210 STRUCTURE OF THE ARTERIES. 



of the artery which is enlarged, but a portion of the vessel is 

 elongated; and this elongation, by producing a curvature of 

 it, renders its motion more visible. 



In the aorta, and probably in its large branches Elasticity 

 seems to be the principal cause of the continuance of the motion 

 which is originally given to the blood by the heart. But there 

 are many circumstances connected with the smaller vessels, 

 which evince that they exert, a power which is very different 

 indeed from elasticity. Thus, the application of local stimu- 

 lants or rubefacients, and of heat, is followed by an increase of 

 action in the arteries of the parts to which they are applied. 

 Neither of these causes could produce their effect by the influ- 

 ence of elasticity : but the effect of these and other similar 

 causes is uniformly produced ; a power of independent motion, 

 or Irritability, is thus proved to exist in these vessels, and 

 seems essentially necessary to the circulation of the blood. 



The Structure of the Arteries 



Is, therefore, a subject of importance, and has received a 

 considerable degree of attention from anatomists. 



They are composed of a dense elastic substance, of a whitish 

 color. Their external surface is rough, and intimately con- 

 nected with the cellular membrane, which every where surrounds 

 it in varying quantities. Internally, they are lined with a 

 thin membrane, which is very smooth and flexible, and is also 

 very elastic. The substance which composes the artery, and 

 is situated between the cellular investment and the internal 

 membrane, consists of fibres, which are nearly, though not 

 completely circular, but so arranged as to constitute a cylinder. 

 These fibres may be separated from each other so as to form 

 lamina, which have been considered as different coats of the 

 arteries ; but there is no arrangement of them which composes 

 regular distinct strata. The coats of arteries may, therefore, 

 be separated into a greater or smaller number of lamina, accord- 

 ing to the thickness of these lamina. 



The fibres which compose these lamina appear to be united 

 to each other in a way which readily allows of their separation, 



