STRUCTURE OF ARTERIES. 



363 



very fine elastic fibres, which are in continuity with the larger elastic fibres of the 

 next layer, occur in it in the aorta. 



.3. Elastic layer (fig. 417, b). The chief substance of the inner coat is formed 

 by elastic tissue, which occurs as longitudinal networks of fibres (fig. 420), consist- 

 ing of one or more layers of different degrees of closeness. Not uncommonly some 

 of these (or one in particular) take on a membranous character, in which case the 

 " perforated " or " fenestrated " membrane of Henle is formed. This consists of a 

 thin and brittle transparent film of elastic tissue. It can be stripped off in small 

 shreds, which have a remarkable tendency to curl in at their borders, and roll them- 

 selves up as represented in fig. 421. The films of membrane are marked by fine 

 lines, following principally a longitudinal direction, and joining each other obliquely 

 in a sort of network. These lines are reticulating fibres formed upon the mem- 

 branous layer and continuous with the reticulating elastic fibres which pervade the 

 muscular coat on the one side and with those which extend into the subepithelial 



Fig. 420. ELASTIC NETWORK OP ARTERY. (Toldt.) 



Fig. 421. PORTION OF FENESTRATED MEMBRANE PROM THE FEMORAL ARTERY, MAGNIFIED 200 



DIAMETERS. (Henle.) 

 a, b, c, perforations. 



layer on the other. The 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 perforated 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 membranous character : 

 indeed it must be borne in mind that every transition is met with between the fene- 

 strated membranes, and the longitudinal elastic network. 



The inner coat in its most developed condition may thus be said to be formed of 

 (1) a layer of flattened epithelial cells (endothelium), (2) a layer of delicate con- 

 nective tissue with branched cells ; and (3) of elastic tissue under two principal 

 forms, namely, the longitudinal elastic networks and the fenestrated membrane ; and 

 these two forms may coexist in equal amount, or one may predominate, the other 

 diminishing or even disappearing altogether. 



Middle coat (Tunica media) (fig. 417, c). This consists of plain muscular 

 tissue, in fine bundles, disposed circularly round the vessel, and consequently tearing 

 off in a circular direction, although the individual bundles do not form complete 

 rings. The considerable thickness of the walls of the arteries is due chiefly to this 

 coat ; in the smaller ones, it is thicker in comparison with the calibre of the vessel. 

 In the larger vessels it is made up of many layers ; and elastic films either finely 

 reticular, or quite similar to the fenestrated membrane of the inner coat, are found 

 between the muscular layers and alternating with them, being also united with one 



VOL. I. 



B B 



