THE BLOODVESSELS VEINS. 507 



2. The medium-sized veins (1 to 8 lines in diameter) have an ex- 

 ternal tunic almost always thicker than the 

 middle coat, often twice as thick, though rare- 

 ly as strong. It is composed of areolar tissue, 

 except in some visceral veins whose trunks 

 contain longitudinal muscular fibres, and into 

 whose branches also the muscular elements 

 extend for some distance. The middle coat 

 has a considerable development of the annular 

 fibrous layer, of a yellowish-red color, as in the Transverse section of the 



u vena sapliena rnagna at the 



arteries. It, however, is never more than - 2 ^ maiieoius. a. striped iameii 



to T f r of an inch thick. Unlike that of the and epithelium of the tunica 



* ' A f mtima. o. Its elastic mem- 



arteries, it consists of longitudinal as well as brane. c. Longitudinal in- 



transverse layers. (Fig. 338.) ternai connective-tissue layer 



J, \ o / of the tunica media, with elas- 



The latter are composed of undeveloped tic fibres, a. Transvers 



areolar tissue and a- large amount of smooth J 



milSCular fibres. In the popliteal, Sapheiia laminated manner. /. Tunica 



, . T , P n . adventitia. Magnified 30 dia- 



major and minor, and the profunda femoris me ters. 



vein, there is a transverse layer of muscular 



fibres with areolar tissue, immediately in contact with the tunica 

 intima, external to which is a regular alternation of longitudinal 

 elastic membranes and transverse muscular fibres; so that the mid- 

 dle coat presents a laminated aspect somewhat like that of the 

 largest arteries. There are from five to ten of these elastic laminae; 

 their interspaces being from 3 oVtr to rsW f an inch. The inner 

 coat of the medium-sized veins is T^'OTT to -%^-Q of an inch thick. 

 Where it is thinnest, it consists of an elastic longitudinal mem- 

 brane, corresponding to the fenestrated membrane of the arteries, 

 and a striated nucleated lamella within it, and an epithelium with 

 shorter, though elongated cells. When the tunica intima is thicker, 

 the striated lamelbe are multiplied, and one or several additional 

 networks of fine elastic fibres appear on the inner surface of the lat- 

 ter, forming the limitary portion of the inner coat. Muscular fibres 

 also have been seen in the inner coat of the veins of the gravid 

 uterus, in the saphena major, and the popliteal vein. (Kolliker.) 



8. In the largest veins the external coat is almost invariably nearly 

 twice as thick as the middle, and sometimes five times as thick, and 

 also contains a considerable amount of longitudinal smooth muscu- 

 lar fibres. (Fig. 243.) These are very distinct in the hepatic portion 

 of the inferior vena cava (Bernard), constituting a network pervading 



