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THE CIRCULATION OF THE BLOOD 



the transmission of the blood from the minute branches of the arteries to the 

 minute veins is effected through a network of capillaries. They may be 

 seen in all minutely injected preparations. 



The point at which the arteries terminate and the capillaries commence 

 cannot be exactly denned, for the transmission is gradual. The capillaries 

 maintain essentially the same diameter throughout. The meshes of the 



FIG. 146. Vein and Capillaries. Silver-nitrate and hematoxylin stain, to show outlines 

 of endothelial cells and their nuclei. (Bailey.) 



network that they compose are more uniform in shape and size than those 

 formed by the anastomoses of the minute arteries and veins. 



The walls of the capillaries are composed of a single layer of elongated 

 or radiate, flattened and nucleated endothelial cells, so joined and dove- 

 tailed together as to form a continuous transparent membrane, figure 146. 



FIG. 147. Network of Capillary Vessels of the Air Cells of the Horse's Lung 

 ^Magnified, a, a, Capillaries proceeding from b, b, terminal branches of the pulmonary 

 artery. (Frey.) 



Outside these cells in the larger capillaries there is a structureless supporting 

 membrane on the inner surface of which they form a lining. 



The diameter of the capillary vessels varies somewhat in the different 

 textures of the body, the most common size being about 1 2 micro millimeters, 

 "giro's" of an inch. Among the smallest may be mentioned those of the 



