THE DIGESTIVE SYSTEM 83 



These micro-organisms break down the resistant vegetable cells 

 and the cellulose disappears. Just what, if any, part is played by 

 the digestive enzymes in cellulose digestion is a mooted question. 

 It seems doubtful whether they exert any action, for when cellulose 

 is broken down not the shghtest trace of sugar can be found, but 

 large amounts of carbon are liberated. It is thought that the 

 carbon combines with hydrogen to form methane, and with oxygen 

 to form carbon dioxid. The expired air contains these gases in 

 relatively large quantities, which has led physiologists to think 

 that, in the horse at least, we can account for their presence in 

 large measure by cellulose destruction. There is reason to believe 

 that the more resistant cellulose-containing portions of the food 

 are not transformed until they have been acted upon by the various 

 agents noted above and reach the last portion of the large colon. 

 On the strength of these facts, it seems unreasonable to attribute 

 much heat value to cellulose destruction. The process may be 

 considered simply an accident in the course of digestion. 



Immediately after passing into the small colon a very marked 

 change occurs in the character of the contents; from a greenish- 

 brown pea-soup-like mass it becomes firmer and resembles quite 

 closely fresh feces. As the material is moved on toward the rec- 

 tum it gets drier and is molded into characteristic balls through 

 the action of the muscular bands in the walls of the small colon. 

 Smith states that the reaction is still alkaline or neutral at this 

 point, but after reaching the rectum a distinctly acid reaction is 

 obtained on the surface of the fecal balls. 



Feces represent the residue of digestion. They have a dis- 

 agreeable, but not exactly a repulsive, odor even in health, due to 

 indol and skatol. When digestive disturbances are present they 

 become extremely offensive. They are collected in the pouch-hke 

 dilatation of the rectum. When the rectum is sufficiently full to 

 cause a reaction in its walls from pressure it is voluntarily emptied. 

 The act is termed defecation. Voluntary defecation becomes im- 

 possible when paralysis of the rectum exists. The feces contain 

 approximately 76 per cent, water, 21 per cent, organic matter, and 

 3 per cent, mineral matter. Grass and other succulent foods result 

 in soft excreta, while dry foods, especially concentrates, have a 

 constipating effect. The color varies with the character of the 

 food. A thin coating of mucus covers the feces and gives them a 

 shiny appearance. Smith observed that horses on a diet consisting 



