228 THE VITAL FUNCTIONS. 



and in Mammalia these valves are perfectly 

 effectual, and are exceedingly numerous, giving 

 to the lacteals, when distended with fluid, the 

 appearance of strings of beads. The effect of 

 these flood-gates, placed at such short intervals, 

 is that every external pressure made upon the 

 tube, assists in the propulsion of the fluid in the 

 direction in which it is intended to move. Hence 

 it is easy to understand how exercise must tend 

 to promote the transmission of the chyle. The 

 glands are more numerous and concentrated in 

 the Mammalia, than in any other class. 



From the mesenteric glands the chyle is^ con- 

 ducted, by the continuation of the lacteals, into 

 a reservoir, which is termed the receptacle of the 

 chyle: whence it ascends through the thoracic 

 duct* which passes along the side of the spine, 

 in a situation affording the best possible pro- 

 tection from injury or compression, and opens into 

 the great veins leading directly into the heart. 



In invertebrated animals having a circulatory 

 system of vessels, the absorption of the chyle is 

 performed by veins instead of lacteal vessels. 



The sanguification of the chyle, or its conver- 

 sion into blood, takes place during the course 

 of the circulation, and is princij^ally effected by 

 the action of atmospheric air in certain organs, 

 hereafter to be described, where that action, or 



* This duct is occasionally double. 



