676 SPECIAL HISTOLOGY. 



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 formative cells of elastic tissue. Transversely 

 striped muscle is found only at the openings of the largest veins into 

 the heart ; whilst, on the contrary, smooth muscles are extensively dis- 

 tributed, 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 0'04 of a line, and that they are 

 united, either directly, or with the intervention of connective 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 meta- 

 morphosed epithelium. It consists of pale, usually striped, or it may 

 I be, homogeneous lamella^ with elongated (long-oval) nuclei disposed 

 parallel to the long axis of the vessel, and which may not unfrequently 

 be broken up into slender nucleated fusiform fibres, similar to certain 

 epithelial cells, or at all events, into fibres ; but at other times are more 

 homogeneous and without nuclei, or else appear to be transformed into 

 extremely delicate fibrous membranes, like the closest and finest elastic 

 networks. The similarity of these layers, which I shall term the 

 striped lamellce of the t. intima, or rather of the fibrous cells of which 

 they arc fniiclamcntally constituted, to the epithelium of the vessels, is 

 nevertheless insufficient to justify the supposition of their being derived 

 from the latter, since there are no facts to show that the true epithelial 

 cells and the striped lamellce stand in any genetic connection of such a 

 kind, as that the latter were at one time a true epithelium and the 

 innermost layer in the vessel, being afterwards successively pushed 

 back, and their elements made to coalesce ; on the contrary, it seems 

 to be allowable to regard the epithelial cells and the formative cells of 

 these layers as originally equivalent,, and that in the course of develop- 

 ment, the one set are transformed in one direction, and the other in 

 another, and, in this way, ultimately become tissues of more or less 

 different kinds. 



The epithelium of the vessels (Fig. 14, p. 76) presents two forms : 

 firstly, especially in the great veins, it appears under that of a tessel- 

 lated epithelium, with polygonal, mostly somewhat elongated cells ; and 

 secondly, as in most of the arteries, as a fusiform epithelium, with 

 acuminated, slender cells, 0-01-0-02 of a line long. Normally, it 



