THE RABBIT. 19 



At intervals along the ileum are oval patches (five or six 

 in all) called Peyer's patches. In these, and in the sacculus 

 rotundus, the structure of the wall is modified in a way to 

 be described later (chap, vi., 13), and they seem to have 

 some special function in relation to absorption. 



14. Absorption in the Large Intestine. The entry 

 from the sacculus rotundus into the colon is guarded by a 

 valve (the ileo-colic valve : often called ileo-caecal) : this 

 prevents the return of any of the chyme that once passes 

 it. The chyme has now a choice of two courses along 

 the colon, or into the caecum and back again. The ar- 

 rangement of a spiral ingrowth along the wall of the 

 caecum and its connexion with the ileo-colic valve seem to 

 show that normally the chyme passes into the caecum. 

 Whether the caecum has any special function is doubtful 

 it is very large in the rabbit and other herbivorous animals, 

 and small in carnivores like the dog : it has therefore been 

 suggested that in it some special ferment is secreted for 

 the digestion of cellulose a compound abundant in plant- 

 material, and not acted on by the other ferments. There is, 

 however, no evidence of this, and, as we have seen, cellulose 

 is decomposed into soluble compounds and gases by bacteria. 

 It seems probable, therefore, that the caecum acts merely as 

 an additional lengthening of the canal, for more complete 

 absorption. It ends in the blind vermiform appendix, whose 

 walls have exactly the structure of Peyer's patches, ancl are 

 therefore probably absorptive in a high degree. 



The movement of the chyme along the intestine is due to 

 a squeezing of the walls in just such a way as you might 

 squeeze an india-rubber tube with your fingers and slide 

 them along the tube. This is called peristaltic action. 



The unabsorbed residue of the chyme becomes less and 

 less liquid as water is absorbed, and in the rectum assumes 

 the form of fceces. These consist partly of materials taken 

 in with the food which have remained unaltered all along, 

 partly of the excretory compounds of the bile, in very small 

 degree of compounds produced during digestion (such as 

 the insoluble soaps), partly of the products of bacterial 

 decomposition. 



