42 A COURSE ON ZOOLOGY. 



the latter ; besides, it exerts an action on fatty sub- 

 stances, converting them into emulsions, mixtures in 

 which the fat is broken up into microscopic drops. 



The action of the bile is not as certainly determined. 

 It is probable that it aids the pancreatic juice in emul- 

 sifying fats, at the same time partially saponifying 

 them, that is, converting them into soluble soaps. 



While the food undergoes the action of the bile and 

 the pancreatic juice, it is pushed forward by the contrac- 

 tions of the muscular tissue, these contractions, having 

 been first started by the introduction of the bolus into 

 the oesophagus, resulting in what are called peristaltic 

 movements, and continues its passage through the 

 many turnings of the intestinal tube. Absorption takes 

 place during this passage by the aid of innumerable 

 villi which cover the mucous membrane, and the liquids 

 pumped in by these little suckers enter vessels, called 

 lacteals or chyle-ducts. These vessels cover the mesen- 

 tery with an extensive net-work, and finally unite to form 

 one large canal, called the thoracic duct, which passes 

 upward along the vertebral column, and pours into the 

 left subclavian vein a whitish, milky liquid, which is 

 usually called chyle ; this owes its opacity and its white 

 color to the fat globules it holds in emulsion, just as 

 milk is white because of the numerous little butter 

 globules suspended in it. 



Those parts of the food which have resisted all diges- 

 tive action pass on into the large intestine, whose length 

 is but a small fraction of that of the small intestine, and 

 are finally expelled as refuse. 



