THE CAPILLARIES. 171 



arrangement, a single capillary projecting from the com- 

 mon network into some prominent organ, and returning 

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

 tongue and skin. Whatever be the form of the capillary 

 network in any tissue or organ, it is, as a rule, found to 

 prevail in the corresponding parts of all animals. 



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 net- 

 work 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 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 cellular 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 ; that 

 is, the closer is its capillary network and the larger its 



