320 



THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



developed in arteries of the middle and smaller size, and may be 

 most easily separated from the fibrous tissue in arteries which 

 have undergone slight decomposition. They are then seen to 

 consist of long fusiform fibres of much delicacy, with a minute 

 nucleus in most of them. In the mass they have the appearance 

 represented in fig. 191 . 



The external layers of the circular fibrous coat become gradually 

 more and more like the ordinary yellow elastic tissue, the penniform 

 and the muscular fibres cease, and the true yellow elastic branching 

 fibre becomes continuous with that which is found in sparing quan- 

 tity in the external coat. 



Epithelial Layer. The interior of the arteries 

 is covered by a single layer of delicate oval epi- 

 thelial particles, which separate very soon after 

 death, and must, therefore, be sought for in quite 

 recent subjects. They may be .best seen by 

 scraping the inner surface of the artery. The long 

 axis of each of these particles is parallel to that of 

 the vessel. They are pointed, or, as it were, 

 drawn out at their extremities ; and, according to 

 Henle, they are sometimes elongated into fusi- 

 form fibres. They are remarkable for the large 

 size and the distinctness of their nuclei which are 

 often visible when the cell-wall cannot be detected. 

 unstriped muscular The particles seem to rest immediately upon the 



fibres from the aorta , i /> .-, i . ji - i i 



of the home, Mag- innermost layer of the longitudinal fibrous tunic, 

 which bears the relation of a basement mem- 

 brane to them ; in this, when detached, minute apertures appear, 

 constituting the fenestrated membrane of Henle. It is possible, 

 as suggested by Henle, that this membrane arises from the 

 transformation of the epithelium, which is ever drawing the ma- 

 terials of its formation and nutrition from the blood contained in 

 the artery. Thus it is not improbable that the innermost layers 

 of the arterial wall, at least, are nourished from the blood flowing 

 through the artery, and not from the blood of the vasa vasorum, 

 which does not seem to penetrate to them. And this view is sup- 

 ported by observing that these innermost layers of the artery, i. e. 

 the longitudinal fibrous tunic, are the seat of the atheromatous 

 deposits which are so common in peculiar diatheses, or at an 

 advanced period of life; these deposits being doubtless derived 

 from the blood which traverses the artery. 



From the preceding description, it would appear that the fol- 



