CH. XVIII.] 



THE VEINS 



217 



collapsed, owing to their want of elasticity. They are usually dis- 

 tributed in a superficial and a deep set which communicate fre- 

 quently in their course. 



Structure. In structure the coats of veins bear a general 

 resemblance to those of arteries (fig. 228). Thus, they possess outer, 

 middle, and internal coats. 



(a) The outer coat is constructed of areolar tissue like that of the 

 arteries, but it is thicker. In some veins it contains muscular fibres, 

 which are arranged longitudinally. 



Endothelium. 

 Sub-endothelial layer. 

 Elastic intima. 



Middle coat. 



FIG. 22(3. Transverse section of aorta through internal and about half the middle coat. 



(b) The middle coat is considerably thinner than that of the 

 arteries ; it contains circular unstriped muscular fibres, mingled with 

 a few elastic fibres and a large proportion of white fibrous tissue. 

 In the large veins, near the heart, namely, the vence cavce and pul- 

 monary 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 

 membrane, which may be absent in the smaller veins. The 



