172 THE CIRCULATION. 



supply of blood. Hence the narrowness of the interspaces 

 in all glandular organs, in mucous membranes, and in 

 growing parts ; their much greater width in bones, liga- 

 ments, 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 

 arranged 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 mi- 

 croscope (fig. 52), the blood 

 flows with a constant equable 

 motion. In very young ani- 

 mals, the motion, though 

 continuous, is accelerated 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 the periodic motion, and to remain stationary in the 



* Fig. 52. Capillaries in the web of the frog's foot magnified. 



