94 



NINTH LECTURE. 



In the smaller arteries, the serosa and adventitia remain toler- 

 ably unchanged. Still, there frequently occur in the former 

 reticular perforated elastic layers, so-called " fenestrated mem- 

 branes," or free elastic longitudinal reticula; the media pre- 

 sents several layers of transversely directed muscles, lying over 

 each other, and an elastic net-work is also developed in the 

 fihrillated outer layer. 



In the larger branches, the stratification of the inner and 

 middle layers increases. In the latter, elastic plates with 

 transverse fibres, are now interpolated between the muscular 

 layers, and the elastic reticulum of the adventitia becomes 

 thicker. 



The largest arteries (Fig. 90') show under the endothelium 

 (a), strongly stratified, the group of the inner vascular mem- 

 brane (b). The several lamel- 

 lae, in varying texture, present 

 the entire multifariousness of 

 the elastic tissue. Inwards, 

 towards the endothelial cover- 

 ing, one may, indeed, meet 

 with more homogeneous, or 

 more striated layers with cellu- 

 lar reticula imbedded over each 

 other (Langhans, von Ebnen. 

 In the more middle group 

 of layers the membranous 

 character of the elastic fibrous 

 net-work (d) becomes more 

 and more prominent. Their 

 fibres may be thinner or thick- 

 er; the membranous connect- 

 ing substance may appear 

 whole or perforated. The num- 

 ber of these elastic layers may 

 increase to 30, 40, 50, and 

 more. The muscles of the 

 middle layer (e) appear unequally developed ; frequently not 



e m 



Fig. 90. — Transverse section through the 

 walls of a large artery : a, endothelium ; /. 

 sa ; c. outer layer of the same ; d. elastic, e, mus- 

 cular layers of the media : g, adventitia ;/, their 

 elastic fibro-reticula. 



