304 SPECIAL HISTOLOGY. 



which three tissues, in these veins, always accompany each 

 other; to this succeeds a regular alternation of longitudi- 

 nal, elastic, reticulated membranes, always in a single layer, 

 and transverse muscles with connective tissue, so that the 

 t. media of these veins presents a laminated aspect, somewhat 

 resembling that of the largest arteries. It should, however, be 

 remarked, that the elastic, reticulated membranes, although 

 frequently very closely interwoven, nevertheless never form 

 homogeneous, elastic membranes ; moreover, that they are occa- 

 sionally interrupted, and, as longitudinal sections manifestly 

 show, are invariably continuous with one another through the 

 whole t. media. The number of these elastic lamella fluctuates 

 between five and ten, and their interspaces vary in width from 

 0*004 — 001'". The t. intima of the medium-sized veins is 

 001 — , 04 /// thick, and consists, where it is thinner, only of 

 an epithelium with shorter, though elongated cells, a striated, 

 nucleated lamella, and an elastic longitudinal membrane, corre- 

 sponding to the elastic inner membrane of the arteries, but 

 which scarcely ever appears as a truly homogeneous, fenestrated 

 membrane, but most usually as an extremely close, areolated 

 network of finer and coarser elastic fibrils. Where the t. intima 

 is thicker, the striated lamella are multiplied, and, above all, one 

 or even several additional networks of elastic, fine fibres make 

 their appearance on the inner aspect of the above-described 

 elastic membrane, which forms the limitary portion of the 

 t. intima, I have also seen smooth muscles in the t. intima, in 

 the veins in the gravid uterus, as well as in the saphena major and 

 popliteal vein ; and Remak has confirmed their existence in the 

 visceral veins of certain Mammalia. The t. adventitia of these 

 veins, is almost invariably thicker than the t. media, often twice 

 as thick, rarely of equal strength. Usually it contains only longi- 

 tudinal, much interwoven, often very well marked elastic net- 

 works with thick fibres, and common connective tissue, but in 

 the case of those visceral veins, the trunks of which have longi- 

 tudinal muscles in the t. adventitia, similar muscular elements 

 also extend for a certain distance into the branches (vid. seq). 

 The largest veins are distinguished from those of the medium 

 diameter chiefly by the sparing development of the /. media, and 

 especially of its muscular elements; a deficiency, however, it is 

 true, often counterbalanced by the presence of contractile elements 



