LACTEAL ABSORPTION. 227 



by very minute orifices, in incalculable numbers, 

 and unite successively into larger and larger 

 vessels, till they form trunks of considerable size. 

 They pass between the folds of a very fine and 

 delicate membrane, called the mesentery, which 

 connects the intestines to the spine, and which 

 appears to be interposed in order to allow them 

 that degree of freedom of motion, which is so 

 necessary to the proper performance of their 

 functions. In the mesentery, the lacteals pass 

 through several glandular bodies, termed the 

 mesenteric glands, where it is probable that the 

 chyle undergoes some modification, preparatory 

 to its conversion into blood. 



The mesenteric glands of the Whale contain 

 large spherical 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 stnicture can be de- 

 tected 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 

 passage 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 



