TEXTURE OF THE VEINS. 173 



of the ascending than of the descending cava; it is also thicker 

 in the superficial than in the deep-seated veins. In some sub- 

 jects it is much better developed than in others. In certain parts 

 of the body it is entirely deficient, as in the sinuses of the aura. 

 mater, and has its place supplied by this membrane; the same 

 deficiency exists in the sinuses of the bones. 



The Internal Coat is more delicate and extensible than the 

 corresponding one of the arteries, is less liable to rupture, and 

 less disposed to ossification. It is thrown into a considerable 

 number of duplications, forming valves. Each valve is of a se- 

 micircular shape; is connected by its convex edge to the vein, 

 while the straight edge is loose, and turned towards the heart. 

 When the veins are injected backwards, these valves may be 

 forced in the larger trunks, and give them a knotted appearance. 

 The valves are commonly in pairs, but in certain veins, as the 

 crural and the iliac, there are three of them together; very rare- 

 ly do they amount to four. In some instances there is but a 

 single one; this arrangernent is more frequent at venous orifices. 

 as the great coronary vein of the heart, the vena cava ascen- 

 dens, the vena azygos. They are frequently found reticulated 

 as if they had been lacerated, whence it has been supposed that 

 the fibres which cross the sinuses of the dura mater are an ele- 

 mentary approach to them. 



The valves are more abundant in the superficial than in the 

 deep-seated veins, but they do not exist every where. There are 

 none in the branches of the vena portarum, excepting the vasa 

 brevia: none in the spine, in the umbilical vein, the cervical veins, 

 the kidneys, womb, ascending and descending cava, or in the 

 median vein. The valves are proportionately more abundant in 

 the lower extremities. 



From the tenuity of the parietes of the veins, the blood may 

 be readily distinguished circulating through them. Their coats, 

 like those of the arteries, are vascular, or have the vasa vaso- 

 rum. The arteries come from the nearest small -trunks, while 

 the corresponding veins do not empty immediately, but second- 

 arily, into the trunk, whose parietes they supply. They are well 

 furnished with veins. 



Their elasticity, both transversely and longitudinally, is well 



16* 



