310 CIRCULATORY SYSTEM. 



The tunica adventitia consists of bundles of white connective 

 tissue and elastic fibers, and gives to the artery its strength. This 

 coat merges with the sheath of the artery, which is composed of 

 fibro-areolar tissue, and in this are the blood-vessels which supply 

 the arteries, the vasa vasorum. 



THE CAPILLARIES, 



The capillaries are minute vessels, having in general a diameter 

 of 12 f2j though this differs very considerably in the different 

 organs of the body. They are smallest in the brain and intestinal 

 mucous membrane, and largest in the skin and bone-marrow, 

 where they have a diameter of about 20 //. 



Their arrangement is also subject to great variation ; thus in the 

 lungs and mucous membranes they form rounded meshes, while in 

 muscles and nerves the form of the mesh is elongated. In some 

 organs they are very close together, as in the lungs, while else- 

 where they are separated to a considerable extent, as, for instance, 

 in the external coats of arteries. In general, where an organ is 

 active, as is the kidney, there the number of capillaries is the 



FIG. 162. Endothelial cells of capillary (a) and precapillary (b) from the mesentery 

 of a rabbit; stained in silver nitrate (Huber). 



greatest ; and where it is inactive, as is the case with bone, the 

 capillaries are correspondingly lacking. 



The walls of the capillaries consist of endothelial cells joined 

 edge to edge by a cement-material. 



From a physiologic standpoint this portion of the circulatory 

 apparatus is the most important, as all the changes between the 

 blood and the tissues take place while the blood is passing through 

 the capillaries. 



THE VEINS. 



The structure of the veins is in many respects similar to that 

 of the arteries. They are likewise composed of three coats, but 



