190 MANUAL OF PHYSIOLOGY. 



form in which the food is presented to the mucous membrane for 

 absorption. It resembles somewhat by its whiteness the fluid 

 called chyle which flows in the lacteals, and formerly was con- 

 sidered to be identical with it. This creamy lining is the chief 

 material found in the upper part of the small intestine, the coarser 

 parts of the food being hurried on by peristaltic action to the 

 large intestine. 



In the large intestine the secretion of the long, closely-set Lie- 

 berkiihn's follicles is the only one of importance. Its reaction 

 and that of the mucous membrane is alkaline, but the contents 

 of the colon are acid, owing to certain fermentative changes which 

 go on in this part of the intestine. 



Of the changes brought about in the large intestine by the 

 agency of the digestive juices we know but little. Judging from 

 the large size of the csecum and colon in herbivorous animals, 

 we are prompted to conclude that vegetable substances, possibly 

 cellulose, may be dissolved here, but we do not know how this is 

 accomplished. Although devoid of villi, the large intestine can 

 certainly absorb readily such materials as are in solution. As 

 the insoluble materials pass along the small intestines the supply 

 of fluid is kept up to about the same standard, the absorption 

 and secretion being about equal ; but in the large intestine, the 

 absorption of the fluid so exceeds the secretion in amount that 

 the undigested materials are gradually deprived of their fluid, 

 and are converted into soft, solid masses which pass on to be added 

 to the faeces. 



Owing to its absorbent power the large intestine is a ready 

 and rapid channel by which materials can be introduced into 

 the system in cases in which the stomach is too irritable to retain 

 food. 



The quantity of faeces evacuated in the day depends upon the 

 kind of diet, being greater with a vegetable than meat diet, 

 averaging about 150 grammes a day (60-250 grms.). This 

 amount may be greatly increased by largely partaking of indi- 

 gestible forms of food. The more rapid the passage of the ingesta 

 through the intestine the greater is the amount of fluid remaining 

 with the faeces, so that any stimulant to the intestinal movements 



