220 A MANUAL OF VETERINARY PHYSIOLOGY 



that the contents are subjected to great compression. The entire 

 contents of the colon are yellow in colour or yellowish-green, 

 becoming rapidly brown or olive-green on exposure to the air ; 

 the colour being due to the chlorophyll of the food. The contents 

 of the colon are normally alkaline throughout ; we once, however, 

 found them acid. 



Digestive Changes. — Large Intestines. 



The changes food undergoes in the large intestine have never 

 excited the same interest as those in the small. The absence of 

 any secretion from the large bowel other than the succus may help 

 to account for this, and may also assist in explaining why the 

 large bowels have been regarded in the light of reservoirs for 

 ingesta, rather than as active centres of digestion. As a matter 

 of fact, the large intestines of the horse are actively employed in 

 dealing with cellulose, not by means of any known enzyme 

 peculiar to the body, but rather by the process of bacterial 

 disintegration, the result of decomposition.^ It is known that 

 bacteria may hydrolyse cellulose, and render ir fit for absorption. 

 In the case of oats, we mentioned (p. 193) that they probably fur- 

 nished their own cellulose enzyme, but this has not been proved 

 for all vegetable material. The cellulose of hay is, probably, only 

 utilised after prolonged maceration in the large intestines and 

 the subsequent attack of bacteria. By some it has been con- 

 sidered that the epithelial cells of the intestine are capable of 

 dealing with cellulose, but on this point no definite statement can 

 be made. Cellulose yields energy to the body on oxidation, but 

 there is another reason for the extensive preparations made for 

 its digestion in herbivora — viz., the cellulose encloses the protein, 

 starch, and fat of vegetable substances in a framework, and until 

 this is broken down these substances cannot be acted upon. 

 We know that considerable cellulose solution must occur before 

 the material arrives at the large intestines, otherwise neither in 

 the stomach nor small intestine could digestion occupy the 

 prominent position it does. The digestion of protein, fat, and 

 sugar are largely, though not entirely, dealt with in the stomach 

 and small intestine, but there must be a certain amount of these 

 substances so firmly locked up in their cellulose envelope that 

 they are not liberated until after prolonged maceration and 

 digestion in the large intestines. We may, therefore, safely 

 assume that protein, fat, starch, and cellulose are capable of being 

 acted upon and absorbed from the large intestines of the horse. 



As the result of cellulose digestion carbonic acid and marsh-gas 

 are formed in equal volumes. We have in our description of the 

 large intestines drawn attention to the appearance of the caecum 



