PHYSIOLOGICAL ANATOMY OF THE ARTERIES. 



63 



at right angles to the course of the vessels, nearly all of the arteries in the 

 human subject are provided with longitudinal and oblique muscular fasciculi, 



FIG. 23. Small artery from the mesentery of the frog, showing endothelium and circular muscular 

 fibres; magnified 500 diameters (from a photograph taken at the United States Army Medical 

 Museum). 



which are sometimes external, sometimes internal and sometimes on both 

 sides of the circular layers. 



The middle coat is composed of circular muscular fibres, without any ad- 

 mixture of elastic elements. In vessels -^ of an inch (254 p) in diameter, 

 there are two or three layers of fibres ; but nearer the capillaries and as the 

 vessels lose the external fibrous coat, these fibres exist in a single layer. 



The internal coat presents no essential difference from the coat in other 

 vessels, with the exception that the endothelium is rather less distinctly 

 marked. 



A tolerably rich plexus of vessels is found in the external coat of 

 the arteries. These are called vasa vasorum and come from the adjacent 

 arterioles, generally having no direct connection with the vessel on which 

 they are distributed. A few vessels penetrate the external layers of the mid- 

 dle coat, but none are ever found in the internal' coat. 



Nervous filaments accompany the arteries, in all probability, to their re- 

 motest ramifications. These are not distributed in the walls of the large 

 vessels, but follow them in their course, their filaments of distribution being 



