DIGESTION OP POOD. 35T 



from the stomach, and carrying on the more elaborated con- 

 tents to points where they can receive fresh attention, both 

 digestive and absorptive. 



Comparative. — In man, the carnivora, and some other groups, 

 it is likely that digestion in the large intestine is slight, the work 

 being mostly completed — ^at all events, so far as the action of 

 the secretions is concerned — ^before this division of the tract is 

 reached, though doubtless absorption goes on there also. The 

 muscular strength of this gut is important in the act of defe- 

 cation. 



But the great size of the large intestine in ruminants — in 

 the horse, etc. — together with the bulky character of the food 

 of such animals, points to the existence of possibly extensive 

 processes of which we are ignorant. It is generally believed 

 that food remains but a short time in the stomach of the horse, 

 and that the cascum is a sort of reservoir in which digestive 

 processes are in progress, and also for water. 



Fermentations go on in the intestine, and probably among 

 ruminants they are numerous and essential, though our actual 

 knowledge of the subject is very limited. 



The gases found in the stomach are atmospheric air (swal- 

 lowed) and carbon dioxide, derived from the blood. Those of 

 the intestine are nitrogen, hydrogen, carbonic anhydride, sul- 

 phuretted hydrogen, and marsh-gas, the quantity varying con- 

 siderably with the diet. In herbivora the quantity of COa and 

 CH4 is large. 



Although our knowledge of the actual processes by which 

 food is digested in the domestic animals is meager, there are 

 certain considerations to which it may be well to give promi- 

 nence at this point. 



The whole subject becomes clearer and the way is paved for 

 more exact and comprehensive knowledge if it be borne in 

 mind that the entire alimentary tract has a common embryo- 

 logical origin from the splanchnopleure (Fig. 325, etc.), consist- 

 ing of outer mesoblast and lining hypoblast, the former giving 

 rise to the muscular and other less essential structures, the lat- 

 ter to the all-important glandular epithelium. But of all re- 

 gfions the alimentary tract has been modified ia relation to 

 the development and habits of the animal group. It can not 

 be too well remembered that digestion is highly complex, with 

 one organ and one process supplementary to another. 



If mastication is imperfect, as in the carnivora, gastric diges- 



