THE CIRCULATION". 



119 



with its torrent of blood rushing down from the heart, 

 secondly, the vein, with its slow, steady stream flowing in 

 the opposite direction ; and between them lies the network 

 of capillaries, so fine that the corpuscles can only pass 

 through "in single file." The current has here an uncer- 

 tain or swaying motion, hurrying first in one direction, then 

 hesitating, and then turning back in the opposite direction, 



FIG. ;. WEB op A FROG'S FOOT, FIG. .33. MARGIN OP FROG'S W 

 slightly magnified. magnified 30 diameters. 



and sometimes the capillaries contract so as to be entirely 

 empty. Certain of the tissues are destitute of capillaries ; 

 such are cartilage, hair, and a few others on the exterior of 

 the body. In all other structures, networks of these vessels 

 are spread out in countless numbers : so abundant is the 

 supply, that it is almost impossible to puncture any part 

 with the point of a needle without lacerating tens, or even 

 hundreds of these vessels. 



41. The capillaries are elastic, and may so expand as to 

 produce an effect visible to the naked eye. If a grain of 

 sand, or some other foreign particle, lodge in the eye, it will 

 become irritated, and in a short time the white of the eye 

 will be "blood-shot." This appearance is due to an in- 



41. Elasticity of the capillaries? Grain of sand in the eye? Blush? Other 

 cases ? 



