THE CIRCULATION. 



299 



larger branches and trunks, from the periphery toward the centre ; 

 and although the arteries always present inosculation in certain 

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

 secondary feature in both vascular systems. The arteries are essen- 

 tially diverging tubes to distribute the blood from within outward ; the 

 veins are converging channels to transport it from without inward. 



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

 constant and repeated intercommunication ; uniting with each other at 

 such short intervals, as to form an interlacing network, known as the 

 capillary plexus. The vessels of the plexus vary somewhat in size, 

 number, and arrangement in different parts; but in every vascular 

 tissue there are certain spaces or islets, surrounded by the capillaries, 

 and into which they do not penetrate. Such intervascular spaces must 

 therefore obtain their nourishment by exudation and absorption through 

 the capillary walls and the intervening tissue. 



FIG. 77. 



CAPILLARY BLOOD-VESSEL, from the tail of the tadpole ; showing the outlines of its epithelium- 

 like cells, rendered visible by the action of silver nitrate. (Kolliker.) 



The special arrangement of the capillary blood-vessels, and the form 

 and size of the meshes of their network, are, in general, characteristic 

 of each organ and tissue. In the muscles, the intervascular spaces are 

 long parallelograms, corresponding with the muscular fibres ; in the 

 mucous membrane of the stomach, they are hexagonal or irregularly 

 circular, inclosing the orifices of the gastric follicles; in the papillae, 

 of the tongue and skin, and in the placental tufts, the capillaries form 

 twisted vascular loops ; in the glomeruli of the kidneys, convoluted 

 coils ; in the connective tissue, irregularly shaped figures, like those 

 included by the fibrous bundles which they supply. 



