THE CAPILLARY CIRCULATION. 



345 



the larger capillaries, of 8 to 13 mmm. in diameter, where the calibre of 

 the vessel is surrounded by three or four cells placed side by side, they 

 are shorter and wider in form, like those of ordinary pavement epithe- 

 lium. The arrangement of these microscopic forms in the wall of the 



Fig. 122. 



CAPILLARY BLOODVESSEL, from the tail of the tadpole; showing the outlines of 

 its epithelium-like cells, rendered visible by the action of silver nitrate. (KOlliker.) 



capillary bloodvessels has given rise to the opinion, entertained by some 

 histologists, that the vascular system is to be regarded as a series of 

 intercellular canals, provided, in different regions, with varying addi- 

 tional layers of muscular, elastic, and connective tissue. 



The capillary bloodvessels ure further distinguished from both arteries 

 and veins by their frequent inosculation. The arteries constantly divide 

 and subdivide, as they pass from within outward, while the veins as 

 constantly unite with each other, to form larger and less numerous 

 branches and trunks, as they converge from the periphery toward the 

 centre ; and although the arteries always present inosculations in certain 

 regions, and the veins more frequently still, this feature is, nevertheless, 

 a secondary or incidental one in both vascular systems. The arteries 

 are essentially diverging tubes to distribute the blood from within out- 

 ward ; the veins are converging channels to collect and transport it from 

 without inward. 



The capillaries, on the other hand, are mainly characterized by their 

 constant and repeated intercommunication. They are vascular canals 

 which penetrate the solid organs and tissues, uniting with each other at 

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