300 SPECIAL HISTOLOGY. 



membranes 0-001 — 0-0012"' thick, and the number of which 

 may amount to from 50 to 60, regularly alternate, at distances 

 of 0003 — O'OOS'", with transverse layers of smooth muscle, 

 which are pervaded by connective tissue and networks of 

 medium-sized elastic fibres; nevertheless, they are not to be 

 regarded merely as tubes inserted regularly one within the other 

 and having the interspaces occupied by muscles ; for, in the 

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

 finer elastic network pervading the muscular tissue, sometimes 

 more frequently, sometimes more sparingly; and, in the second 

 place, they are not unfrequently interrupted in spots, or re- 

 placed by common elastic networks. The plates are seen most 

 Fig. 283. distinctly and most regularly disposed 



» A in the abdominal aorta, the a. innomi- 



u a J fl nata, the common carotid and the 



M % I \ smallest of their immediate branches, 



£1 fc*k /©J although these conditions vary con- 

 I H 1 siderably in different individuals, so that 



M in the want of very extended researches 



u W y no general statement with respect to 



* '* them can be propounded. 



Another characteristic of the t. media is the slight develop- 

 ment 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 constitute only a very 

 inconsiderable part of that membrane (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 001'", and 0*004 — 0-006'" 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 nuclei. In 

 the outer layers the fibre-cells are more slender and more 

 elongated, up to 0-02'", and at the same time more like the 



Fig. 283. Muscular fibre-cells from the innermost layer of the axillary artery in 

 Man, x 350 diam.: a, without; b t with, acetic acid; a, nucleus of the fibres. 



