COMPARATIVE ANATOMY 81 



area with its long axis disposed at right angles to the 

 ileal patch. (Fig. 23.) A terminal branch of the ileo- 

 caecal artery marks the ileo-csecal junction on the ex- 

 terior and sends twigs to both patches. 



The lymphoid tissue massed in the csecum however 

 usually occurs at its apex and is present in very many 

 different species. And the form in which it presents 

 itself seems to be associated with the animal's diet 

 rather than with the position of the species in the 

 zoological scale. 



The caecum is the proximal part of the large intestine. 

 Its walls secrete no . digestive juices and one of its 

 chief functions seems to be that it provides a recep- 

 tacle in which digestion by bacterial action can be 

 carried on. The normal alimentary juice of higher 

 animals can break up all food substances with one 

 exception — namely cellulose. The body can deal satis- 

 factorily with protein, carbohydrate and fat without 

 bacterial assistance so long as these foodstuffs are not 

 enclosed in cellulose envelopes. 



In cows and other herbivora, known as ruminants, 

 bacterial decomposition of the cellulose takes place 

 while the food is incubated in the pouch or first 

 stomach. Then the food is chewed once more and 

 swallowed, finally, a second time. In rodents such as 

 the rabbit the bacterial decomposition of the cellulose 

 takes place in a truly enormous caecum provided with 

 a number of spiral folds increasing the area for ab- 

 sorption. 



In carnivorous animals the proximal part of the large 

 intestine is of small size. 



Thus in the same class of animals one may find 

 varying degrees of caecal development correlated to 

 some extent with the diet of the particular species. 

 Furthermore, it may be added, analogous modifications 



