THE BLOODVESSELS. 687 



pearance on the inner aspect of the above-described elastic membrane, 

 which forms the limitary portion of the t. intima. I have also seen 

 smooth muscles of the t. intima, in the veins of the gravid uterus, as 

 well as in the saphena major and popliteal vein ; and Remak has con- 

 firmed their existence in the visceral veins of certain Mammalia. The 

 t. adventitia of these veins is almost invariably thicker than the t. media, 

 often twice as thick, rarely of equal strength. Usually it contains only 

 longitudinal, much interwoven, often very well-marked elastic networks 

 with thick fibres, and common connective tissue, but in the case of those 

 visceral veins, the trunks of which have longitudinal muscles in the t. 

 adventitia, similar muscular elements also extend for a certain distance 

 into the branches (vid. seq.) 



The largest veins are distinguished from those of the medium diameter 

 chiefly by the sparing development of the t. media, and especially of 

 its muscular elements ; a deficiency, however, it is true, often counter- 

 balanced by the presence of contractile elements in the t. adventitia. 

 The thickness of the t. intima is usually O'Ol of a line, when it presents 

 the same conditions as in the medium-sized veins. More rarely, as oc- 

 casionally in the vena cava inferior, in the trunks of the hepatic veins, 

 and in the vena innominata, it amounts to 0-02 and 0-03 of a line, 

 which increase of thickness is due to striped lamellce 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 of a 

 line, but may occasionally, as at the commencement of the trunk of the 

 vena portai, 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 particulars, 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 

 transverse muscles, also, are scanty and indistinct, even where the t. 

 m,edia 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. portce ; they 

 appeared to me to be wholly wanting in the abdominal portion of the 

 inferior vena cava below the liver, in certain spots, and also in the sub- 

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

 vence cavce. 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 consider- 

 able quantity of longitudinal muscles. These are very distinct, as was 

 pointed out by Bernard (" Gaz. Med. de Paris," 1849, 17, 331), in the 



