304 CIRCULATION. 



lower extremities, by the inferior vena cava, and from the 

 substance of the heart, by the coronary veins. 



Structure and Properties of the Veins. The structure of 

 the veins is somewhat more complex and difficult of study than 

 that of the arteries. Their walls, which are always much 

 thinner than the walls of the arteries, may be divided into 

 quite a number of layers ; but for convenience of physiologi- 

 cal description, we shall regard them as presenting three dis- 

 tinct coats. These have properties which are tolerably distinc- 

 tive, though not as much so as the three coats of the arteries. 



The internal coat is a continuation of the single coat of 

 the capillaries and the internal coat of the arteries. It is a 

 simple, homogeneous membrane, somewhat thinner than in 

 the arteries, lined by a delicate layer of epithelium. 



The middle coat is divided by some into two layers : an 

 internal layer, which is composed chiefly of longitudinal 

 fibres ; and an external layer, in which the fibres have a cir- 

 cular direction. These two layers are intimately adherent, 

 and are quite closely attached to the internal coat. The 

 longitudinal fibres are composed of the white fibrous tissue 

 mingled with a large number of the smallest variety of the 

 elastic fibres. This layer contains a large number of capil- 

 lary vessels (vasa vasorum). The circular fibres are com- 

 posed of the elastic tissue, some of the same variety as found 

 in the longitudinal layer, some of medium size, and some in 

 the form of the " fenestrated membrane." In addition, there 

 are white inelastic fibres interlacing in every direction and 

 mingled with capillary blood-vessels, and the unstriped or 

 involuntary muscular fibres, which are always circular in 

 their direction. The muscular fibres are relatively much 

 less numerous than in the arterial system. They are most 

 abundant in the superficial veins. 



The external coat is generally composed simply of the 

 white fibrous tissue, like the corresponding coat of* the arte- 

 ries. In the largest veins, particularly those of the abdominal 



