CIRCULATION OF THE BLOOD. 135 



according as the vessels composing them have a straight or tortuous form. 

 Sometimes the capillaries have a looped arrangement, a single capillary 

 projecting from the common network into some prominent organ, and 

 returning after forming one or more loops, as in the papillae of the tongue 

 and skin. 



The number of the capillaries and the size of the meshes in different 

 parts determine in general the degree of vascularity of those parts. The 

 parts in which the network of capillaries is closest, that is, in which the 

 meshes or interspaces are the smallest, are the lungs and the choroid 

 membrane of the eye. In the iris and ciliary body, the interspaces are 

 somewhat wider, yet very small. In the human liver the interspaces are 

 of the same size or even smaller than the capillary vessels themselves. 

 In the human lung they are smaller than the vessels; in the human 

 kidney, and in the kidney of the dog, the diameter of the injected capil- 

 laries, compared with that of the interspaces, is in the proportion of one 

 to four, or of one to three. The brain receives a very large quantity of 

 blood; but the capillaries in which the blood is distributed through its 

 substance are very minute, and less numerous than in some otherj^arts. 

 Their diameter, according to E. H. Weber, compared with the long diam- 

 eter of the meshes, being in the proportion of one to eight or ten ; com- 

 pared with the transverse diameter, in the proportion of one to four or 

 six. In the mucous membranes for example in the conjunctiva and in 

 the cutis vera, the capillary vessels are much larger than in the brain, 

 and the interspaces narrower, namely, not more than three or four times 

 wider than the vessels. In the periosteum the meshes are much larger. 

 In the external coat of arteries, the width of the meshes is ten times that 

 of the vessels (Henle). 



It may be held as a general rule, that the more active the functions of 

 an organ are, the more vascular it is. Hence the narrowness of the inter- 

 spaces in all glandular organs, in mucous membranes, and in growing 

 parts; their much greater width in bones, ligaments, and other very 

 tough and comparatively inactive tissues; and the usually complete 

 absence of vessels in cartilage, and such parts as those in which, prob- 

 ably, very little vital change occurs after they are once formed. 



THE VEINS. 



Distribution. The venous system begins in small vessels which are 

 slightly larger than the capillaries from which they spring. These vessels 

 are gathered up into larger and larger trunks until they terminate (as 

 regards the systemic circulation) in the two venae cavae and the coronary 

 veins, which enter the right auricle, and (as regards the pulmonary circu- 

 lation) in four pulmonary veins, which enter the left auricle. The capac- 

 ity of the veins diminishes as they approach the heart; but, as a rule, 



