THE CAPILLARIES. 135 



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, com- 

 pared with the long diameter of the meshes, being in the pro- 

 portion of one to eight or ten ; compared with the transverse 

 diameter, in the proportion of one to four or six. In the mu- 

 cous 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 supply of 

 blood. Hence the narrowness of the interspaces in all glandu- 

 lar organs, in mucous membranes, and in growing parts ; their 

 much greater width in bones, ligaments, and other very tough 

 and comparatively inactive tissues ; and the complete absence 

 of vessels in cartilage, the dense tendons of adults, and such 

 parts as those in which, probably, very little organic change 

 occurs after they are once formed. But the general rule must 

 be modified by the consideration, that some organs, such as 

 the brain, though they have small and not very closely ar- 

 ranged capillaries, may receive large supplies of blood by 

 reason of its more rapid movement. When an organ has 

 large arterial trunks and a comparatively small supply of 

 capillaries, the movement of the blood through it will be so 

 quick, that it may, in a given time, receive as much fresh 

 blood as a more vascular part with smaller trunks, though at 

 any given instant the less vascular part will have in it a 

 smaller quantity of blood. 



In the Circulation in the Capillaries, as seen in any trans- 

 parent part of a living adult animal by means of the micro- 

 scope (Fig. 52), the blood flows with a constant equable motion. 

 In very young animals, the motion, though continuous, is ac- 

 celerated at intervals corresponding to the pulse in the larger 

 arteries, and a similar motion of the blood is also seen in the 

 capillaries of adult animals when they are feeble : if their 

 exhaustion is so great that the power of the heart is still more 

 diminished, the red corpuscles are observed to have merely 



