336 VEINS OF THE HEAD AND NECK. 



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 difference 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 instances 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 

 become 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 ap- 

 pearance. The valves are most numerous in the veins of the extremities, 

 particularly 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 cerebral 

 veins : they are also absent in the large trunks, as in the vense cavse, venae 

 azygos, innominatse, and iliac veins. 



Sinuses are venous channels, excavated in the structure of an organ, 

 and lined by the internal coat of the veins ; of this structure are the sinuses 

 of the dura mater, whose external covering is the fibrous tissue of the 

 membrane, and the internal, the serous layer of the veins. The external 

 investment of the sinuses of the uterus is the tissue of that organ ; and that 

 of the bones, the lining membrane of the cells and canals. 



Veins, like arteries, are supplied with nutritious vessels, the vasa vaso- 

 rum ; and it is to be presumed that nervous filaments are distributed in 

 their coats. 



I shall describe the veins according to the primary division of the body, 

 taking first, those of the head and neck ; next, those of the upper extre- 

 mity ; then, those of the lower extremity ; and lastly, the veins of the 

 trunk. 



VEINS OF THE HEAD AND NECK. 



The veins of the head and neck maybe arranged into three groups, viz. 

 1. Veins of the exterior of the head. 2. Veins of the diploe and interior 

 of the cranium. 3. Veins of the neck. 



The veins of the exterior of the head are the 



Facial, 



Internal maxillary, 



Temporal, 



Temporo-maxillary, 



Posterior auricular, 



Occipital. 



The Facial vein commences on the anterior part of the skull in a venous 

 plexus, formed by the communications of the branches of the temporal, 

 and descends along the middle line of the forehead, under the name of 

 frontal vein, to the root of the nose, where it is connected with its fellow 

 of the opposite side by a communicating trunk which constitutes the nasal 

 3.rch. There are usually two frontal veins, which communicate by a trans- 



