SECT. 2J5.] STRUCTURE OF LARGEST VEINS. 495 



uterus, as well as in the saphena major and popliteal veins ; and 

 this observation has been confirmed by llemak in the visceral veins 

 of certain mammalia. — The tunica adventitia of these veins is almost 

 invariably thicker than the tunica media, frequently twice as thick, 

 more rarely of the same strength. As a rule, it contains only 

 elastic tissue and ordinary connective tissue, longitudinally ar- 

 ranged; the elastic elements are variously connected together, 

 often forming very beautiful elastic reticulate coats, with thick 

 fibres. Those visceral veins, however, which will be presently 

 described as possessing longitudinal muscular fibres in the outer 

 coat, also present similar elements for a certain distance on their 

 branches. 



The largest veins are distinguished from those of medium dia- 

 meter by the slight development of the tunica media, and especially 

 of its muscular fibres ; but their loss in this situation is often 

 compensated by the appearance of contractile elements in the 

 adventitia. — The tunica intima measures, as a rule, o*oi'" in 

 thickness, and presents the same structure as in the veins of 

 medium size. More rarely, as, occasionally, in the vena cava 

 inferior, in the trunks of the hepatic, and in the innominate vein, 

 it increases to 00 2'" and even 0*03'"; this increase in thickness 

 being due to the striped lamellce with nuclei, and to fine, longi- 

 tudinal, elastic networks, never to the development of any muscular 

 layer. — The tunica media measures, on an average, o - 02"' to 004'", 

 but may amount to o - o6"' to 0"i2'" in exceptional cases, as in the 

 commencement of the portal trunk, in the uppermost part of the 

 abdominal portion of the inferior cava, and at the mouths of the 

 hepatic veins. On the other hand, it may be entirely wanting, as 

 in the greatest part of the vena cava on the liver, and in the 

 further course of the largest hepatic veins. Its structure is essen- 

 tially the same as in the former instances, only that the longi- 

 tudinal elastic networks are variously connected with each other, 

 and are less distinctly disposed in lamellre, or are even not so dis- 

 posed at all; moreover, the transverse muscular fibres are scanty 

 and indistinct (even where the tunica media possesses the con- 

 siderable thickness above-mentioned), and are intermingled with 

 more numerous transverse bundles of connective tissue. I have 

 observed the muscular fibres of the tunica media most developed 

 in the splenic and portal veins ; they appeared to me to be com- 

 pletely absent at certain spots of the abdominal vena cava below 

 the liver, and they are wholly wanting, also, in the subclavian vein 

 and in the terminal portions of the superior and inferior vena cava. 



