49 6 



STRUCTURE OF THE 



[sect. 215. 



— The tunica adventitia of the largest veins is invariably thicker 

 than the middle coat, being often twice to five times as thick, and 

 exhibits the important peculiarity in its structure, pointed out by 



Fig. 203. 



mm 



1 





Longitudinal section of the inferior vena cava, at the liver ; magnified 30 times, a. 

 Tunica intiraa; b. tunica media, without muscular fibres, only containing areolar 

 tissue and elastic fibres ; c. internal laminae of the tunica adventitia; a. longitudinal 

 muscular fibres of the same ; |3. circular areolar tissue of the same layer ; d. external 

 part of the tunica adventitia without muscular fibres. 



Remah, that in certain of these veins, at least, the outer coat 

 contains a large number of longitudinal muscular fibres. These 

 are most conspicuous in the hepatic part of the inferior vena cava, 

 where they were first pointed out by Bernard (Gaz. Mid. de Paris, 

 1849, xvn - 33 1 )- Here they are arranged in fasciculi, o - oi'" to 

 0-04'" in diameter, which form a network traversing the inner half 

 or inner two-thirds of the outer coat, and attaining sometimes a 

 thickness of o"22'" ; where the middle coat is absent, this muscular 

 layer of course lies directly upon the tunica intima. Besides, in 

 this situation, Remak and myself have also found these longitudinal 

 contractile fasciculi very well developed in the trunks of the hepatic 

 vein, in the trunks of the vena portre, and in the upper part of the 

 inferior vena cava ; and I have tracked them as far as the splenic 

 vein, the superior mesenteric, the external iliac and renal veins. 

 In these situations, the longitudinal fasciculi never contain any 

 connective tissue, but probably elastic fibres in a certain number. 

 The vena azygos also exhibited a few muscular elements in its 

 outer coat, but they were absent throughout in the upper veins. 

 It was only in the renal veins and the vena portse that these mus- 

 cular fibres extended through the whole thickness of the tunica 

 adventitia, for, in the other veins mentioned, a larger or smaller 

 part of it externally remained free from muscle, and consisted, as 

 usual, of longitudinal connective tissue and elastic fibrous networks. 

 This circumstance gave to the muscular layer of the tunica adven- 

 titia the appearance of a special vascular coat, and might have 

 produced a confusion between it and the tunica media, which in 



