BLOOD-VESSELS. 1 25 



The walls of the larger arteries and veins contain 

 blood and lymph-vessels, called vasa vasorum. 



The valves of the veins consist of bundles of fibril- 

 lar connective tissue arranged to form a membran- 

 ous projection from the walls of the vessels the 

 bundles being arranged, in general, in a direction 

 parallel with the free edge of the valve. They con- 

 tain also a net-work of elastic fibres, which, at that 

 surface of the valve which is exposed to the blood- 

 current, form a dense layer like the intermediary 

 layer of the vein-wall itself. The whole free surface 

 is covered with endothelium like that lining the 

 general surface of the vessel. 



ENDOCARDIUM AND VALVES OF THE HEART. 



The endocardium^ which differs somewhat in thick- 

 ness and structure in different parts of the heart, 

 consists, in general, of a membranous expansion of 

 fibrillar connective tissue, with elastic fibres and 

 smooth muscle-elements, which lines the cavities of 

 the heart, and is covered on its free surface with a 

 layer of endothelial cells. 



If we examine sections made perpendicular to 

 the surface of the endocardium, we find that just 

 beneath the endothelium is a layer of fibrillar con- 

 nective tissue, with flattened cells, reinforced by a 

 net-work of elastic fibres, which become coarser and 

 more abundant in that portion of the layer lying far- 

 thest from the heart-cavities. In this layer are smooth 



