King's Applied Anatomy of the Abdomen and Pelvis of Domestic Mammals. Geoff Skerritt

and is suspended as festoons by the peritoneal sheets of the mesentery. (ii) Crescentic folds of the internal lining membrane (the mucous membrane) occur, known as the plicae circulares (or the valves of Kerkring). (iii) Tiny finger‐like projections of the lining mucous membrane are present in vast numbers along the length of the small intestine. They are covered by absorptive and mucus‐secreting cells, and with a core of connective tissue, smooth muscle cells, blood vessels and lacteals, channels for transport of fat. These minute projections, known as villi, enormously increase the area for absorption. (iv) Finally, at the cellular level, at the luminal surface of each absorptive cell, the cell membrane is itself thrown into submicroscopic projections known as microvilli, which further increase the available absorptive surface.

2.3 Regions of the Alimentary Tract (Figure 2.1)

Figure 2.1 The general arrangement of the intestines of the domestic mammals. The diagrams of the mammalian gastrointestinal tracts are drawn from the dorsal view although the ruminant intestinal spiral and the abomasum are displaced to the right by the rumen. The duodenum is V‐shaped with the ascending limb lying dorsally on the right at the level of the base of the caecum (mauve). The stomach and the duodenum are both coloured green on the diagram. The jejunoileum is not shown on the diagram but occupies a large space between the duodenum and the colon mainly on the right. The transverse and descending colon retain their basic position. The transverse colon passes cranial to the root of the mesentery which contains the cranial mesenteric artery (red circle).

      In the higher vertebrates the regions of the tract comprise the oesophagus, stomach, small intestine, large intestine and rectum. In ruminant herbivores (e.g. the ox) the stomach is very large and divided into four distinct regions, each with its own structure and function (see Sections 4.1 and 4.2.). The small intestine comprises the duodenum, jejunum and ileum (although these two divisions are ill‐defined and are termed the jejunoileum). The large intestine in most species comprises a fairly long portion known as the colon, with a relatively short blind‐ending diverticulum known as the caecum. However, there are wide species variations, and in some herbivores (e.g. the horse), the caecum is very large (see Section 6.2.1).

In lower vertebrates marked variations from the above pattern occur. For example, in several varieties of fish no stomach is present, there being only a single tube between the pharynx and the intestine, which may be regarded as equivalent to the oesophagus. In fact the oesophagus only becomes well defined structurally in land vertebrates with the development away from gill breathing, i.e. an adaptation for the extraction of oxygen from water.

      Important variations occur also in connection with the modifications to increase surface area mentioned earlier. Of particular importance, in view of its obvious functional success, is the so‐called spiral intestine that characterises elasmobranch fishes. In these species the intestine is cigar‐shaped and has a fold of internal epithelium projecting into the lumen. This projection twists spirally along the length of the intestine, thereby increasing the internal surface area. However, it seems that this adaptation is less successful than the greatly elongated intestine that is found in higher forms, from the teleost fishes to land vertebrates.

      One feature that is common to the digestive tract of all vertebrates is the presence of a constriction known as the pylorus, which in most species occurs at the distal end of the stomach where the latter is continuous with the small intestine. The pylorus is a point of demarcation; that part of the tract cranial to the pylorus is known as the foregut and the region caudal to it is the hindgut. Primitively therefore the foregut was merely a short link between the pharynx and the intestine, with the latter being solely responsible for chemical treatment and absorption, but in higher orders the stomach has taken on an important role in this connection. It has developed as a storage area and for both the physical and chemical treatment of food. It seems likely that the development of this organ in an evolutionary sense is related to changed feeding habits, developing from primitive food strainers.

2.4 Clinical Conditions Affecting Gastrointestinal Function

      The clinical presentation of gastrointestinal disease involves one or more of the following clinical signs (see Sections 4.3, 5.3, and 6.3):

      1 Acute diarrhoea

      2 Chronic diarrhoea

      3 Vomiting

      4 Weight loss

      5 Abdominal discomfort

      6 Anorexia

      7 Presence of fresh or altered blood in the faeces.

      Acute diarrhoea may be mild with no other clinical signs and due to a dietary trigger. Severe diarrhoea may result in dehydration, abdominal pain and a body temperature increase, and it may be due to an infection. Chronic diarrhoea, lasting more than 2 weeks, can have a variety of aetiologies ranging from neoplasia to hyperthyroidism (feline). Clinical signs such as increased or decreased frequency of defaecation or the presence of blood in the faeces are all diagnostically significant and may help in the localisation of intestinal disease.

A vomiting centre, located in the medulla oblongata of the hindbrain, mediates the vomiting reflex. The differential diagnosis of vomiting can be quite involved, although more so in dogs and cats rather than in the larger animals. Vomiting can be acute or chronic and may originate in the stomach or the small intestine; the possible causes are many and serious or trivial. Retching is followed by abdominal contractions resulting in forceful expulsion of oesophageal and gastric contents.

      There are many possible causes of weight loss in animals. In dogs and cats disorders of malabsorption (see Section 5.3.5) may result from inflammatory bowel disease or severe parasitism. In farm animals there are a number of specific diseases that may cause weight loss, e.g. Johne’s disease in cattle and sheep (see Sections 5.3.2 and 5.3.3).

      Abdominal discomfort in animals is usually detected by the patient’s restlessness and pain on palpation. In dogs gastroenteritis and gastric dilatation and volvulus (GDV) are severe causes of abdominal pain requiring urgent attention (see Section 4.3.5). In horses colic is a serious cause of abdominal pain (see Section 6.3).

      Loss of appetite (anorexia) may have many causes in dogs ranging from dietary preference to a variety of causes warranting clinical assessment. In horses the explanation for a loss of appetite may be dental pain.

      Haemorrhagic gastroenteritis is potentially a serious cause of blood in the faeces of dogs. Other possible causes are parasitism, parvovirus infection, E. coli infection and neoplasia.