328 STRUCTURE OF VEINS. 



side of the artery, and named vence comites. The larger arteries, as 

 the axillary, subclavian, carotid, popliteal, femoral, &c., are accom- 

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

 structure, and also in their mode of distribution, being confined to 

 especial organs, situated within their substance. The principal venous 

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

 structure of bone, and of 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 trans- 

 verse branches which pass from one to the other. These communi- 

 cations 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 are composed of three tunics, external, middle, and in- 

 ternal. 



The External or Cellular coat is dense and resisting, and resem- 

 bles the cellular tunic of arteries. The middle coat is fibrous like 

 that of arteries, but extremely thin ; so that its existence is ques- 

 tioned by some anatomists. The internal coat is serous, and also 

 similar to that of arteries ; it is continuous with the lining membrane 

 of the heart at one extremity, and with the lining of the capillaries 

 at the other. At certain intervals the internal coat forms folds or 

 duplicatures, which constitute valves. The valves of veins are 

 generally composed of two semilunar folds, one on each side of 

 the cylinder of the vessel, occasionally of a single duplicature, 

 having a Spiral direction, and in rare instances of three. The free 

 extremity of the valvular folds is concave, and directed forwards, 

 so that while the current of blood would be permitted to flow freely 

 towards the heart, the valves would become distended and the current 

 intercepted if the stream became retrograde in its course. Upon the 

 cardiac side of each valve the vein is expanded into two pouches 

 (sinuses), corresponding with the segments 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, particularly in the 

 deeper veins, and they are generally absent in the smaller veins, 

 and in the veins of the viscera, as in the portal and cerebral veins : 

 they are also absent in the large trunks, as in the venae caves, venae 

 azygos, innominate, and iliac veins. 



Sinuses are venous channels, excavated in the structure of an 

 organ, and lined by the interns,! coat of the veins ; of this structure 

 are the sinuses of the dura matter, whose external covering is the 

 fibrous tissue of the membrane, and the internal, the serous layer of 



