THE BLOOD-VESSELS. 301 



well-marked muscular fibre-cells of other organs, though even 

 there retaining somewhat of a rigid and peculiar aspect. In the 

 carotids, subclavian, axillary and iliac arteries, the contractile 

 elements have become more developed, whence also the t. media 

 of those arteries does not present the pure yellow colour of that 

 of the largest vessels, but begins to assume a reddish tinge. The 

 /. adventitia of the largest arteries is, relatively and absolutely, 

 thinner than in the smaller, amounting to 0*04 — 0*02'". Its 

 structure, upon the whole, is the same as in other vessels, 

 although the elastic inner layer is less developed, and also very 

 indistinctly defined from the thick elastic elements of the 

 t. media. 



[The t. intima of certain arteries also contains smooth muscles, 

 asl have found in the axillary and popliteal arteries in Man, and as 

 has been lately demonstrated, particularly in the visceral arteries 

 of the Mammalia. In the largest arteries in Man this tunic is 

 very frequently thickened, in which condition a vast increase 

 of the striped lamellae in particular takes place. In the /. media 

 of no artery is the muscular element wholly wanting, and Henle 

 erroneously adduces the arteries of the retina in opposition to 

 this, for it occurs in those arteries even in branches of 0*03'", 

 and is not absent in any above 002'". In Animals the t. adven- 

 titia of the large arteries contains muscles, but not in Man.] 



§ 216. 



Veins. — The veins also admit of being divided into three 

 groups, small, medium-sized, and large, which, however, are not 

 so abruptly defined as is the case with the arteries. The veins, 

 without exception, have thinner walls than the arteries, which 

 depends just as much upon their containing a less considerable 

 quantity of contractile, as upon a more sparing development 

 of the elastic elements; whence also the venous walls col- 

 lapse more readily, and are less contractile. The t. intima, in 

 the larger veins is frequently not thicker than it is in those of 

 medium size; it is less developed than in the arteries, but in other 

 respects, of the same structure. The t. media, which is never 

 yellow, but usually greyish-red, contains far more connective 

 tissue, fewer elastic fibres and muscles, and, what chiefly charac- 

 terises it, always presents together with the transverse, longitudinal 



