300 COMPARATIVE ZOOLOGY 



This surplus overflow is returned to the blood by the 

 lymphatics. The current is kept up by the movements 

 of the body, and in many vertebrates, as frogs and 

 fishes, by lymph hearts. 



Like the roots of plants, the absorbent vessels do not 

 commence with open mouths ; but the fluid which enters 

 them must traverse the membrane which covers their 

 minute extremities. This membrane is, however, porous, 

 and the fluids pass through it by the processes of filtra- 

 tion and diffusion, or dialysis. How the fat gets into 

 the lacteals is not yet well understood, but the lacteals 

 are themselves rhythmically contractile, and force the 

 absorbed chyle toward the heart. The valves of the 

 lymphatics prevent its return. 



