336 VETERINARY PHYSIOLOGY 



undigested constituents of the food and excretory products. 

 The undigested material usually forms the greater part of 

 the accumulation, especially in herbivora. It has been 

 shown that rabbits on a diet freed from cellulose cease to 

 defecate and die of intestinal obstruction. 



The higher centres are stimulated by excitement, which, 

 especially in timid animals, may produce diarrhoea. Muscular 

 exercise helps to induce the reflex, probably partly by a 

 sympathetic increased muscular action of the colon, and 

 partly by the contraction of the abdominal muscles forcing 

 material into the lower bowel. The influence of exercise is 

 well seen in dogs, which often defsecate after being released 

 and allowed a run. 



When defecation takes place, all the various muscles 

 which can increase the pressure in the abdomen are called 

 into play. A deep inspiration is taken, and then, with the 

 glottis closed, expiratory efforts are made. The levator ani 

 which supports the anus is relaxed, and the upper part of 

 the rectum is brought more into line with the lower part of 

 the pelvic colon. Faeces may thus be forced into the 

 rectum, and the reflex act of defaecation with contraction of 

 the colon and relaxation of the sphincters is effected. The 

 act is completed by the contraction of the internal sphincter 

 from above downwards and by the contraction of the levator 

 ani and external sphincter. 



B. DIGESTION IN HERBIVORA. 



The food of herbivora is characterised by its bulk, and 

 by the fact that the digestive material is for the most part 

 enclosed in cells whose cellulose walls are not dissolved by 

 the enzymes of the digestive tract. The distinctive feature 

 about digestion in herbivora is therefore the provision for 

 detaining a large quantity of food in a specially developed 

 part of the alimentary canal, where, under the influence of 

 bacteria, the cellulose is dissolved and the enclosed digestible 

 material liberated. 



