152 THE CIRCULATION OF THE BLOOD 



The walls of the capillaries are composed of a single layer of elongated 

 or radiate, flattened and nucleated endothelial cells, so joined and dove- 

 tailed together as to form a continuous transparent membrane, figure 147. 

 Outside these cells in the larger capillaries there is a structureless supporting 

 membrane on the inner surface of which they form a lining. 



The diameter of the capillary vessels varies somewhat in the different 

 textures of the body, the most common size being about 12 micromillimeters, 

 -joinr f an mcn - Among the smallest may be mentioned those of the 

 brain and of the follicles of the mucous membrane of the intestines; among 

 the largest, those of the skin, and especially those of the medulla of the bones. 



The form of the capillary network differs in the different organs of the 

 body, but is usually adjusted to the structural arrangement of the cells of 

 any given organ. 



The capillary network is closest in the lungs and in the choroid coat of 

 the eye. In the human liver the interspaces are of the same size, or even 

 smaller than the capillary vessels themselves. In the human lung the spaces 

 are smaller than the vessels; in the human kidney, and in the kidney of the 

 dog, the diameter of the injected capillaries, compared with that of the inter- 

 spaces, is in the proportion of one to four, or of one to three. The brain 

 receives a very large quantity of blood; but its capillaries are very minute 

 and are less numerous than in some other parts. In the mucous mem- 

 branes, 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 and in the external coat of arteries the meshes are much larger, 

 their width being about ten times that of the vessels. It may be held as a 

 general rule that the more active the functions of an organ are, the more 

 vascular it is. 



The Veins. 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 in the two 

 venae cavse and the coronary vein which enter the right auricle, and in' four 

 pulmonary veins which enter the left auricle. The total capacity of the 

 veins diminishes as they approach the heart; but their capacity exceeds by 

 two or three times that of their corresponding arteries. The pulmonary 

 veins, however, are an exception to this rule. The veins are found after 

 death more or less collapsed, and often contain blood. They are usually 

 distributed in a superficial and a deep set which communicate frequently 

 in their course. 



The coats of veins bear a general resemblance to those of arteries, figure 

 150. Thus, they possess outer, middle, and inner coats. The outer coat is 

 constructed of areolar tissue like that of the arteries, but is thicker. In some 

 veins it contains muscular cells arranged longitudinally. The middle coat 



