TEXTURE OF THE ARTERIES. 165 



tainty, because of the close succession of branches which they 

 send off. The vertebral arteries afford a striking example of 

 the same. j 



Arteries have within themselves a power of increase con- 

 nected with the exigencies of the part to which they go: thus, 

 the uterine arteries increase much in their capacity during preg- 

 nancy, while the hypogastric, from which they are derived, 

 augment inconsiderably, and the primitive iliacs not in an ap- 

 preciable manner. In animals of the deer kind, whose horns 

 are deciduous, the same augmentation of arterial trunks occurs 

 while the horn is growing. Tumours are supplied in the same 

 way. But in all these cases, after the exigency is passed, the 

 vessels diminish to their primitive size. 



With the exception of the semi-lunar valves at the orifice of 

 the pulmonary artery and of the aorta, there are no others in 

 the whole arterial system. These valves permit the blood to 

 pass in the direction of the circulation, but not backwards, as 

 they are closed immediately upon the cessation of the contrac- 

 tion of the ventricles. The tricuspid valve of the heart, and 

 the semi-lunars of the pulmonary artery, are naturally not so per- 

 fect in their closure as those on the other side of the heart, but 

 permit a small quantity of blood to retrograde.* As life ad- 

 vances, the valves of the aorta are much disposed to ossifica- 

 tions and derangements of different kinds, which render them 

 much less perfect than those of the pulmonary artery. 



CHAPTER II. 



OF THE TEXTURE OF THE ARTERIES. 



THE arteries are composed of three coats; an external, a mid- 

 dle, and an internal. 



The External Coat, also called Cellular, is, in fact, condensed 

 cellular substance formed into a cylinder. Its fibres run in eve- 

 ry direction, so as to be perfectly interwoven with one another. 

 The exterior periphery of this coat is continued into the adja- 



* Hunter, loc. cit. 



