168 



BLOOD-VESSELS. 



works of vaiying fineness. This layer is said to be most constant in 

 the larger arteries : it exists however in the medium-sized ones, and is 

 to be looked upon as of considerable pathological importance, as being 

 that in which, under certain conditions, cell proliferation is most apt 

 to occur. 3. Elastic layers (fig. 107, b). These form the chief sub- 

 stance of the inner coat. The elastic tissue commonly forms longitu- 

 dinal networks of fibres (fig. 110), which consist of several layers of 

 different degrees of closeness. Not uncommonly some of these layers 

 take on a membranous character, in which case they form the 

 "perforated" or "fenestrated" membrane of Henle. This consists of 

 a thin and brittle transparent film, and may exist in one or several 

 layers ; and in that case it may be stripped olf in small shreds, which 

 have a remarkable tendency to curl in at their upper and lower borders, 

 and roll themselves up as represented in fig. HI. The films of 



Fig. 110. 



Fig. 111. 



mm 



Fig. 110. —Elastic Network of Akterv. 350 Diameters (Kolliker). 



Fig. 111. — Portion of Fenestrated Membrane from the Crural Artery, magnified 

 200 Diameters (Henle). 



o, h, c, perforations. 



membrane are marked by very fine pale streaks, following principally 

 a longitudinal direction, and joining each other obliquely in a sort of 

 network. Henle considers these lines to be reticulating fibres formed 

 upon the membranous layer. This membrane is further remarkable 

 by being perforated with numerous round, oval, or irregularly shaped 

 apertures of different sizes. In some parts of the arteries the per- 

 forated membrane is very thin, and therefore difficult to strip off; in 

 other situations it is of considerable thickness, consisting of several 

 layers ; in which case it tends in the outer layers to lose its membra- 

 nous character : indeed it must be borne in mind that every transition 

 is met with between the fenestrated membrane, as above described, 

 and the longitudinal elastic netAvorks before mentioned. 



The inner coat may thus be said to be formed of a layer of flattened 

 epithelioid cells ; a layer of delicate connective tissue with branched 

 cells ; and clastic layers : the latter consisting of elastic tissue under 

 two principal forms, namely, the longitudinal elastic networks and 

 the fenestrated membrane ; and these two forms may coexist in equal 



