PHYSIOLOGICAL ANATOMY OF THE ARTEKIES. 245 



distinct division, as regards the middle coat, between the 

 largest arteries and those of medium size. As we recede from 

 the heart, muscular fibres gradually make their appearance 

 between the elastic layers, progressively increasing in quan- 

 tity, while the elastic element is diminished. 



In the smallest arteries the external coat is thin, and dis- 

 appears just before the vessels empty into the capillary sys- 

 tem ; so that the very smallest arterioles have only the inner 

 coat and a layer of muscular fibres. 



The middle coat is composed of circular muscular fibres, 

 without any admixture of elastic elements. In vessels y^ 

 of an inch in diameter, we have two or three layers of fibres; 

 but as we near the capillaries, and as the vessels lose the ex- 

 ternal fibrous coat, these fibres have but a single layer. 1 



The internal coat presents no difference from the coat in 

 other vessels, with the exception that the epithelium is less 

 distinctly marked, and is lost near the capillaries ; the mem- 

 brane being studded with longitudinal oval nuclei. 



A tolerably rich plexus of vessels is found in the external 

 coats of the arteries. These are called the vasa vasorum, and 

 come from the adjacent arterioles, having no direct connec- 

 tion with the vessel on which they are distributed. A few 

 vessels penetrate the external layers of the middle coat, but 

 none are ever found in the internal coat. 



Nervous filaments, principally from the sympathetic sys- 

 tem, accompany the arteries, in all probability, to their re- 

 motest ramifications ; though they have not yet been demon- 

 strated in the smallest arterioles. These are not distributed 

 in the walls of the large vessels, but rather follow them in 

 their course ; their filaments of distribution being found in 

 those vessels in which the muscular element of the middle 

 coat predominates. When we come to treat of the physiology 

 of the organic system of nerves, we shall see that the " vaso- 



i 



1 The structure of the smallest arteries can be beautifully exhibited in fresh 

 microscopic preparations of the pia mater, in which the various points to which 

 we have alluded can be easily studied. 



