HANDBOOK OF PHYSIOLOGY. 



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, } ? et 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 kid- 

 ney, and in the kidney of a 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 the capillaries in which the blood is distributed through its 

 substance are very minute, and less numerous than in some other parts. 

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

 diameter of the meshes, being in the proportion of one to eight or ten; 

 compared 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 

 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, prob- 

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



IV. The Veins. 



Distribution. The venous system begins in small vessels which are 

 slightly larger than the capillaries from which they spring. These ves- 

 sels are gathered up into larger and larger trunks until they terminate 

 (as regards the systemic circulation) in the two venas cavas and the coro- 

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

 circulation) in four pulmonary veins, which enter the left auricle. The 

 total capacity of the veins diminishes as they approach the heart ; but, 

 as a rule, their capacity exceeds by twice or three times that of their 

 corresponding arteries. The pulmonary veins, however, are an excep- 

 tion to this rule, as they do not exceed in capacity the pulmonary arte- 

 ries. The veins are found after death as a rule to be more or less col- 



