684 SPECIAL HISTOLOGY. 



nuclei. In the outer layers the fibre-cells are more slender and more 

 elongated, up to 0'02 of a line, and at the same time more like the 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 color of that of the largest vessels, but begins to assume 

 a reddish tinge. The t. adventitia of the largest arteries is, relatively 

 and absolutely, thinner than in the smaller, amounting to 0-04-0-02 of a 

 line. Its structure, upon the whole, is the same as in other vessels, 

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

 tinctly defined from the thick elastic elements of the t. media. 



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

 have found in the axillary and popliteal arteries iru^Jan, 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 t. 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 0-02 of a line. 

 Animals the t. adventitia 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 collapse 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 grayish-red, contains far more connective tissue, 

 fewer elastic fibres and muscles, and, what chiefly characterizes it, 

 always presents together with the transverse, longitudinal layers also. 

 It is in general thin, but absolutely stronger in the medium-sized veins 

 than in the larger, and in these also the muscular element is most 

 vigorously developed. The t. adventitia, lastly, is usually the thickest 

 of the three coats, its relative and absolute thickness usually increasing 

 with the size of the vessels. In constitution it precisely resembles that 



