96 COMPARATIVE ZOOLOGY. 
minute extremities. The pores of this membrane (which, 
though invisible, are demonstrable) are so many short cap- 
illary tubes; so that the passage 
of the chyle is a physical neces- 
sity, and is not dependent on a 
vital or some other mysterious 
7 pk Nae a i a principle. But as capillary at- 
ithelial to nucleated cells. traction can not produce motion 
beyond the capillary tubes, another force is called into 
play, namely, d7ffusion. The chyle, having passed through 
the membrane, meets a fluid (lymph), with which it is mis- 
cible, and diffuses itself into that fluid, leaving room for a 
fresh supply to enter the membrane.” 
In this way a slow 
but strong current is kept up, which is probably aided by 
a rhythmical contraction of successive portions of the lac- 
teals themselves. In Fishes and Reptiles, the absorbents 
are provided with pulsating sacs, called lymphatic hearts. 
In Birds and Mammals, they are furnished with valves, 
“which prevent the backward flow of the contents. 
CHAPTER XII. 
THE BLOOD OF ANIMALS. 
The Blood is that peculiar fluid derived from digested 
food which carries tu the living tissues the materials nec- 
essary to their growth and repair. The great bulk of the 
body is occupied with apparatus for the preparation and 
circulation of this vital fluid. 
The blood of the lower animals (Invertebrates) differs 
so widely from that of Man and other Vertebrates, that 
the former were long supposed to be without blood. In 
them the blood is commonly colorless; but it has a bluish 
