The oropharyngeal cavity consists of the teeth and mucosa, which is continuous with the gingiva, inner surfaces of the cheeks and palate. The teeth are described in Chapter 5.

The inner cheeks of most domestic species are smooth, while in ruminants, they are studded with hundreds of conical keratinized papillae that assist with chewing coarse plant material while protecting against shear trauma (Figure 8.2). The tongue occupies the ventral aspect of the oral cavity, while the rostral hard plate and caudal soft palate form the roof. The tongue is composed of skeletal muscle covered by keratinized squamous epithelium interspersed with various papillae important in moving food particles and taste. In the cat, the tongue is covered by thick brush-like filiform papilla with caudally oriented keratinized spines important for grooming. The oral cavity also includes tonsils, which are discussed in Chapter 14.

The salivary glands consist of multiple paired secretory units, including the parotid, mandibular, and sublingual glands which are found in all domestic mammalian species. The parotid glands are located within the subcutaneous tissues along the back of the vertical ramus of the mandible. Ventral and slightly caudal and medial to the parotid glands are the mandibular glands. The sublingual salivary glands lie rostrally under the oral mucosa besides the frenulum of the tongue.

The esophagus is a tubular organ that joins the pharynx to the stomach. It opens dorsal to the larynx at the upper esophageal sphincter or pharyngoesophageal limen, a circumferential plication of the mucosa that forms a shallow ridge in the dog and cat. The cervical portion of the esophagus extends along the neck, dorsal to the trachea, and passes through the thoracic inlet into the thoracic cavity. The thoracic part of the esophagus lies to the left of the trachea at the thoracic inlet and extends dorsal to the trachea in the thoracic cavity. Within the thorax, it lies above the heart before passing through the esophageal hiatus, where it joins the stomach at the lower esophageal sphincter to the left of the medial plane and ventral to thoracic vertebra 11–12. In ruminants, the forestomaches (reticulum, omasum, and rumen) are interposed between the esophagus and the abomasum. In these species, the esophagus empties into a muscular fold of the inner wall of the reticulum, the reticular groove, which allows food to bypass the reticulo-rumen and enter directly into the omasum-abomasum. Camelids also have forestomachs (C1, C2, and part of C3) between the esophagus and the glandular stomach. Running along most of the outer length of the esophagus are the left and right vagal nerves. The mucosa of the esophagus has longitudinal folds and is uniformly smooth in all species, except in the cat, where the distal one-third has shallow circumferential ridges that impart a herring-bone appearance to the mucosal surface (Figure 8.3). The outer tunica muscularis layer of the esophagus is composed of striated muscle the entire length of the esophagus in dogs, rabbits, rodents, camelids, and ruminants, while in the cat and the horse, only the cranial two-third to four-fifth is composed of striated muscle with the caudal portion consisting of smooth muscle. In the pig, the cranial third is composed of striated muscle, while the distal third consists of smooth muscle, and the middle third is mixed.

The greater omentum is a large, sac-like, fibrofatty membrane that encloses the gastrointestinal tract (Figure 8.4). It extends from the greater curvature of the stomach and proximal duodenum and envelopes the distal small intestine and large intestine, before doubling back onto itself as it ascends to the transverse colon and caudal abdominal wall forming the omental bursa. The omental bursa communicates with the abdominal cavity through the epiploic foramen (foramen of Winslow), a small potential opening located in the right dorsal cranial abdomen. The epiploic foramen is bordered dorsally by the caudal vena cava and caudate lobe of the liver, and ventrally by the portal vein, the right lobe of the pancreas, the gastro-pancreatic fold, and the hepatoduodenal ligament. The lesser omentum connects the liver to the lesser curvature of the stomach.

The mesentery consists of a fibrofatty membrane like the omentum; however, it also contains enteric nerves, splanchnic blood, and lymphatic vessels, along with a chain of mesenteric lymph nodes located along the root. The mesentery connects the gastrointestinal tract, pancreas, spleen, and other abdominal organs to the dorsal abdominal wall. The mesentery at the junction between the ileum and cecum contains up to two colic lymph nodes (Figure 8.5). The mesocolon is the portion of mesentery that connects parts of or the entire large intestine to the dorsal abdominal wall.

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