VEIN. 



1373 



veins, the cerebral sinuses, Breschet's veins of 

 the bones, and the venous cells of the corpora 

 cavernosa. 



External coat of longitudinal fibres. The ex- 

 ternal coat consists of a mass of areolar tissue, 

 adhering together with more or less compact- 

 ness, and running in a longitudinal direction. 



This tunic occupies full two-thirds of the 

 entire thickness of the whole wall of a vein. 

 Its internal boundary is irregular at the union 

 with the equally irregular outer limit of the 

 middle coat, and its external boundary is the 



Fig. 858. 





Transverse vertical Section of Sulclavian Vein 

 of Ox. 



a, internal coat ; b, middle coat ; c, part of external 

 coat. (Magnified 200 diameters.) 



loose cellular tissue into which it degenerates, 

 and which constitutes the "sheath" of the 

 vessel. This coat, like the preceding, gradu- 

 ally diminishes in density and compactness as 

 it is examined further and further from its 

 inner limit, though its density is everywhere 

 inferior to that of the middle ; indeed, it may 

 be said that the tissue of which the vessel is 

 composed becomes looser and looser in pro- 

 ceeding outwards from the inner boundary of 

 the middle coat, this quality passing gradu- 

 ally from one coat to the other. 



The textural arrangement of this tunic 

 must be examined, like the preceding, from 

 longitudinal and transverse sections. In the 

 former it is seen that bundles of rods of yel- 

 low elastic tissue are disposed in alternate 

 lamina; with white fibre, or rather, that the 

 former are embedded in a mass of the latter, 

 and that they are a continuation of the longi- 

 tudinal fibres of the middle coat, which gradu- 

 ally become arranged more and more in strata, 

 and at increasing distances. (/ Y V. 856. c.) 



The characters of the white fibrous tissue 

 between the bundles of yellow, become more 

 conspicuous as the laminae are wider apart. 



The stratified arrangement of the fibres, 

 and their correspondence with the longitudinal 

 rods of the middle coat are very well seen in 

 the specimen treated with acetic (Jig- 857.). 



When this tunic is viewed in transverse 

 section, the relative thickness and proportion 

 of the lamella;; are better seen than in a longi- 

 tudinal cut : the yellow element is seen to 

 occupy but a small relative thickness in com- 

 parison with the white. 



When viewed with a high magnifying power 

 the divided extremities of the rods of yellow 

 fibrous tissue form undulating, and somewhat 

 imbricated lines of discs, while the white 

 fibrous tissue, cut across, shows slight, indis- 

 tinct, wavy indications. (Fig- 858. c.) 



Seen with a low power, the ends of the 

 yellow fibrous tissue appear as series of dark 

 undulating dots on a field of white. 



Minute veins. When very small veins, a 

 few removes from capillaries, are examined 

 microscopically, they merely present faint stri- 

 ations in the longitudinal direction. When, 

 however, these vessels are treated with acetic 

 acid, nuclei and fibres are distinctly displayed, 

 whose long axis may be assumed to indicate 

 the direction of the tunic (or set of fibres) 

 which they represent, in which they are em- 

 bedded, and of which they form a part. 



Henle was, I believe, the first to point out 

 this mode of textural dissection of vessels, and 

 nothing can be more satisfactory than the 

 analysis that it makes. By this means small 

 vessels alone can be examined, but the scru- 

 tiny may be carried up to those the third of a 

 line in diameter, which are sufficiently trans- 

 parent, when entire, to exhibit the nuclei. 

 Small vessels may be conveniently obtained 

 from the pia mater and mesentery ; they are 

 there free from other structures, and their 

 form is not interfered with. 



I have found, however, that it is in the pia- 

 mater somewhat difficult to trace the small 

 veins; the arteries about them are more de- 

 finite and conspicuous, and less injured by 

 manipulation, and generally catch the atten- 

 tion of the observer. It has been more con- 

 venient to obtain isolated cerebral veins from 

 the surface of the ventricles of the brain ; the 

 small veins on the corpus striatum may be 

 raised and torn away with the points of fine 

 forceps, and sufficient capillaries will generally 

 be found attached to their extremities to ex- 

 hibit the structure of these vessels of all size?. 

 The accompanying figures were made from 

 specimens thus obtained, about whose venous 

 character there could he no mistake. 



Small veins vary in structure in different 

 regions, and according to their size ; some ap- 

 approach the structure of arteries *, whilst 



* It may be convenient here to give a brief out- 

 line of the anatomy of capillary, and minute, arteries. 

 Their structure is more definite than that of veins; 

 their walls are thicker and their cavity less in pro- 

 portion. The larger ones exhibit both a longitu- 

 dinal and a transverse striation. When treated 

 with acetic acid the nuclear corpuscles are numerous, 

 sha> jih/ defined, and obvious in {hair ilireclion. 



Capillaries, up to about 1-lGCOth of an inch in 



