STRUCTURE OF VEINS. 367 



the artery, and named venae comites. The larger arteries, as the 

 axillary, subclavian, carotid, popliteal, femoral, &c., are accompanied 

 by a single venous trunk. Sinuses differ from veins in their structure ; 

 and also in their mode of distribution, being confined to especial organs 

 and situated within their substance. The principal venous sinuses 

 are those of the dura mater, the diploe, the cancellous structure of 

 bones, and the uterus. 



The communications between veins are even more frequent than 

 those of arteries, and they take place between the larger as well as 

 among the smaller vessels ; the venae comites communicate with each 

 other very frequently in their course, by means of short transverse 

 branches which pass across from one to the other. These communica- 

 tions are strikingly exhibited in the frequent inosculations of the 

 spinal veins, and in the various venous plexuses, as the spermatic 

 plexus, vesical plexus, &c. The office of these inosculations is very 

 apparent, as tending to obviate the obstructions to which the veins are 

 particularly liable from the thinness of their coats, and from their 

 inability to overcome much impediment by the force of their current. 



Veins, like arteries, are composed of three coats, external or cellulo- 

 fibrous, middle or fibrous, and internal or serous. The external coat is 

 firm and strong, and resembles that of arteries. The middle coat 

 consists of two layers, an outer layer of contractile fibrous tissue dis- 

 posed in a circular direction around the vessel, and an inner layer of 

 organic muscular fibres arranged longitudinally. This latter resem- 

 bles the inner layer of the middle coat of arteries, but is somewhat 

 thicker, and is not unfrequently hypertrophied. The internal coat, as 

 in arteries, consists of a striated or fenestrated layer, and a layer of 

 epithelium ; it is continuous with the internal coat of arteries through 

 the medium of the lining membrane of the heart on the one hand, 

 and through the capillary vessels on the other. The differences in 

 structure, therefore, between arteries and veins relate to the differ- 

 ence of thickness of their component layers, and to the absence of the 

 elastic coat in the latter. Moreover, another difference occurs in the 

 presence of valves. The valves of veins are composed of a thin layer 

 of fibrous membrane, lined upon its two surfaces by epithelium. The 

 segments or flaps of the valves of veins are semilunar in form and 

 arranged in pairs, one upon either side of the vessel ; in some in- 

 stances there is but a single flap, which has a spiral direction, and 

 occasionally there are three. The free border of the valvular flaps is 

 concave, and directed forwards, so that while the current of blood is 

 permitted to flow freely towards the heart, the valves are distended 

 and the current intercepted if the stream becomes retrograde in its 

 course. Upon the cardiac side of each valve the vein is expanded 

 into two pouches (sinuses), corresponding with the flaps of the valves, 

 which give to the distended or injected vein a knotted appearance. 

 The valves are most numerous in the veins of the extremities, particu- 

 larly in the deeper veins, and they are generally absent in the very 

 small veins, and in the veins of the viscera, as in the portal and cere- 



