494 MIDDLE-SIZED VEINS. [SECT. 2 1 5. 



readily separable from each other (the so-called nuclear fibres), 

 Fig. 202. and contains a large number of smooth mus- 



cular fibres, whose fusiform elements mea- 

 sure 0-02'" to 0"04'" in length, and 0*004'" 

 to 0007'" in breadth, and present the ordi- 

 nary characters of the contractile fibre-cells ' 

 on the other hand, the longitudinal laminae 

 consist of true reticulated elastic fibres, some 

 Transverse section of the thicker than others, but all of the coarser 

 Seoitf°ZSd att so variety. With regard to the mode of ar- 

 and e epitheifum P of tuTnit raugement of these tissues with one another, 

 oftoe in saSe^% 6 'io3iuctoai tnere follows upon the tunica intima in cer- 

 K%££ZS% tain veins (Popliteal, profunda femoris, sa- 

 SSSZ&Z&TiSZ P» ffi ™ major and minor), a longitudinal 

 fi^tataSoiSTS % er ; o -01 '" t0 0-04'" thick, formed entirely 

 nica adventitia. f connective tissue with fine elastic net- 



works ; while in certain other veins, muscular elements extend even 

 as far as the innermost layers. In this case, there is found immedi- 

 ately external to the inner coat a transverse layer of muscular fibres, 

 mixed with connective tissue and elastic fibrils — these three tissues 

 always accompanying each other in such veins ; next there follow 

 layers of longitudinal elastic network, alternating regularly with 

 layers of transverse muscular fibres and connective tissue, so that the 

 middle coat of these veins somewhat resembles that of the largest 

 arteries. It is, however, to be remarked, that these elastic networks, 

 although densely interwoven, are yet never converted into homoge- 

 neous membrane ; they are, moreover, interrupted here and there, 

 and, as longitudinal sections show, are invariably connected with one 

 another throughout the tunica media. The number of these elastic 

 lamellae varies from five to ten, and their interspaces measure 0"004'" 

 to o'oi'". — The tunica intima of the middle-sized veins measures 

 o*o \'" to o"04'" in thickness, and consists, in its thinner form, of only 

 an epithelium, with shorter, though still elongated cells, of a striped, 

 nucleated lamella, and of an elastic longitudinal layer, which cor- 

 responds to the elastic inner coat of the arteries, though it scarcely 

 ever appears as a truly homogeneous, fenestrated membrane, but 

 rather as an extremely dense, extended network of finer and 

 coarser elastic fibrils. Where the tunica intima is thicker, the 

 striped lamellae multiply and appear in variable number on the 

 inner aspect of the elastic longitudinal layer, before described as 

 forming the outer limit of the tunica intima. I have also observed 

 smooth muscular fibres in the inner coat of the veins of the gravid 



