22 A COURSE ON ZOOLOGY. 



The internal membrane of the veins is provided through- 

 out all their lengths (excepting in the veins of the head) 

 with innumerable little folds, shaped much like bird's 

 nests, and designed to prevent the flowing back of the 

 blood. These valves of the veins do important work in 

 the circulation: they prevent a backward flow of the 

 blood, which is far from its starting-point, and, having 

 lost a part of its initial speed, meets resistance in rising 

 towards the heart. The arteries are under different con- 

 ditions, and, with the exception of the large ones that 

 start from the heart, do not possess valves. 



The arteries are much less numerous than the veins. 

 Yery few of them are superficial ; they are generally 

 situated in deep places, and each is usually accompanied 

 by two veins that are called satellites. The venous sys- 

 tem, on the contrary, independently of the veins associ- 

 ated with the arteries, includes a very large number of 

 superficial vessels. 



The capillary system may be easily understood as the 

 common continuation of the other two systems. Each 

 of the last arterial ramifications ends in a vessel of very 

 small diameter, and after travelling a certain distance 

 this vessel becomes a prolongation of one of the last 

 venous ramifications. Multiply this example innumer- 

 ably, and we have an idea of the vast net-work whose 

 ramifications extend in one manner or another to the 

 smallest and most remote particles of our bodies. 



The capillary vessels differ in their diameters, which 

 are often microscopic ; some are so narrow that the red 

 corpuscles of the blood cannot pass through, and only 

 the plasma circulates in them. Others allow the red cor- 

 puscles to pass through only when for any reason the 

 vessels are dilated. 



