LACTEAL ABSORPTION. 201 



between the folds of a very fine and delicate mem- 

 brane, called the mesentery, which connects the 

 intestines with 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 glandu- 

 lar bodies, termed the mesenteric glands, where it 

 is probable that the chyle undergoes some modifi- 

 cation, 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 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 suffi- 

 ciently extended to prevent the free passage of their 

 fluid contents in a retrograde direction. The lac- 

 teals 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 nume- 

 rous ; 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 



