THE BLOOD VESSELS. 



291 



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transitionary forms are here met with ; and besides this, we may- 

 notice every degree of transformation of the latter, or of the 

 elastic reticulated membranes into true 

 elastic membranes, which either betray 

 their derivation, in an elastic, more or 

 less indistinct, fibrous network per- 

 vading them, and distant openings, or 

 are transformed, either partially or en- 

 tirely, into perfectly homogeneous plates, 

 exhibiting more or fewer openings (fig. 

 25, vol. I, p . 62) . In the smallest vessels, 

 instead of the elastic elements, there 

 occasionally occur, especially in the 

 t. adventitia, fusiform cells, which can 

 only be regarded as undeveloped forma- 

 tive cells of elastic tissue. Transversely 

 striped muscle is found only at the 



openings of the largest veins into the heart j whilst, on the con- 

 trary, smooth muscles are extensively distributed, especially in 

 vessels of a medium size, and also to some extent in the larger 

 vessels. The elements of these muscles, or the contractile fibre- 

 cells, in the majority of the vessels, present nothing peculiar, 

 except that their length never exceeds O04/", and that they 

 are united, either directly, or with the intervention of con- 

 nective tissue and elastic fibrils, into flattened bundles and 

 muscular membranes, more rarely into reticulated muscular 

 tissue. In the larger arteries, in place of these elements, we 

 find shorter plates resembling epithelium cells, always with 

 elongated nuclei ; and in the smallest arteries and veins, slightly 

 elongated, or even roundish cells ; both which forms may be 

 regarded as being in a less developed condition. 



A peculiar fibrous tissue is contained in the t. intima of the 

 larger vessels, which, since Henle, has generally been regarded 

 as a metamorphosed epithelium. It consists of pale, usually 

 striped, or it may be, homogeneous lamellae, with elongated 

 (long-oval) nuclei disposed parallel to the long axis of the 

 vessel, and which may not unfrequently be broken up into 



Fig. 278. Muscular fibre-cells from the human arteries, x 350 diam. 1, from the 

 popliteal artery: a, without, 6, with, acetic acid. 2, from a twig, %" in diam., of 

 the anterior tibial artery : a, nuclei of the cells. 



