CAPILLARIES. 287 



of a delicate elastic homogeneous membrane lined with a single 

 layer of endothelial cells. The intima may be said to be contin- 

 uous throughout all the vessels and the heart cavities. 



It is thus seen that the large arteries have extremely elastic 

 and firm walls, capable of sustaining considerable pressure. The 

 smaller the arteries become in calibre, the more the general 

 property of elasticity and resiliency is reinforced by that of vital 

 contractility, due to the greater relative number of muscle cells 

 contained in the middle coat. 



FIG. 126. 



Capillary network of a Lobule of the Liver. 



The frequently-branching arterioles finally terminate in the 

 capillaries, in which distinct branches can no longer be recog- 

 nized, but these thin canals are united and interwoven into a net- 

 work of blood channels, the meshes of which are all made up of 

 vessels having about the same calibre. They communicate indefi- 

 nitely with the capillary meshworks of the neighboring arteri- 

 oles, so that any given capillary area appears to be one continuous 

 network of tubules, connected here and there with distinct ar- 

 terioles, and thus is fed with blood from several different sources. 



