THE BLOOD-VESSELS. 



305 



in the/, adventitia. The thickness of the t. intima is usually 

 O'Ol/", when it presents the same conditions as in the medium- 

 sized veins. More rarely, as occasionally in the vena cava infe- 

 rior, in the trunks of the hepatic veins, and in the vena innomi- 

 nata, it amounts to , 02 /// and O'OS"', which increase of thickness 

 is due to striped lamella with nuclei, and fine, elastic, longitudinal 

 networks, never to those composed of muscles. The t. media, on 

 the average, presents a thickness of 0*02 — 0'04'", but may occa- 

 sionally, as at the commencement of the trunk of the vena 

 porta, in the uppermost part of the abdominal portion of the v. 

 cava inferior, and at the orifices of the hepatic veins, measure 

 0*05 — 0-12'", or be wholly wanting, as in the greater part of 

 the v. cava inferior in the liver, and in the further course of the 

 largest hepatic veins. Its structure, in all essential parti- 

 culars, is the same as in the previous class of vessels, except 

 that the longitudinal elastic networks are intricately connected 

 together, and less distinctly, or not at all, laminated ; the trans- 

 verse muscles, also, are scanty and indistinct, even where the 

 t. media possesses the considerable thickness above stated, and 

 are more abundantly intermixed with bundles of connective 

 tissue. I have noticed the muscles to be most developed in the 

 splenic vein and v. porta; they appeared to me to be wholly 

 wanting in the abdominal portion of the inferior vena cava 



Fig. 286. 



below the liver, in certain spots, and also in the subclavian 

 vein, and in the terminal portions of the superior and inferior 



Fig. 286. Longitudinal section of the inferior vena cava, in the liver, x 30 diam.: 

 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. 



ii. 20 



