Page 197 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

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Digestive system

(apparatus digestorius)

The organs of the digestive system are responsible for oral cavity by the lips and tongue. In the oral cavity, it is
decomposition of ingested nutrients into constituents reduced into smaller components by the teeth and mixed
that can subsequently be absorbed. The process of diges- with secretions produced by the salivary glands. The food
tion begins with the mechanical breakdown of foodstuffs bolus is partially covered in mucus and prepared for swal-
within the oral cavity, aided in some species by enzymes lowing. In pigs, and to a lesser extent in horses, digestion of
secreted by the salivary glands. After swallowing, diges- carbohydrates commences within the oral cavity through
tion of carbohydrates continues in the intestine. Additional exposure to enzyme-rich salivary gland secretions.
secretions (gastric and pancreatic secretions, bile) result in To withstand the mechanical impact of the predomi-
the breakdown of proteins and fats. The end products, nantly solid diet of domestic mammals, the lining of the
including monosaccharides, amino acids, fatty acids and oral cavity is robust and has a high regenerative capacity. In
glycerides, are absorbed through the epithelium of the addition, the underlying tissue is richly vascularised. The
intestinal mucosa and are utilised as energy sources in wall of the oral cavity exhibits the tri-layered structure
cellular metabolism. In addition to these enzymatically characteristic of the intestinal tract, incorporating various
derived molecules, water, electrolytes and vitamins are special features:
taken up across the intestinal wall.
The liver serves as an accessory organ to the intesti- · Tunica mucosa:
nal tract, performing a central role in metabolism. As the − mucosal epithelium (epithelium mucosae):
largest gland in the body, the liver synthesises and stores − stratified squamous,
endogenous substances and is responsible for producing − pigmentation,
bile. − sensory receptors,
End products of metabolism are excreted via the intes- − immune receptors,
tinal tract, kidneys, lungs and skin. − regional specialisation: gums (gingiva),
The digestive system of ruminants is characterised − lamina propria (lamina propria mucosae):
structurally and functionally by the multi-chambered − collagen and elastic fibres,
forestomach. Microbial fermentation in the reticulum and − numerous vessels and nerves,
rumen gives rise to volatile fatty acids, saturated and non- − distinct papillae (interdigitations with epithelium),
saturated fatty acids, ammonia, carbon dioxide, microbial · Tela submucosa:
proteins and lipids that are absorbed in the forestomach − glands (labial, buccal and lingual),
– particularly the rumen – and in the intestine (refer to − lymphatic tissue and
veterinary physiology texts). · Tunica muscularis:
Macroscopically, the digestive system can be divided − skeletal muscle (lips, cheek, tongue).
into a proximal portion associated with the head (oral
cavity, salivary glands and pharynx), a distal portion The oral cavity is lined by mucosa composed of stratified
(oesophagus, stomach, intestine and anal canal) and acces- squamous epithelium underlaid by a connective tissue
sory glands of the intestine (liver and pancreas) (Figure layer (lamina propria).
10.1; see Veterinary Anatomy of Domestic Mammals: Textbook The stratified squamous epithelium (thickness 200–500
and Colour Atlas). Minor modifications of this basic schema μm) may be keratinised in places. Thickening of the epi-
are observed in the avian digestive system. thelium occurs in areas subjected to greater mechanical
strain. Heavy keratinisation is observed on the dorsum
Oral cavity (cavum oris) of the tongue and on the palate. In some breeds of cattle
The oral cavity serves to prepare ingested foodstuffs for and dog, this epithelium may be strongly pigmented. Free
enteral digestion. Food is prehended and directed into the nerve endings and Merkel cells in the epithelium serve as

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