THE VEINS 



173 



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

 distributed in a superficial and a deep set which communicate 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 miter coat is 

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

 veins it contains muscular cells arranged longitudinally. 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 bv endothelium. The 



iv ** 



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

 off by lateral channel. (Dalton.) 



fenestrated membrane may be absent in the smaller veins. The veins are 

 supplied with valves in certain regions of the body, especially in the veins of 

 the arm 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 sometimes 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 in- 

 action, when the venous blood is flowing in its proper direction, they lie by 



