1374 



VEIN. 



others are fur removed from them in com- 

 position. 



Venous capillaries do not, that I am aware 

 of, differ from arterial. They consist of tubes 

 of homogeneous membrane, studded here and 

 there with nuclei of a more or less oval form, 

 and placed generally with their long axis cor- 

 responding with that of the vessel. In passing 

 to larger veins the change in structure of the 

 vessel (its increase and character) depends 

 upon the region from which it is obtained, 

 and whether or not it is endowed with muscle- 

 cells. 



In the cerebral veins, which have no muscle, 

 in passing from small capillaries to larger 

 vessels, all that is observed is the superaddi- 

 tion of a tunic of areolar tissue, surrounding 

 the lining membrane ; and when this tissue is 

 treated with acetic acid, all that is observable 

 is a series of nuclei, oval, irregular, and longi- 

 tudinal, embedded in the substance of the 

 parietes, which have a perfect resemblance to 

 the external, or cellular coat of small arteries. 

 (fig. 859. A.) In none of the microscopical 

 cerebral veins have I been able to discover 

 any nuclear or yellow fibres. In veins of 

 the T-iy to T i~g- of an inch the vascular wall has 

 been composed of a tissue rendered trans- 

 parent by acetic acid, and displaying oval and 

 irregular nuclei having a longitudinal course, 

 without any admixture of elastic fibre. The 

 most internal of the nuclei appeared elongated, 

 but there was no appearance of fibres upon 

 the internal membrane, so as to produce a 

 striated membrane. 



In those veins which are endowed with 



diameter, consist of tubes of homogeneous mem- 

 brane, studded here and there with nuclei of an 

 oval form, whose long axis is that of the vessel, 

 except in the larger ones, where a few are placed 

 transversely. These corpuscles are elongated, and 

 more numerous, in the larger capillaries. 



In arteries about 1 -600th of an inch, the walls 

 consist of a homogeneous tube, with the nuclei on 

 the outer surface still more elongated ; and external 

 to this, a set of nuclei, club-shaped and sharply 

 defined, placed at right angles to the former, and 

 representing the circular (muscular) tunic in which 

 they are embedded. 



In larger arteries a cellular tissue, still more ex- 

 ternal, is seen, whose corpuscles oval and irregu- 

 lar are longitudinal. In such arteries (say, about 

 1 -300th of an inch in diameter"), the tunics are, 

 1st. a homogeneous tube covered with elongated 

 longitudinal nuclei; 2nd. a circular tunic, repre- 

 sented by the club-shaped transverse nuclei ; and 

 3rd. a longitudinal tunic of cellular tissue, in which 

 are imbedded oval and irregular nuclei. In passing 

 to larger vessels, the nuclei of the internal coat 

 elongate and disappear ; and the internal membrane 

 then consists of perfect striated membrane, the 

 homogeneous tube being covered with very fine 

 longitudinal fibres of elastic tissue. Such may be 

 seen in arteries of 1 -100th to l-60th of an inch in 

 diameter, and in some much smaller. Fibres make 

 their appearance shortly afterwards in the circular 

 tunic, intermixed with the club-shaped nuclei. 



The proportion existing between the diameter 

 of an artery and the thickness of its walls is sub- 

 ject to considerable variety ; and the same may be 

 said of the relative thickness of the several tunics 

 which compose the wall. 



Henle' has given good figures of the structure of 

 arteries. 



muscular tissue, small club-shaped nuclei are 

 observed placed externally to the lining mem- 



Fig. 859. 



A, Minute Cerebral Vein (about 1-2001& of an inch 

 diameter), from Sheep, treated with Acetic Acid. 



a, wall ; b, cavity. 



B, Venous Capillaries (a, about l-1600//t of an inch 

 b, about 1-600*A of an inch} treated with Acetic 

 Acid. 



In both these figures persistent nuclei of areolar 

 tissue and epithelial nuclei are alone visible. (Mag- 

 nified 200 diameters.) 



brane, and transversely as it regards the ves- 

 sel ; these show themselves very distinctly on 

 the addition of acetic acid, and are character- 

 istic of the " muscular fibre-cells" of Kolliker. 

 Such may be seen in the small veins of the 

 mesentery, which are more easily examined 

 than any other veins of this class. In these 

 vessels, two or three removes from capillaries, 

 the muscular nuclei make their appearance, 

 at first, few in number and irregular ; and in 

 still larger, but microscopical veins, these 

 nuclei are seen to be mixed up with, or 

 covered in by, more or less areolar tissue, 

 which at first is destitute of nuclear fibre, but 

 contains conspicuous nuclei, oval and irregular 

 in shape and longitudinal in direction. In 

 the accompanying drawing (fig. 860. B) is 

 a small vein from the mesentery of a rabbit, 

 measuring about T> ^ of an inch in diameter, 

 and treated with acetic acid. On the lining 

 membrane are seen a few elongated nuclei 

 and longitudinal striations, constituting a sort 

 of incipient striated membrane. External to 

 this is a thin layer of muscular tissue, and 

 without that, again, is cellular tissue, which 

 constitutes the main bulk of the vascular wall. 

 The muscular coat is less distinct than in 

 arteries of the same size and thinner in pro- 

 portion to the areolar coat. 



Such veins have, however, a strong resem- 

 blance to small arteries. Henle states that 

 the two systems of vessels are not to be dis- 

 tinguished in those measuring up to 6 ^ ri of an 

 inch ; and he appears to consider thinness of 

 wall as the main characteristic in larger micro- 

 scopical veins. He instances one vessel, which 

 he considers as venous, measuring ^ of an 

 inch in entire diameter, having an annular 

 tunic ^5-, and a cellular ^^ of an inch. It 

 has appeared to me that the differences of 

 these vessels are principally these, micro- 

 scopical veins have thinner walls ; the mus- 



