OF THE ARTKRIES. 279 



dinous, proper membrane, &c. is thick amKof a yellowish co- 

 lour, and is formed of nearly circular or annular fibres. This 

 membrane, the thickest of the three, is very apparent in the 

 trunks; it augments proportionally in thickness, as the arte- 

 ries diminish in volume. Its thickness is inconsiderable in 

 the arteries of certain viscera, and particularly in the arteries 

 of the brain. It can be divided into several layers by dissec- 

 tion; this, probably, has led into error those who have ad- 

 mitted more than three arterial membranes. The external 

 fibres are less close, the deeper seated still more so, and thus 

 progressively. These fibres do not all encircle the vessel. 

 The longitudinal and spiral fibres which have been admitted 

 in the middle membrane, do not exist. In the places where 

 the arteries divide, the circular fibres of the trunk separate 

 and form on each side a half ring, and the annular fibres of 

 the branches succeed these latter. The middle membrane is 

 intimately united to the outer one. 



The middle membrane has so great a degree of strength, 

 that when detached from the others it retains its cylindrical 

 form; it is to it that the arteries owe the faculty of remaining 

 open when they are empty. When isolated, it possesses a 

 feeble power of resistance and elasticity, in the longitudinal 

 direction of the arteries, but is very tenacious and elastic in 

 the direction of its fibres, i. e. in that of the circumference of 

 the vessel. The firmness and elasticity of the fibres, which 

 form it, successively diminish from the large towards the 

 small arteries. It has been, by turns, compared and likened 

 to the muscular fibre in general, the muscular fibre of the 

 uterus, and the fibrous or ligamentous tissue; it constitutes a 

 species of elastic tissue, a peculiar tissue, but participating of 

 the characters of the muscular and ligamentous fibres. 



412. The inner membrane of the arteries, which is also 

 named the nervous, arachnoid, common, &c. is the thinnest of 

 the three. It is continued from the ventricles of the heart 

 into the arteries; by it the greater part of the semi-lunar valves 

 of the arteries are formed. In the larger branches, when empty, 

 it presents some longitudinal folds, and small transverse ones 

 in the arteries of the ham and elbow-joint; it is equally wrin- 

 37 



