Diseases of the digestive tract account for nearly half the deaths which occur in horses; despite this, very few investigations have been iiiade into the functions of this system. The work described here was undertaken with the object of providing some information on this subject. It is hoped that the information obtained will be of value in understanding some of the pathalogical conditions and will also advance knowledge of digestion in herbivorous mammals. In this last respect, studies on the horse provide a useful comparison with those on the ruminant.
Since the size and cost of horses made acute experiments impracticable, it was necessary to devise other methods of investigation. h supply of fresh intestines was available from horses slaughtered for food. This was utilized and a technioue developed for the perfusion of isolated segments of intestine (Alexander, 1949). The movements of the ileum and small colon were studied in this fashion and also the effect on them of various humoral agents. hen the ileum was perfused with isotonic solutions containing oxygenated red blood cells, it showed propulsive movements. This was studied in more detail (Alexander, 1950) and it was found that the magnitude of the movements varied with the supply of oxygen. The effects of the inorganic constituents of the perfusion fluid were investigated and, of these, only sodium ions seemed to have a specific effect. During the course of these experiments a fast propelling mechanism was observed which propelled balloons through the ileum at rates of 1 -4 cm./sec. This mechanism could be shown only in a small proportion of preparations and differed from the slow propulsion described in the first paper.
The movements of the horse stomach were investigated (Alexander, 1951a) and it was found that the perfused isolated stomach did not contract strongly enough to expel its contents. Attention then turned to the living animal and a method devised to record gastric contractions by means of a tube passed down the oesophagus. Gastric contractions were unaffected by histamine, carbamylcholine or posterior pituitary extract.
By mixing carbon granules of suitable size with food, it was possible to measure the rate of passage of food residues through the alimentary tract of the horse (Alexander, 1946). In view of the length and complexity of this system in the horse it was surprising to find that the time taken by digesta to traverse the tract was approximately the same as in man. A more detailed study became possible with the development of radiographic methods (Alexander and Benzie, 1951). So far as could be ascertained, this was the first publication of radiographs of the equine digestive tract. It was of particular interest to discover that digesta remained in the stomach and small intestine of the suckling foal for a longer time than in the weaned animal.
Since abdominal surgery in the horse has been regarded as particularly hazardous, the study of digestion by means of permanent fistulae into the intestine had not been attempted. However, the development of a suitable technique seemed essential and a method was evolved for the fistulation of the caecum (Alexander and 1)onald,1944) The success of this procedure encouraged the extension of the intestinal surgery to include fistulation and exteriorisation of the small intestine (Alexander, 1951b) and now it has been found possible to fistulate the caecum, ventral and dorsal colons in the same animal (Alexander, 1952a).
The ileal fistulae allowed the renstition on the living animal of some of the experiments on the perfused ileum.. There appeared to be an association between ileal motility and blood flow (ítlexander, 1952b) which supported the observations on the in vitro preparations. The fistulae of the large intestine were used to study movements, products of fermentation and cellulose digestion. Three types of movement were recognised but no evidence of antiperistalsis was obtained. Large amounts of the lower fatty acids were found in the caecum and large colon and the proportion of the various acids differed between the dorsal and ventral colon. It appeared that witholding food depressed cellulose digestion (Alexander, 1952a).
The experiments on the small intestine had been confined to a study of the mechanical factors. However, by utilizing the fistulated animals it was possible to study absorption. It was found that feeding glucose produced a hyperglycaemia which was associated with the presence of a high glucose concentration in the ileum. Since no glucose or its j fermentation products appeared in the caecum, it seemed probable that it was completely absorbed in the small intestine. This may explain, in part, the finding that the concentration of glucose in horse blood is about 50% higher than in ruminants (Alexander,1954a).
Since the horse is a good example of a. herbivore in which microbial digestion is carried out in the large intestine, studies of digestion in the horse are complementary to those on the ruminant, animals in which microbial digestion takes place in the stomach. The digestive processes in these species have been reviewed (Alexander, 1954b) and it is suggested that the horse is particularly suited for studies of cellulose digestion.
During the development of the surgical techniques described, considerable experience was gained in anaesthetising ponies for long periods. A variety of anaesthetics were tried and eventually a satisfactory technique evolved. This has been described in detail and an account given of the anaesthetic deaths which occurred during the course of the operations (Alexander, 1954c).
The development of the techniques described has encouraged investigation of the microbial activities of the horse's digestive tract. So far, a specific streptococcus, a lactate fermenting organism (Veillonella gazogenes) (Alexander, Macpherson and Oxford, 1952) and a glucuronide decomposing enzyme have been isolated from the horse colon. This enzyme resembled others which occurred at sites of cellulose in various herbivores (Marsh, Alexander and Levvy,1952). It has been suggested that the enzyme is concerned with the digestion of hemicellulose.
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| 2. The Action of Some Humoral Agents on the Horse Intestine. quart. J. exp. Physiol. (1949), 35, 11.
| 3. Factors influencing the Motility of the Perfused Horse intestine. quart. J. exp. Physiol. (1950), 36 1.
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| 14. Glucuronide in the Tract. (with C.A. Marsh & G.A. Levvy) Nature (1952), 170, 163.