CAPILLARIES. 281 



tissue gradually becoming less abundant and the muscle elements 

 more important in proportion as the capillaries are approached. 

 3. The internal lining (tunica intima') of the arteries is composed 

 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 



FIG. 126. 



Capillary network of a Lobule of the Liver. 



smaller the arteries become in calibre, the more the general prop- 

 erty of elasticity and resiliency is reinforced by that of vital con- 

 tractility, due to the greater relative number of muscle cells con- 

 tained in the middle coat. 



The frequently-branching arterioles finally terminate in the 

 capillaries, in which distinct branches can no longer be recognized, 

 but these thin canals are united and interwoven into a network 

 of blood channels, the meshes of which are all made up of ves- 

 sels having about the same calibre. They communicate indefi- 



24 



