CIRCULATION OF THE BLOOD. 



537 



margins. These tubules divide and reunite to form net-works which 

 differ in shape and arrangement in different organs and tissues. Their 

 diameter varies considerably, but is as a rule about that of the single red 

 blood-corpuscle. In the lungs and brain, the capillaries are smaller than 

 those of the skin ; in the retina and muscle, are smaller than in bone, 



FIG. 225. SMALL PORTION OF FROG'S LIVER, VERY HIGHLY MAGNIFIED, 

 AFTER HUXLEY. (Yeo.) 



A, wall of capillary vessels ; B, tissue lying between the capillaries ; C, epithelial cells of the skin, 

 only shown in part of specimen where the surface is in focus ; D, nuclei of the epithelial cells ; E, pigment- 

 cells, contracted ; F, red blood-corpuscles : G, H, red corpuscles squeezing their way through a narrow 

 capillary, showing their elasticity ; I, white blood-cells. 



marrow, river, and the choroid tunic of the eye. In all probability the 

 walls of the capillaries are not contractile, although they are capable of 

 undergoing variations in diameter, this change in all probability being 

 of a passive nature, owing to similar phenomena taking place in the 

 small arteries and veins. Thus, the pulse which is evident in inflamed 



