306 SPECIAL HISTOLOGY. 



venae cava. The t. adventitia of the largest veins, is almost 

 invariably nearly twice as thick as the middle tunic, or even 

 as much as five times as thick, and exhibits, in its structure, 

 the important difference, at least in certain veins, as Remak 

 correctly states, that it contains a considerable quantity of 

 longitudinal muscles. These are very distinct, as was pointed out 

 by Bernard (' Gaz. Med. de Paris/ 1849, 17, 331), in the hepatic 

 portion of the inferior vena cava, where their fasciculi, 0*02 — 

 O04'" 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 022'". Besides this, I 

 have found, as Remak has also done, these contractile, longitu- 

 dinal bundles, (which never contain 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, 

 external iliac and renal veins. Some were also to be found in 

 the vena 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 enumerated, a greater or 

 less portion of it, contained no muscular element, and con- 

 sisted as usual, of longitudinal connective tissue, and elastic 

 networks composed of strong fibres. The muscular layer of 

 the t. adventitia 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, which might be readily avoided by tracing the 

 conditions of the smaller veins. The muscular layer of the 

 /. adventitia, besides the contractile elements, having a length 

 of 0*02 — 0*04/", which present the common character, 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, 



