SECT. 2I4.] 



LARGEST ARTERIES. 



49I 



dense networks of clastic fibres, sometimes truly fenestrated mem- 

 branes with indistinct fibrillation. Tlicse membranes, o'ooi'" to 

 o - oo 1 2'" in thickness, may amount to fifty or sixty in number, and 

 alternate regularly at distances of o'oo3'" to 0008'", with layers 

 of smooth muscular fibres running transversely; among which is 

 sccu connective tissue, with networks of elastic fibres of medium 

 size. Tlicse clastic membranes are not, however, to be regarded as 

 tubes regularly encased in muscular fibres, which fill their inter- 

 spaces, and thus isolate them from each other. They are rather 

 to be considered as connected with each other, and with the fine 

 elastic network, which, in variable amount, traverses the mus- 

 cular fibres ; and they are not unfrequently interrupted at dif- 

 ferent places, or represented by ordinary elastic networks. The 

 elastic plates are seen to most advantage and most regularly 

 arranged in the abdominal aorta, the innominate artery, the 

 common carotid, and in the smaller arteries of the class wc are 

 now considering ; their disposition, however, varies so much in 

 different individuals, that without being in possession of very ex- 

 tended investigations, it is scarcely possible to state anything 

 generally applicable. — The tunica media Fig. 201. 



of the largest arteries is further distin- 

 guished by the slight development of 

 its muscular fibres. Contractile fibre-cells 

 are, indeed, to be found in the largest 



\ 



Muscular fibre-cells, from the 

 innermost layers of the human 

 axillary artery; magnified 350 

 times. " a. Without, 6. with acetic 

 acid ; a. nucleus of the fibres. 



arteries throughout all the laminse of the 

 tunica media ; but, compared with the 

 other elements of this coat, viz., the 

 elastic plates, the connective tissue, and 

 the finer elastic networks, the muscular 

 element constitutes only an unimportant 

 part (uot more than one-third or one- 

 fourth) ; and again, the cells of the muscu- 

 lar tissue are so undeveloped, that it appears very doubtful whether 

 they possess any power of contraction worthy of mention. In the 

 aorta and the trunk of the pulmonary artery especially, the fibre- 

 cells are often found in the inner layers of the tunica media not 

 longer than o - oi'", and o'004'" to croo6"' broad, quite fiat (so that 

 they are not unlike to certain epithelial cells), irregular in form, 

 rectangular, and fusiform or club-shaped, but still they preserve 

 the well-known cylindrical nuclei. In the outer lamina? of these 

 vessels, the fibre-cells become narrower and longer, up to 0-02'", 

 and, at the same time, more similar to the fully-developed mus- 



