before egestion, the pellet is moved by esophageal antiperistalsis. This process does not use abdominal or duodenal muscles. Fig. 17.17 Slow waves. Slow, rhythmic depolarizations and repolarizations, called slow waves or basic electrical rhythm, are initiated by the interstitial cells of Cajal. They establish the background rhythm for peristalsis, which involves spikes superimposed on the slow waves resulting in depolarizations above threshold that cause smooth muscle contractions. Gastric secretions are controlled by neural and hormonal mechanisms. The nervous control includes both long and short nerve reflexes involving the vagus nerve. Stimulation of the vagus nerve (i.e., parasympathetic nervous system) increases the secretory activity of the stomach. In contrast, sympathetic stimulation inhibits stomach secretion. Gastric secretions are controlled at three levels, including the central nervous system, stomach, and small intestine. Controls from these three sites are the cephalic phase, gastric phase, and intestinal phase of gastric secretion, respectively (Fig. 17.18). These control mechanisms can either increase or decrease gastric secretions. Fig. 17.18 Phases of gastric secretion. (A) Cephalic phase. (B) Gastric phase. (C) Intestinal phase. Figure modified from Martini (2004). The cephalic phase causes an increase in gastric secretions prior to the arrival of food. This stage is controlled by the central nervous system, and it prepares the stomach for the arrival of food. The sight, smell, and taste of food stimulate the parasympathetic nervous system to send signals via the vagus nerve that synapses on the submucosal plexus located in the wall of the stomach. This stimulates the postganglionic parasympathetic fibers innervating mucous cells, chief cells, parietal cells, and G cells in the stomach, thus increasing gastric secretions. This phase is short, lasting minutes. Emotional responses associated with activation of the fight‐or‐flight response decrease gastric secretions and gastric motility. Beginning with the arrival of food in the stomach, this phase further stimulates gastric secretion and motility. This phase accounts for about two‐thirds of gastric secretions. Stimuli for the gastric phase include distention of the stomach, an increase in gastric pH, and the presence of undigested food, especially proteins and peptides. The arrival of protein in the stomach increases the pH since proteins act as buffers. Activation of stretch receptors sends signals to the myenteric plexus (short loop reflex) and the medulla via the vagus (long loop reflex). These reflexes result in the release of ACh, which stimulates gastric secretions. Chemical stimuli, such as partially digested proteins and increasing pH, also directly activate G cells to secrete gastrin, which strongly stimulates HCl but also has a weaker effect of increasing pepsinogen secretion. A decrease in the luminal pH below 2 inhibits gastrin secretion. Finally, the local release of histamine in the lamina propria, presumably from mast cells, also stimulates parietal cells to secrete HCl. Therefore, there are three chemicals that can stimulate parietal cells to release HCl. Involving neural and hormonal signals, the intestinal phase functions to decrease gastric motility. Stimulation of chemoreceptors and stretch receptors triggers the enterogastric reflex. This reflex inhibits gastrin production and gastric motility and stimulates contraction of the pyloric sphincter, thus slowing gastric emptying into the duodenum. The enterogastric reflex has three components: (1) inhibition of vagal nuclei in the medulla, (2) inhibition of local reflexes, and (3) sympathetic stimulation of the pyloric sphincter causing it to tighten. The enterogastrone reflex is a hormonal reflex. The arrival of lipids (especially medium‐ and long‐chain fatty acids) and amino acids (especially tryptophan) cause the release of CCK and gastric inhibitory peptide (GIP). CCK inhibits gastric secretion of acid and enzymes, while GIP inhibits gastric secretions as well as gastric motility. These reflexes act to prevent the excessive decrease in pH of the small intestine, as well as slow gut motility to facilitate digestion and absorption from the small intestine, particularly in response to lipids. A decrease in duodenal pH below 4.5 also stimulates secretin release by enteroendocrine cells in the duodenum. Secretin further inhibits gastric HCl and pepsinogen release in the stomach. A summary of these intestinal inhibitory effects on motility is shown in Fig. 17.19. In addition to these inhibitory effects occurring during the intestinal phase, there is an excitatory component. The presence of partially digested proteins in the duodenum stimulates G cells in the duodenal wall to release gastrin that circulates to the stomach to facilitate enzyme secretion. This gastrin is referred to as intestinal (enteric) gastrin. The excitatory phase is short because it is overridden by the inhibitory intestinal phase mechanisms described above. Fig. 17.19 Neural and hormonal inhibition of gastric emptying. Inhibitory signals originating in the duodenum slow gastric emptying allowing the duodenum time to buffer the acid and to absorb nutrients. Ruminants are those animals that ruminate (i.e., chew their cud). They have a specially modified stomach that consists of three nonsecretory forestomachs and a secretory “true” stomach. The forestomachs include the reticulum, rumen, and omasum; the true stomach is the abomasum (Fig. 17.20). The forestomachs serve as a large fermentation chamber where microbial digestion occurs, allowing the ruminant to digest feedstuffs not available to nonruminants. The fermentation end products, such as volatile fatty acids (VFAs), are absorbed and used as metabolic substrates (Gheorghe et al., 2004). Fig. 17.20 Ruminant stomach. The right (A) and left (B) aspects of the stomach of the goat. Reprinted from Constantinescu (2001). Used by permission of the publisher. The esophagus connects with the reticulum at the cardiac opening (Fig. 17.20). The reticulum is separated from the heart by only the diaphragm. As a result, any hardware such as nails or wire entering the reticulum can puncture the pleural and pericardial spaces, or the liver (hardware disease). Often a magnet is placed in the reticulum to attract hardware and to prevent its migration to the remainder of the stomach. The lining of the reticulum has a honeycomb arrangement of ridges (Fig. 17.21). The reticulum is separated from the rumen by the ruminoreticular fold or groove. While this separates the two chambers, there remains an opening connecting the two. Therefore, the rumen and reticulum act as a functional unit, the reticulorumen, which is lined with keratinized, stratified squamous epithelium. In young animals, the reticuloomasal fold (Fig. 17.22), sometimes called the reticular groove, prevents food from entering the rumen and instead directs it to the omasum. Since the milk ingested during suckling does not require fermentation, it is shunted directly to the abomasum through the omasal canal. This groove closes because of a reflex initiated by stimulation of receptors in the mouth and pharynx. This reflex diminishes with age, thus attenuating the reticular groove. Fig. 17.21 Inside of Stomach ruminant inside of Reticuloomasal fold (reticular groove) Rumen Reticulum ruminant stomach Omasum. (A) The inside of the reticulum (honeycomb) and rumen. Note the hardware found in the rumen that was ingested as the animal grazed. (B) The reticuloomasal fold (reticular groove). (C) The many plies shown in the omasum. Fig. 17.22 The internal configuration of the reticulum and rumenal atrium of the goat. Reprinted from Constantinescu (2001). Used by permission of the publisher. Fig. 17.23 Internal configuration of the rumen of the goat. Reprinted from Constantinescu (2001). Used by permission of the publisher. The rumen, sometimes called the “pouch,” occupies almost the entire left side of the abdominal cavity. The rumen is divided into a ventral and dorsal sac by the cranial and caudal pillars as well as by the right and left longitudinal pillars (Fig. 17.23). The dorsal sac is further divided into the cranial sac found between the ruminoreticular fold and cranial pillar, the dorsal sac, and a caudodorsal blind sac. The luminal surface of the rumen is lined with papillae to increase its surface area. The omasum is kidney shaped and lies to the right of the ruminoreticulum and is located between the rumen‐reticulum and abomasum. Its lining consists of many leaflike folds, or plies, attached to the greater curvature with their free edges extending into the omasal canal. It is therefore sometimes called the “book stomach” since its interior looks like the pages of a book. The leaves have small papillae, thus further increasing the surface area. Food enters the omasum via the reticuloomasal orifice and exits to the abomasum via the omasoabomasal orifice. The abomasum consists of two glandular regions equivalent to the fundus and pyloric region of the monogastric stomach. The cardiac region is confined to the area adjacent to the omasoabomasal orifice. The interior of the abomasum has about 12 rugae (folds) that spiral over the fundus and body but are absent from the pylorus. A constriction in the pylorus separates this region from the duodenum. The ruminant stomach provides several advantages compared to the monogastric stomach: (1) it allows animals to use feedstuffs too fibrous for monogastric; (2) cellulose can be broken down and used by ruminants; (3) it allows the use of nonprotein nitrogen sources (urea and uric acid), which are converted by the ruminal microbes to high‐value organic nitrogen compounds; and (4) it provides B complex vitamins due to the action of microbes as long as cobalt is present in the diet. However, there are also disadvantages associated with ruminant digest: (1) animals must spend a considerable part of each day ruminating (chewing); (2) a large amount of alkaline saliva is necessary; and (3) considerable amounts of volatile acids are released into the environment. There is also considered the release of methane and important greenhouse gas. The mixing, or A, sequence spreads across the reticulorumen in a “Z” pattern. It begins with a double contraction in the reticulum that proceeds across the dorsal rumen to the caudodorsal area. The contraction then propagates through the ventral region of the rumen. This sequence provides extensive mixing of the rumen contents, which disrupts the layering of luminal contents that would allow gas to collect at the top, with coarse solids floating on the surface and finer particles suspended below. Soil and sand gather in the ventral region. See Fig. 17.24 for further explanation of this mixing sequence. Fig. 17.24 Mixing and eructation sequence of rumen motility. (A) The mixing, or A, sequence, begins in the reticulum in a biphasic, or double, contraction (1,1′). The contraction is next seen in the rumen in the following sequence: anterior dorsal sac (2), caudal region (3), ventral rumen (4), and finally, in the caudoventral sac (5). (B) The eructation sequence allows the elimination of gas produced by fermentation. The sequence begins in the caudodorsal blind sac (1) and proceeds to the craniodorsal blind sac (2) and dorsal sac area (3), the cardia (4), and then the ventral rumen (5). Reprinted from Constantinescu (2001). Used by permission of the publisher. With fermentation comes the production of gas, which must be removed from the animal. The burp, or eructation, sequence moves gas from the rumen toward the oral cavity (Fig. 17.24). This sequence allows the formation of a gas bubble, which is eventually forcibly ejected into the esophagus by contraction of the ventral rumen. Excess accumulation of gas in the reticulum and rumen leads to bloat. While grazing, ruminant animals quickly move feed into the rumen before it is completely masticated. This feed is then returned to the oral cavity through a process called rumination. Rumination is a series of coordinated events involving the respiratory muscles, larynx, pharynx, esophagus, oral cavity, and reticulum. At the height of a single contraction of the reticulum, the animal inhales while the glottis is closed so that air cannot flow into the lungs. This generates negative pressure in the thorax. The transfer of this negative pressure to the esophagus allows a bolus of reticular contents to move through the cardia and, by a process of reverse peristalsis, to move into the oral cavity. Immediately, there is a swallowing event that carries the liquid portion of the bolus back to the rumen. The remaining residue is chewed, saliva is added, and it is again swallowed. Time spent ruminating varies with the diet. A cow consuming a coarse hay diet will spend approximately 8 hours/day. Fermentation involves the anaerobic action of bacteria and protozoa with bacteria accounting for about 80% of rumen metabolism. Primary bacteria are those that break down the dietary constituents; secondary bacteria further break down the end products of the primary bacteria. Secondary bacteria include those that produce propionate from lactate, and methane‐producing bacteria. The protozoa consume bacteria, plant starch granules, and perhaps linoleic and linolenic acids. Fig. 17.25 Rumenal carbohydrate fermentation. Complex carbohydrates are fermented by microorganisms within the rumen. Those compounds displayed in blue do not accumulate; those shown in green are the end products. Products of the bacteria and protozoa carbohydrate digestion include short‐chain VFAs, carbon dioxide, and methane (Fig. 17.25). The major VFAs are acetic, propionic, and butyric acids, which are produced in the following proportions: 60–70% acetic acid, 15–20% propionic acid, and 10–15% butyric acid. The percentage of propionic acid increases when the animal is fed concentrates with soluble sugars or starch. The rumen epithelium can absorb glucose and VFAs. Rumen microorganisms hydrolyze dietary proteins into peptides and amino acids. In addition, these microorganisms can make amino acids from nonprotein nitrogen sources such as uric acid, urea, and ammonia. As a result, as much as 50% of the diet of ruminant animals can include poultry waste since the rumenal bacteria will convert the uric acid into amino acids. These amino acids are then absorbed and used by the animal. Triglycerides are hydrolyzed by rumenal bacteria yielding glycerol and fatty acids. The glycerol is generally metabolized to propionic acid while the fatty acids pass to the duodenum where they are absorbed (Box 17.1).
17
Digestive System
Digestive System Overview
Functions of the Digestive Tract
Peritoneal Cavity
Mesenteries
Blood Supply of the Digestive Organs
Histology of the Digestive Tract
Mucosa
Submucosa
Muscularis Externa
Serosa
Enteric Nervous System
Functional Anatomy of the Digestive System
Species
Part of the Digestive Tract
Relative Length of Intestines (%)
Average Length (m)
Relative Capacity (%)
Absolute Capacity (L)
Horse
Stomach
8.5
17.96
Small intestine
75
22.44
30.2
63.82
Cecum
4
1.00
15.9
33.54
Large intestine
21
6.47
38.4
81.25
Pig
Stomach
29.2
8.00
Small intestine
78
18.29
33.5
9.20
Cecum
1
0.23
5.6
1.55
Large intestine
21
4.99
31.7
8.70
Sheep and goats
Rumen
52.9
23.40
Reticulum
4.5
2.00
Omasum
2.0
0.90
Abomasum
7.5
3.30
Small intestine
80
26.2
20.4
9.00
Cecum
1
0.36
2.3
1.0
Large intestine
19
6.17
10.4
4.6
Cat
Stomach
69.5
0.34
Small intestine
83
1.72
14.6
0.11
Cecum
Large intestine
17
0.35
15.9
0.12
Chicken
Small intestine
79
1.08
Cecum
9
0.13
Large intestine
5
0.068
Dog
Stomach
62.3
4.33
Small intestine
85
4.14
23.3
1.62
Cecum
2
0.08
1.3
0.09
Large intestine
13
0.60
13.1
0.91
Mouth
Lips, Cheeks, and Gums
Tongue
Salivary Glands and Saliva
Teeth
Species
Deciduous
Permanent
Cat
2 (I 3/3 C 1/1 P 3/2) = 26
2 (I 3/3 C 1/1 P 3/2 M 1/1) = 30
Dog
2(I 3/3 C 1/1 P 3/3) = 28
2 (I 3/3 C 1/1 P 4/4 M 2/3) = 42
Horse
2(I 3/3 C 0/0 P 3/3) = 24
2(I 3/3 C 1(0)/1(0) P 3(4)/3 M 3/3) = 36–42
Pig
2(I 3/3 C 1/1 P 3/3) = 28
2 (I 3/3 C 1/1 P 4/4 M 3/3) = 44
Ruminant
2(I 0/4 P 3/3) = 20
2(I 0/4 P 3/3 M 3/3) = 32
Term
Meaning
Floating
The filing off of sharp edges (points) of the horse’s cheek teeth
Needle teeth
The pig’s deciduous third incisors and canines. They are often nipped off in newborn pigs to benefit the sow suckling
Parrot mouth
Seen in a horse when the mandible is shorted
Scissor mouth
Seen in a horse when an oblique angle to the incisors appears on the occlusal surface due to uneven wear
Shear mouth
Seen in a horse in which there is a narrow lower dental arch requiring frequent floating
Sow mouth
Seen in a horse when the mandible is elongated
Tusks
The canine teeth of the pig. The lower tusks are larger than the upper
Wolf teeth
The horse’s rudimentary upper first premolars. They are usually absent 
Pharynx
Swallowing
Esophagus
Histology of the Esophagus
Stomach
Anatomy of the Monogastric Stomach
Histology of the Monogastric Stomach
Gastric Secretions
Gastric Motility
Vomiting and Egestion
Regulation of Gastric Secretions and Emptying
Cephalic Phase
Gastric Phase
Intestinal Phase
Anatomy of the Stomach of Ruminants
Motility of the Ruminant Stomach
Ruminal Microbial Fermentation
Rumenal Carbohydrate Digestion
Rumenal Protein Digestion
Rumenal Lipid Digestion
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
