278 THE BLOODVESSELS, BY C. J. EBERTH. 



minal viscera, and of the upper extremity, which are about upon 

 an equality ; and finally those of the thorax and neck. 



The longitudinal muscular coat is most developed in the 

 inferior vena cava below the liver, in the iliac, portal, renal 

 and mesenteric veins. 



The thoracic portion of the inferior vena cava has no con- 

 tractile fibres in man, the ox, sheep, pig, and rabbit, whilst the 

 hepatic portion of the same vessel in these animals possesses a 

 strong circular muscular layer, 



In the superior vena cava of man, in opposition to that of 

 the ox and sheep, there are no muscular fibres, and they first 

 appear in the upper branches of the common jugular vein. 

 Here, in consequence of the fixed position of the vessels, those 

 obstacles are absent which render the passage of the current 

 in the inferior cava difficult. On the other hand, according 

 to Remak, in the superior cava of the ox and sheep there 

 are internal transverse and external longitudinal muscles, an 

 arrangement that may, perhaps, be rendered requisite by the 

 different position in which the head is maintained, 



THE TUNICA ADVENTITIA of the veins, like that of the arte- 

 ries, consists of bundles of decussating fibrils, the direction of 

 which is for the most part longitudinal. As a general rule 

 their diameter increases with that of the vessel, but there are 

 many exceptions. The tunica adventitia of the veins is dis- 

 tinguished from that of the arteries by its greater thickness 

 and the small amount of elastic fibres it contains, as well as by 

 the presence of longitudinal muscles in certain vessels. The 

 external layer of longitudinal muscles belongs exclusively to the 

 tunica adventitia. To whatever extent the longitudinal fibres 

 may be developed, they never form a distinct coat as in the 

 tunica adventitia of arteries, but only a coarse network con- 

 structed of larger or smaller fibres, which are chiefly found in 

 the middle and internal layers of the tunica adventitia, and 

 diminish towards the outer. The limits between the layers of 

 muscular and elastic fibres are never very well defined. 



THE VALVES OF THE VEINS cannot be regarded as true 

 duplications of the internal tunics. The elastic finely fibrillated 



