30 CIRCULATION OF THE BLOOD. 



Describe the structure of the arteries. 



They are surrounded by a dense fibrous coat externally, and 

 lined internally by a smooth serous (endothelial) lining ; between 



FIG. 4. 



Valves of the Heart. 



these is an elastic layer of fibrous tissue, which has interlaced in 

 its structure muscle-cells. Each artery has its own vasa vasorum, 

 or nutrient vessels, and is usually enmeshed in a plexus of sympa- 

 thetic nerves, " vaso-motor nerves." 



Describe the capillaries. 



The capillary blood-vessels are channels of very small but 

 variable size, but usually of about sufficient calibre to just permit 

 the passage of the red and white corpuscles. They are usually 

 composed of a single layer of endothelial cells joined at the edges, 

 though near the arteries and veins there is sometimes an elastic 

 fibrous coat. A sympathetic nerve-plexus surrounds these vessels. 

 The capillaries form a complicated network in the tissues, and the 

 mesh of the net varies in shape and size greatly with the vascu- 

 larity and function of the tissue. 



Describe the characteristics of the veins. 



In structure the veins are similar to the arteries, but much less 



