688 



SPECIAL HISTOLOGY. 



hepatic portion of the inferior vena cava, Avhere their fasciculi, 0-01- 

 0-04 of a line thick, constitute a network pervading the inner half or 

 two inner thirds of the external membrane, which, where the t. media is 

 absent, rests immediately upon the t. intima, and may attain a thickness 

 of as much as 0-22 of a line. Besides this, I have found, as Remak has 

 also done, these contractile, longitudinal bundles (which never contain 



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connective tissue, though probably a certain number of elastic fibres), 

 still very well developed, in the trunks of the hepatic veins, in that of 

 the vena porta', and in the remaining portion of the inferior vena cava, 

 and have traced them as far as the splenic, superior mesenteric, exter- 

 nal iliac, and renal veins. Some were also to be found in the ve7ia 

 azygos, whilst they were altogether absent in the superior veins. These 

 muscles extended through the entire thickness of the t. adventitia, only 

 in the renal vein and vena porta', whilst in the other veins above enume- 

 rated, a greater or less portion of it, contained no muscular element, 

 and consisted as usual, of longitudinal connective tissue, and elastic 

 networks composed of strong fibres. The muscular layer of the t. ad- 

 ventitia therefore appears to be a special membrane of the vessels, and 

 occasion would be afforded to confound it with the undeveloped, or, as 

 has been stated, even absent t. media; an error, however, Avhich might 

 be readily avoided by tracing the conditions of the smaller veins. The 

 muscular layer of the t. adventitia, besides the contractile elements, 

 having a length of 0-02-0-04 of a line, which present the common cha- 

 racter, and numerous elastic longitudinal networks, invariably contains 

 .a certain amount of connective tissue, which, as it seems, is always 

 arranged transversely, so that the transverse elements, even in these 

 large veins, are compensated for though not exactly, by muscle. All 

 the large veins, which open into the heart, are furnished, for a short 

 distance, with an external annular layer of muscles similar to those of 

 the heart itself, and which also present anastomoses of the primitive 



Fig. 280. — Longitudinal section of the inferior vena cava, in the liver, magnified 30 

 diameters: a, t. intima; b, t. media, without muscles, containing only connective tissue and 

 elastic fibres ; c, inner layer of the t. adventitia; a, its longitudinal muscles; /3, transverse 

 connective tissue of the same layer ; d, external portion of the t. adventitia, without muscles. 



