THE BLOODVESSELS. 683 



lamella, which, when nucleated, often appear to consist 

 of coalesced epithelial cells ; and when homogeneous and without nuclei, 

 to approach pale elastic membranes. In the annular fibrous layer, the 

 largest arteries present, as a new element, peculiar elastic membranes 

 or plates, which, except in the transverse disposition of their fibres, are 

 constituted in all essential respects in the same way as the elastic inner 

 membrane, particularly of the smaller arteries, and sometimes appear 

 as very thick networks of strong elastic fibres, sometimes as true fenes- 

 trated membranes with a less evident fibrous structure. These mem- 

 branes 0-001-0-0012 f 'of a line thick, and the number of which may 

 amount to from 50 to 60, regularly alternate, at distances of 0-003- 

 0-008 of a line, with transverse layers of smooth muscle, which are per- 

 vaded by connective tissue and networks of medium-sized elastic fibres ; 

 nevertheless, they are not to be regarded merely as tubes inserted regu- 

 larly one within the other and having the interspaces occupied by muscles ; 

 for, in the first place, they are connected with each other and with 

 thejiner elastic network pervading the muscular tissue, sometimes more 

 frequently, sometimes more sparingly ; and, 

 in the second place, they are not frequently 

 interrupted in spots, or replaced by com- 

 mon elastic networks. The plates are seen 

 most distinctly and most regularly disposed 

 in the abdominal aorta, the a. innominata, 

 the common carotid, and the smallest of 

 their immediate branches, although these 

 conditions vary considerably in different 

 individuals, so that in the want of very 

 extended researches no general statement with respect to them can -be 

 propounded. 



Another characteristic of the t. media is the slight development of its 

 muscular element. Contractile fibre-cells, it is true, may be found also 

 in the largest arteries throughout all the layers of the middle tunic ; 

 but, in the first place, compared with its other elements, the elastic 

 plates, the connective tissue, and the finer elastic networks, they con- 

 stitute only a very inconsiderable part of that membrane (J-J) ; and, 

 secondly, are so undeveloped in their elements, that it appears very 

 doubtful whether they possess any notable contractile power. For in 

 the aorta and the trunk of the pulmonary artery, the fibre^cells in the 

 inner layers of the t. media are often not longer than 0-01, and 0-004- 

 0-006 of a line broad, and quite flat, so that they are not unlike certain 

 epithelial cells ; at the same time their figure is irregular, rectangular, 

 fusiform or clavate, though they still contain the well-known, rod-like 



Fia. 283. Muscular fibre-cells from the innermost layer of the axillary artery in Man ; 

 magnified 350 diameters : a, without ; 6, with, acetic acid ; a, nucleus of the fibres. 



