PERIPHERIC ORGANS OF THE CIRCULATION. 163 



Veins. The structure of the veins closely resembles that 

 of the arteries ; they are, however, distinguished from them 

 by containing much less elastic tissue, and thus have no 

 tendency to remain open, even in the dead body, after the 

 blood has run out. 



These vessels are, nevertheless, very contractile, . but the 

 muscular element in them is unequally distributed. Their 

 contractions may be easily observed ; for instance, we see the 

 veins of the hand contract and shrink when immersed in cold 

 water; a sudden blow, or slight percussion of a subcutaneous 

 vein, produces immediate contraction, followed shortly by 

 paralysis which causes the vessel to dilate ; and we sometimes 

 see these two phenomena succeed each other irregularly. 

 These contractions of the veins assist the circulation, but 

 their rhythm is never regularly intermittent ; there is really 

 no systole or diastole, properly so called. The effect of the 

 contraction is to diminish the size of the vessel, and to drive 

 the blood always in the same direction, on account of the 

 valves, of which we shall speak presently. 



The veins are very dilatable, owing to the elasticity of the 

 muscular elements which compose their coats, and we may 

 say that one of their principal functions is to promote the 

 easy flow of blood from the capillaries. Thus we see that 

 the veins, beside taking the part of conduits, also serve as a 

 reservoir, especially at the summit of the venous cone, in 

 the auricle. For this purpose, the veins are sometimes 

 developed in the form of plexus, and this arrangement in- 

 creases their capacity as a whole ; these plexus may also be 

 sometimes intended to warm the parts in which they are 

 situated, as we shall see is the case with the choroid plexus 

 (heating apparatus of the retina) ; but their object is gener- 

 ally to prevent stagnation in the capillaries, and they are 

 therefore arranged and grouped in parts where they will riot 

 be subject to compression, as, for instance, behind the body 

 of the vertebras (between this body and the posterior com- 

 mon ligament.) Moreover, the ramified form and the anas- 

 tomoses of these plexus prevent any partial and local com- 

 pression from impeding the return of the circulation, the 

 blood finding always an easy passage through the vessels 

 which have remained open. Finally, there are some veins 

 whose coats are inextensible and incompressible, so that 

 nothing can hinder the circulation in them ; and, on the other 

 hand, they cannot swell, so as to compress the adjacent 



