THE CIRCULATION OF THE BLOOD. 



185 



The capillary network is closest in the lungs and in the choroid coat 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 capillaries, 

 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 its capillaries are very minute, and are less numerous than 

 in some other parts. In the mucous membranes for example in the 

 conjunctiva and in the cutis vera, the capillary vessels are much larger 

 thnn in the brain, and the interspaces narrower, namely, not more 



Fig. 158. 



Fig. 159- 



Fig. 158. Network of capillary vessels of the air-cells of the horse's lung magnified, a, a, 

 Capillaries proceeding f rom 6, b, terminal branches of the pulmonary artery. (Frey.) 



Fig. 159. Injected capillary vessels of muscle seen with a low magnifying power. (Sharpey.) 



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. 



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 

 interspaces in all glandular organs, in mucous membranes, and in grow- 

 ing 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, proba- 

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



