VEIN. 



of the bodies of the vertebrae remarkable 

 plexuses exist. (Fig* 87 I.) 



Fig. 871. 



Diploic Veins in a Vertebra. 



a, great anterior spinal veins; 6, posterior spinal 

 veins ; c, transverse branches ; d, venae basis ver- 

 tebrarum ; e, vein on the surface of the body of 

 the vertebra. (After Breschet.) 



Fig. 872. 



t^^^^^^Jr #<* 



Parietal Bone of Foetus at nine months, exhibiting 

 the veins. (After Breschet.) 



IV. FUNCTION OF VEINS. In function 

 veins may be considered as having a triple 

 office : they are passive organs of circulation; 

 they are diverticula or reservoirs for blood ; 

 and they are agents of absorption. 



Circulation in the veins consists of the 

 passive and equable transmission of blood 

 along their tubes. " When we consider the 

 great size of the veins, compared with the 

 arteries, we must conclude that the blood 

 flows but slowly in the venous system ; that, 

 from the narrowness of the trunks of the 

 veins near the heart, the blood must be ac- 

 celerated as it approaches the heart ; and 

 that receiving the impulse from the ventricle, 

 it must take a rapid course through the 

 arteries, until, again approaching the extreme 

 branches of the arteries, and passing into the 

 veins, its motion becomes more languid and 

 slow. In youth, as the size of the veins is 

 not in so great a proportion to the arteries 

 as in advanced life, the blood must be in 

 more rapid circulation : but in old age, owing 

 to the largeness of the veins and the accu- 

 mulation of blood in them, it moves slowly 

 through the venous system, and is almost 

 stagnant in the dilated veins and in the 

 sinuses. 



" There is no pulsation to be observed in 

 the veins, but what the)' receive from con- 

 tiguous arteries. There is no pulsation in the 

 veins because they are removed from the 

 heart ; because they do not receive the shock 

 of the heart's action in their trunk, but only 

 by their widely-spread branches ; because the 

 contraction of the heart and of the arteries so 

 alternate with each other, as to keep up a 

 perpetual and uniform stream of blood into 

 the veins." (Sir C. Bell's Anatomy, vol. ii. 

 p. 284.) 



As diverticula and reservoirs of blood the 

 veins must be considered as performing a most 

 important service in the ceconomy of the cir- 

 culation. There are numerous circumstances 

 both in health and disease which either tem- 

 porarily or permanently disturb the balance 

 of v the circulation, and thereby displace and 

 choke up large quantities of blood ; and, were 

 it not for the reservoirs, diverticula, and lateral 

 compensating channels, which are afforded by 

 large, bulging, and dilatable veins, by the enor- 

 mous area of the common venous cavity, and 

 by the numerous and elaborate inosculations 

 and plexuses which these vessels every where 

 exhibit, such interruptions might lead to 

 consequences, permanent, irremediable and 

 destructive. 



The whole subcutaneous system of veins 

 must be looked upon as a series of compen- 

 satory channels. The cavities of the deep 

 veins are quite sufficient to return the blood 

 that is sent to them by vessels of smaller 

 aggregate calibre. But during muscular ex- 

 ertion, &c., these deep vessels are impeded 

 by pressure, and then the blood finds a colla- 

 teral route by means of the subcutaneous veins. 

 So again, when from cold, or other causes, the 

 skin-muscle contracts and presses on the 

 cutaneous veins they are more or less emptied, 

 and their office is resumed by the deep veins : 

 they thus keep up a mutual reciprocation of 

 function. 



The large internal veins of the body j the 



