292 SPECIAL HISTOLOGY. 



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 trans- 

 formed into extremely delicate fibrous membranes, like the 

 closest and finest elastic networks. The similarity of these layers, 

 which I shall term the striped lamella of the t. intima, or rather 

 of the fibrous cells of which they are fundamentally consti- 

 tuted, 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 shew that the true epithelial cells 

 and the striped lamella stand in any genetic connexion of such 

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

 and the innermost layer in the vessel, being afterwards succes- 

 sively 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 development, the one set are trans- 

 formed 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. 1 4, p. 55, vol. I) presents 

 two forms : firstly, especially in the great veins, it appears under 

 that of a tesselated epithelium , with polygonal, mostly somewhat 

 elongated cells ; and secondly, as in most of the arteries, as a 

 fusiform epithelium, with acuminated, slender cells, 001 — 0'02'" 

 long. Normally, it exists in all vessels, may almost always be 

 pretty readily broken up into its elements, and like other simple 

 epithelia, is not subject to any constant detachment and restora- 

 tion. With Remak, we might describe the epithelium as the 

 cellular membrane of the vessels, since it differs from other 

 epithelia in this respect, that in the large vessels, it is often con- 

 tinuous with the striped lamellae, without any line of demar- 

 cation, so that frequently it cannot be said where the one 

 ceases and the others commence; but I should myself rather 

 be inclined to retain the old name, both because the inner- 

 most cellular layer of the vessels presents in all respects the 

 relations of a simple epithelium and is, in many situations 

 (heart, smaller vessels) abruptly defined from the deeper tissues. 

 Even the circumstance particularly adduced by Remak, that 

 the vascular epithelium does not proceed from the embryonic 

 epithelial membrane, is not with me decisive as to the propriety 



