VEIN. 



1371 



free margin of valves I have occasionally seen, 

 as noticed above, a clear, structureless rim, 

 which I have imagined to be this membrane 

 reduplicated, the membrane being extremely 

 thin, and the interposd fibrous lamina not 

 reaching quite to the margin. 



It is said by Hcnle and others only to exist 

 in a few places, and certainly it is not always 

 to be found, though it is not perhaps right to 

 limit its existence to those spots only where 

 it has been recognised, as its extreme tenuity 

 and firm adhesion to the next coat might ac- 

 count for its not being generally seen.* 



The tissue in question is best seen by slit- 

 ting up a vein and pinning it out on cork. The 

 inner surface is then scratched with a needle 

 after it has been moistened. The fenestrated 

 membrane retracts, and its edges may be 

 ruffled up and a small portion removed by 

 the points of very fine forceps, and thus ob- 

 tained for examination, which is best done 

 without any superimposed glass, as that flattens 

 the coils and folds, and it is impossible to 

 unravel them. 



Fig. 855. 



the relative thickness of the three elementary 

 fibrous coats. 



Specimens for displacing the profile views 

 of the walls of veins are best obtained by 

 slitting up the vessel, pinning it out upon 

 cork, and suffering it to get dry. Sec- 

 tions are then to be made vertical to the 

 surface, either parallel to, or at right angles 

 with, the axis of the vessel. Thin shavings 

 may then be removed with a very flat knife 

 (and for this purpose, a Beer's cornea knife is 

 the best), and when placed on a slip of glass, 

 moistened, and covered with a square of thin 

 glass, are ready for observation. The accom- 

 panying drawings were made from sections 

 thus obtained. 



When a longitudinal section of the in- 

 ternal tunic is examined with a high power, 

 as in^/zg. 856. a, it is seen to consist of very 

 fine yellow elastic tissue, which is peculiarly 

 pale and indistinct, having mainly a longi- 

 tudinal direction, but being much interlaced 

 and matted together, so that its longitudinal 

 course is, in many situations, obscure. This 

 coat is seen to be fine, and dense, and in 

 strong contrast to all the other structures, 

 from which it is separated by a distinct line of 

 demarcation. When thus viewed, the fibres 

 of this tunic are seen to present a succession 

 of waves, not unlike those of white fibrous 

 tissue, but finer and smaller : whether these 

 undulations are from its own inherent pro- 



Fig. 856. 



Transverse vertical Section of the Wall of the 

 Subdavian Vein of an Ox, exhibiting the relative 

 Thickness of the Three Coats. 



a, internal coat ; b, middle coat ; c, the entire ex 

 ternal coat. (Magnified 40 diameters.) 



Internal tunic vf longitudinal fibres. The 

 internal tunic of longitudinal fibres is ex- 

 tremely thin, and occupies but a very small 

 amount of the thickness of the vessels' walls. 

 This is well seen in/"g.855., which is a trans- 

 verse section of the subclavian vein of.an ox, 

 made vertical to the surface, and displaying 



* It appears to me that there are sufficient 

 grounds to acknowledge the existence of a limitary 

 membrane beneath the epithelium in the blood- 

 vessels, and no sufficient reason to question its uni- 

 versal distribution. It must be recollected that 

 there are certain regions, even in the mucous mem- 

 brane (the nasal fossa, for instance), where no base- 

 ment membrane has been found, and I am not aware 

 that, in the skin, it has ever been separated from 

 the chorion, and yet in neither case is its existence 

 doubted. To exhibit the limitary membrane well, 

 some peculiar mechanical arrangements of the sur- 

 face is necessary, as papillae or crypts, and such are 

 not presented on the surface of blood-vessels. It is 

 not to be wondered at, therefore, that there is here 

 great difficulty in displaying the limitary mem- 



Longitudinal vertical Section of Wall of Subclavian 

 Vein of an Ox. 



a, internal coat rather thicker than common ; b, mid- 

 dle coat ; c, part of external coat. (Magnified 200 

 diameters.) 



perties, or whether they are produced by its 

 adhesion to the next tunic, whose elastic con- 

 traction is greater than its own, thus throwing 

 it into folds and waves, I am unable to say. 

 The identity of these fibres with yellow elastic 

 tissue is shown by the action of acetic acid, 

 which does not destroy them, but renders 

 them even more distinct, while the contiguous 



