THE CAPILLAEY CIRCULATION. 



295 



when distended by injection, is about twice as great as that of the 

 entire arterial system. During life, however, the venous system is 

 at no time so completely filled with blood as is the case with the 

 arteries, and, making allowance for this difference, we find that the 

 entire quantity of venous blood is to the entire quantity of arterial 

 blood nearly as three to two. The velocity of the venous blood, 

 as compared with that of the arterial, is therefore as two to three : 

 or about 8 inches per second. It will be understood, however, that 

 this calculation is altogether approximative, and not exact ; since 

 the venous current varies, according to many different circumstances, 

 in different parts of the body ; being slower near the capillaries, 

 and more rapid near the heart. It expresses, however, with suffi- 

 cient accuracy, the relative velocity of the arterial and venous cur- 

 rents, at corresponding parts of their course. 



THE CAPILLARY CIRCULATION. 



The capillary bloodvessels are minute inosculating tubes, which 

 permeate the vascular organs in every direction, and bring the 

 blood into intimate contact with the substance of the tissues. They 

 are continuous with the terminal ramifications of the arteries on 

 the one hand, and with the com- 

 mencing rootlets of the veins on 

 the other. They vary somewhat 

 in size in different organs, and in 

 different species of animals ; their 

 average diameter in the human 

 subject being a little over ^Va of 

 an inch. They are composed of 

 a single, transparent, homogene- 

 ous, somewhat elastic, tubular 

 membrane, which is provided at 

 various intervals with flattened, 

 oval nuclei. As the smaller arte- 

 ries approach the capillaries, they 

 diminish constantly in size by 

 successive subdivision, and lose 

 first their external or fibrous 

 tunic. They are then composed 



only of the internal or homogeneous coat, and the middle or muscu- 

 lar. (Fig. 98, a.) The middle coat then diminishes in thickness, 



SMALL ABTKRT, with its muscular tunic 

 (n}, breaking up into capillaries. From the jt/ut 

 muter. 



