434 ANGIOLOGY. 



Veins are superficial and deep. The former lie immediately 

 below the skin, and are mostly unaccompanied by arteries; the 

 latter are situated deeply, and usually related with arteries, and 

 hence they are called satellite veins. Two veins sometimes 

 accompany one central artery. Veins anastomose more freely 

 than arteries ; and in the head they form dilated pouches or 

 sinuses, and plexuses in the palate and foot ; their coats are 

 thinner, but stronger than those of arteries, and collapse when 

 cut across or emptied. After death, they are usually distended 

 with blood, their walls being thin, semi-transparent, and elastic, 

 generally consisting of three tunics, arranged for the most part 

 like those of an artery. 



The majority of veins are provided with valves, somewhat 

 similar to the semilunar ones at the origins of the great arteries. 

 These are folds of the lining membrane, strengthened by fibrous 

 tissue, and present an adherent and a free semilunar border when 

 the valves are shut. When distended they have a concave 

 surface towards the heart, and a convex one towards the origin 

 of the vein ; they may consist of from one to five, but generally 

 of two flaps, opposed to each other. The walls of the veins 

 immediately above the valves are pouched into .sinuses, which 

 produces a knot or swelling. Their use is to favour the 

 course of the blood and oppose its reflux from the heart. During 

 its normal flow, they are placed against the walls, offering no 

 obstacle to the current. When the current is intercepted, the 

 valves are pressed back, the refluent column of blood gets between 

 the concave surfaces of the flaps and the dilated pouches, pressing 

 the former inwards until their edges, meeting in the centre, 

 arrest the retrograde current. Valves are irregularly distributed, 

 and are most numerous near the convergent points ; they are 

 absent in the pulmonary, cranial, spinal, pedal, and osseous veins, 

 in the foetal umbilical vessels, and venae cavse. They are onl}- 

 rudimentary in the portal, rare in the azygos, spermatic, uterine, 

 and ovarian veins ; but numerous, large, and complete in those 

 of the extremities. Their coats, like the arterial, are supplied 

 Avith capillary vasa vasorum. Their nerves, few and only trace- 

 able on the larger vessels, come from the sympathetic system. 

 Veins may originate from arteries without the intervention of 

 capillaries, as in the erectile tissue of the penis. 



