NORMAL HISTOLOGY. 



thin layer of fibrous tissue and networks of elastic fibres. A distinct internal 

 elastic membrane is seldom present. In some veins, as the cephalic, basilic, 

 mesenteric, iliac, femoral and saphenous, the intima contains bundles of 

 involuntary muscle. The media, the most variable coat of the vein-wall, 

 consists of circularly disposed thin sheets of muscular and fibre-elastic tissue, 

 reinforced by longitudinal strands of fibre-elastic tissue and, sometimes, 

 muscle. In certain veins, as the saphenous, deep femoral and popliteal, 

 these longitudinal strands constitute a distinct zone beneath the intima. 

 While in the larger veins the intima is only exceptionally increased, as in 

 the hepatic portion of the inferior vena cava and the portal vein, the media 

 is often markedly thickened. This increase is due chiefly to excess of the 

 elastic and fibrous tissue, the muscle remaining proportionately scanty. 



Intima 



Med 



Adventi 



FIG. 129. Transverse section of vein of medium size. X 250. 



The splenic and portal veins, however, are particularly rich in muscular 

 tissue. On the other hand, the media may be almost wanting, as in the 

 greater part of the inferior vena cava and the larger hepatic veins, or entirely 

 disappear, as in the superior vena cava and in the veins of the pia and dura 

 mater, of the retina and of bone. The valves, with which many veins are 

 provided, consist of paired crescentic projections of the intima, covered on 

 both sides with endothelium. The two layers of endothelial plates are 

 separated by a thin stratum of delicate fibrous tissue, which contains a dense 

 network of elastic fibres beneath the inner endothelium. 



The Capillaries. The most favorable arrangement for efficient nutri- 

 tion is, manifestly, one insuring the passage of the blood-stream in intimate 

 relations with the tissue-elements and at a reduced rate of speed. These 

 requirements are met in the capillaries, whose collectively increased calibre 

 and thin walls favor slowing of the blood-current and the passage of the 

 plasma and oxygen into .the surrounding tissues. The walls of the capil- 

 laries consist of only the lining plates, the entire vessel being in fact a delicate 

 endothelial tube. The cells composing the latter are elongated lanceolate 

 plates, possessing oval nuclei, united by narrow lines of cement substance. 

 Although the transition from the arterioles is usually gradual, the final 





