LACTEAL ABSORPTION. 165 



The mesenteric glands of the Whale contain large spheri- 

 cal cavities, into which the trunks of the lacteals open, and 

 where the chyle is probably blended with secretions proper 

 to those cavities; but no similar structure can be detected in 

 terrestrial mammalia. 



It is only among the Vertebrata that lacteal vessels are 

 met with. Those of Fishes are simple tubes, either wholly 

 without valves, or if there be any, they are in a rudimental 

 state, and not sufficiently extended to prevent the free pas- 

 sage of their fluid contents in a retrograde direction. The 

 lacteals of the Turtle are larger and more distinct than those 

 of fishes, but their valves are still imperfect, though they 

 present some obstruction to descending fluids. In Birds 

 and in Mammalia these valves are perfectly effectual, and are 

 exceedingly numerous, giving to the lacteals, when distend- 

 ed with fluid, the appearance of strings of beads. The ef- 

 fect 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 intend- 

 ed to move. Hence it is easy to understand how exer- 

 cise 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 conducted, 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 protection from in- 

 jury 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 conversion into 

 blood, takes place, during the course of the circulation, and 



* This duct is occasionally double. 



