2l6 



THE CIRCULATORY SYSTEM. 



[CH. XVIII. 



veins, which enter the left auricle. The total capacity of the 

 veins diminishes as they approach the heart; but, as a rule, 

 their capacity is two or three times that of their correspond- 

 ing arteries. The pulmonary veins, however, are an exception 

 to this rule, as they do not exceed in capacity the pulmonary 

 arteries. The veins are found after death more or less collapsed, 

 owing to their want of elasticity. They are -usually distributed 

 in a superficial and a deep set which communicate frequently in 

 their course. 



Structure. In structure the coats of veins bear a general re- 

 resemblance to those of 

 arteries (fig. 219). Thus, 

 they possess outer, middle, 

 and internal coats. 



(a) The outer coat is con- 

 structed of areolar tissue 

 like that of the arteries, 

 but it is thicker. In some 

 veins it contains muscular 

 fibres, which are arranged 

 longitudinally. 



(b] The middle coat is 

 considerably thinner than 

 that of the arteries; it 

 contains circular unstriped 

 muscular fibres, mingled 

 tfith a few elastic fibres 

 and a large proportion of 

 white fibrous tissue. In the 

 large veins, near the heart, 

 namely, the venae cavce and 



pulmonary veins, the middle coat is replaced, for some distance 

 from the heart, by circularly arranged striped muscular fibres, 

 continuous with those of the auricles. The veins of bones, and of 

 the central nervous system and its membranes have no muscular 

 tissue. 



(c) The internal coat of veins has a very thin fenestrated mem- 

 brane, which may be absent in the smaller veins. The endothe- 

 lium is made up of cells elongated in the direction of the vessel, 

 but wider than in the arteries. 



Valves. The chief influence which the veins have in the 

 circulation, is effected with the help of the valves, contained in all 

 veins subject to local pressure from the muscles between or near 

 which they run. The general construction of these valves is 



Fig. 218. Ramification of nerves and termination 

 in the muscular coat of a small artery of the 

 frog. (Arnold.) 



