850 HUMAN ANATOMY. 



the vessel becomes reduced to the slender a. comes nerval ischiadic! of the adult. Its lower por- 

 tions, which become the popliteal and peroneal arteries, now seem to be the continuation of the 

 femoral (/.^., the saphenous). 



From the lower part of the popliteal a branch arises which anastomoses with the saphenous 

 and, together with the lower part of that artery, forms the posterior tibial, the upper part of the 

 saphenous then disappearing except in so far as it is represented by one of the branches of the 

 anastomotica magna. The anterior tibial is a late formation resulting from the fusion of an upper 

 and lower branch from the peroneal which perforate the interosseous membrane ( C ), the con- 

 nection of the lower branch with the peroneal degenerating after the anastomosis, except in so 

 iar as it persists as the anterior peroneal artery {D). 



THE VEINS. 



The veins are those vessels which receive the blood from the capillary net-work 

 and return it to the heart. 



Compared with the arteries, they present many differences, both of structure 

 (page 677) and arrangement. Their walls are much thinner, so that the color of 

 the blood which they contain shows through, and they are readily compressible to the 

 extent of a complete obliteration of their lumen and are also exceedingly dilatable. 

 Notwithstanding their thinness, they are less easily ruptured by over-distention than 

 are the arteries and are capable of undergoing a remarkable elongation, those of an 

 adult withstanding an extension to at least 50 per cent, more than their original length 

 without losing their elasticity — a property which explains the more direct course 

 taken by the veins as compared with the arteries in mobile portions of the body {e.g., 

 the facial vein as compared with the artery). Indeed, it seems that the veins when 

 in place in the body are always stretched to a considerable extent, the cephalic vein, 

 for example, contracting when removed from the body to 40 per cent, of its length 

 in the extended arm (Bardeleben). 



The most striking structural peculiarity of the veins, however, is the occurrence 

 in them of semilunar valves, arranged usually in pairs, with their cavities directed 

 towards the heart. These valves resemble in their general form the semilunar \'alves 

 of the systemic and pulmonary aortae, and, as in those vessels, the veins are somewhat 

 enlarged immediately above the attachment of each pair, so that the blood may 

 readily flow behind the valves, force their free margins together and so occlude 

 the vessel. These valves play an important part in directing the flow of blood in 

 the veins towards the heart, since, in the event of any pressure, such as that exerted 

 by a contracting muscle, acting on the vein, they will prevent a backward flow of 

 blood towards the capillaries. Valves do not occur in veins of less than i mm. in 

 diameter and are, also lacking in many of the larger trunks, such as the superior and 

 inferior venae cavae, the pulmonary and the portal veins. In general they are more 

 numerous in the veins of the limbs than in those of the trunk and in the deep than in 

 the superficial vessels. 



Their number in any vessel in which they normally occur is subject to con- 

 siderable variation in different individuals and even on opposite sides of the body in 

 the same subject. It seems probable that this variation is brought about by a 

 degeneration of a greater or less number of the pairs originally present, since in the 

 majority of the veins the number of valves diminishes with age (Bardeleben), and 

 even in adult bodies evidence of degeneration may be seen in the insufficiency of 

 some of the valves or even in their perforation. It is possible, therefore, that the 

 arrangement of the valves in the adult is a secondary condition, derived from one in 

 which the valves were much more numerous and were situated at regular intervals 

 along the vessels. In favor of this view it has been found (Bardeleben) that in certain 

 veins the valves in the adult are separated by intervals either of a definite length or of 

 a multiple of this, the length of the intervals standmg in relation to the length of the 

 part or, in general, to the height of the individual m which the vein occurs. Thus, in 

 a man measuring 160 mm. in height, the valves "of the right long saphenous vein were 

 separated by intervals which were all approximately multiples of 6.85 mm. in length, 

 while the intervals separating the valves of the right cephalic vein were approximately 

 multiples of 5.2 mm.; and in a male child 8r cm. in height, the valves of the right 

 long saphenous vein were separated by intervals of 3 mm. or some multiple of this. 



