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

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.

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.

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