FOOD AND DIGESTION 377 



act is completed by the emptying of the canal by the con- 

 traction of the levatores ani muscles. 



B. Intestinal Digestion in the Horse and other Herbivora 



In the horse intestinal digestion is of great importance. 



In the small intestine the processes which go on are much 

 the same as in the carnivora, but in the huge large intestine 

 important changes occur. 



1. In the eseeum some of the food remains for a very 

 considerable time as much as twenty-four hours. It is mixed 

 with the secretion of the intestine and with water which the 

 horse has drunk, and which passes rapidly to the caecum. The 

 reactions of the contents are alkaline, and bacteria are 

 abundant. Here a considerable quantity of protein is digested 

 and cellulose disappears. This disappearance of cellulose is 

 probably largely due to the action of bacteria, and it results 

 in the production of: 



(1) Lower fatty acids, such as acetic and butyric acid, which 

 combine with the alkalies present, and, being absorbed, are oxi- 

 dised to carbonates, yielding possibly a small amount of energy, 

 and being partly excreted by the lungs as C0 2 and partly by 

 the urine as carbonates of sodium and potassium, which help to 

 give the fluid its alkaline reaction. It is said that 100 parts of 

 cellulose in decomposing will yield about 60 parts of these acids. 



(2) Gases, of which carbon dioxide, C0 2 , and marsh gas, CH 4 , 

 are the chief. 



2. In the double colon, which has a mean capacity of no less 

 than 80 to 100 litres, the same processes go on as in the 

 caecum, and when the single colon is reached, a very rapid 

 absorption of fluid and of the products of digestion held in 

 solution takes place, and the residue of the food is formed into 

 the desiccated balls of faeces. So rapid is absorption from the 

 lower bowel in the horse, that the animal is readily killed by 

 rectal injection of strychnine, and may be easily anaesthetised 

 by giving ether per rectum. The rapid absorption also allows 

 of life being maintained on nutritive enemata. 



The time taken for food to pass right along the alimentary 

 canal of the horse is considerable probably about four days. 

 But solid bodies given by the mouth have been found in the 



