4 8 4 



TISSUES OF BLOOD-VESSELS. 



[SECT. 213. 



veins, shorter cells of various shapes are seen, which are, probably, 

 a less developed condition of the muscular elements. 



Certain peculiar lamella? found in the tunica intima of the 

 larger vessels, and consisting of fibres with elongated nuclei, and 

 very delicate fibrous membranes, almost homogeneous, are re- 

 garded by Henle and later observers as being a metamorphosed 

 epithelium. I prefer to consider these striped lamella of the 

 inner coat as derived from the same formative-cells as the epithe- 

 lium, which have developed themselves in a peculiar way. 



The epithelium of the vessels (fig. 6, p. 35) appears in two 

 Fig. i9G. forms. In the great veins especially, 



it is of the pavement variety, with poly- 

 gonal cells, tending to be elongated. 

 In most of the arteries, the epithelium 

 is of the fusiform kind, with small 

 pointed cells, o'Oi'" to 0*02'" in length. 

 An epithelium is present in the nor- 

 mal state of every vessel; but almost 

 without exception it may be broken up 

 into its elements without much diffi- 

 culty ; and like other simple epithelia, 

 it experiences no constant detachment 

 and re-formation. In the larger ves- 

 sels, the epithelium is often continuous 

 with the striped lamella before men- 

 tioned, without any line of demarca- 

 tion between them; in the heart, and 

 in the smaller vessels, it is sharply 

 defined from the subjacent tissues. 

 All the larger vessels, down to those of \'" in diameter and 

 under, possess nutrient vessels, the vasa vasorum s. nutrientia, 

 which arise from small neighbouring arteries, and are chiefly 

 distributed in the tunica adventitia, producing here a rich capillary 

 network with roundish meshes, from which small veins arise and 

 accompany their arteries, or, in the case of the vasa vasorum of the 

 veins, pour their blood directly into the veins, from whose walls they 

 proceed. According to the concurrent testimony of many authors, 

 the tunica media of the larger arteries and veins also contains 

 vessels, although in very small numbers, and only in its outer 

 laminae ; whilst, on the other hand, the inner layer of the middle 

 coat, and the whole of the tunica intima, always appear to me to 

 be non-vascular, although even here some observers believe they 



Muscular fibre-cells from human 

 arteries, magnified 350 times. 1. 

 from the popliteal artery ; a. with- 

 out, b. with, acetic acid ; 2. from a 

 branch, half a line in diameter, of 

 the anterior tibial ; o. nucleus of the 

 cells. 



