178 THE CIRCULATION OF THE BLOOD 



death more or less collapsed and often contain blood. They are usually 

 distributed in a superficial and a deep set which anastomose frequently 

 in their course. 



The coats of veins bear a general resemblance to those of arteries, figure 

 149. Thus, they possess outer, middle, and inner coats. The outer coat is 

 of areolar tissue like that of the arteries, but is relatively thicker. In some 

 veins it contains a few musclar longitudinal cells. The middle coat 

 is considerably thinner than that of the arteries; it contains circular un- 

 striped muscular fibers mingled with a large proportion of yellow elastic and 

 white fibrous connective tissue. In the large veins near the heart the 

 middle coat is replaced for some distance from the heart by circularly 

 arranged striped muscular fibers continuous with those of the auricles. 

 The internal coat of veins consists of a fenestrated membrane lined by 

 endothelium. The fenestrated membrane may be absent in the smaller 

 veins. 



FIG. 150. A, Vein with valves open. B, vein with valves closed; stream of blood passing 

 off by lateral channel. (Dalton.) 



The veins are supplied with valves in certain regions of the body, espe- 

 cially in the arms and legs. The general construction of these valves is 

 similar to that of the semilunar valves of the aorta and pulmonary artery 

 already described. Their free margins are turned in the direction toward 

 the heart, so as to prevent any movement of blood backward. They are 

 commonly placed in pairs, at various distances in different veins. In the 

 smaller veins single valves are often met with, and three or four are some- 

 times placed together or near one another in the larger veins, such as in the 

 subclavians at their junction with the jugular veins. During the period 

 of their inaction, when the venous blood is flowing in its proper direction, 



