330 



ARTERIES, ETC. 



ARTERIES, CAPILLARIES, AA T D VEINS. 

 When the ventricles of the heart close on the blood 

 which they contain, the fluid is pushed into the arte- 

 rial system. The arteries are elastic tubes, with walls of 

 sufficient firmness and thickness to prevent collapse when 

 the blood flows out of them. As the vessels ramify they 

 diminish in size, thickness, and elasticity, acquiring, on the 

 other hand, greater contractile power. The branching occurs 

 in a variety of ways: frequently by bifurcation; at other 

 times a main trunk gives off small branches at acute or right 

 angles, and, in some instances, vessels split up suddenly into 

 several or many small branches. This is seen with the me- 

 senteric arteries. Without entering into the minute anatomy 

 of the blood-vessels, I may mention that the elasticity of the 

 larger arteries is due to elastic tissue, (see Figs. 113, 111); 



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Fig.113. (K6LLIKER.) 



Elastic network from 

 the tunica media of the 

 pulmonary artery of the 

 horse, with holes in the 

 fibres. Magnified 350 

 times. 



Fig.ll4.KoLLTKER.) 



Elastic membrane from 

 tunica media of the ca- 

 rotid of the horse. Mag- 

 nified 350 times. 



whereas their contractility is due to plain contractile fibres 

 described by Kolliker. 



The capillaries are composed of exceedingly delicate tran- 

 sparent membrane, on which nuclei are observed, and, as 



