256 PRACTICAL ANATOMY. 



descending venae cavee, which empty into the right auricle. 

 The pulmonary veins, four in number, convey oxygenated 

 blood from the lungs to the left auricle. Veins, like 

 arteries, have three coats. The outer or areolar, the 

 middle or muscular or fibrous, and the inner or epithe- 

 lial. These coats are relatively thinner than in the arte- 

 ries ; so that a vein, when cut across, will collapse, while 

 an artery remains patulous and cylindrical. Veins inter- 

 communicate freely and often form large plexuses. All 

 veins, with the few exceptions of those in which the blood 

 gravitates to the heart, when the subject is in the erect 

 posture, are supplied with valves, the positions of which 

 can be distinguished on the exterior of the vein by slight 

 constrictions. The area of the venous system is esti- 

 mated to be two and one-half times as much as the ar- 

 terial system, owing to the larger size of the veins and 

 the greater number of the venous radicles. In general 

 the veins are erratic in distribution, and anomalies are 

 constantly met with. The smaller arteries, as a rule, 

 have two accompanying veins called the venae comites ; 

 the larger arterial trunks, as the popliteal, femoral, axil- 

 lary, and subclavian, have but one vein. The veins of 

 the cranium are called sinuses ; of these there are two 

 kinds, those which run in the substance of the bone 

 and those which are formed by a separation of the leaflets 

 of the dura mater. 



The veins are classified into those of the head, neck, 

 upper extremities, trunk, and lower extremities. Those 

 of the head and of the neck are the superficial and deep. 

 The superficial set are temporal, facial, internal maxil- 

 lary, posterior auricular, occipital, and temporo-maxillary. 

 These veins follow the course of the arteries ; they 

 freely communicate with each other, and at the angle of 

 the lower jaw unite with more or less regularity, to form 





