THE VEINS. 851 



A more readily appreciable relation of the valves is that which they bear to the 

 branches which open into the vein, a pair of valves being found immediately distal to 

 the entrance of each collateral vein ; and, furthermore, a pair, or at least a single valve, 

 very generally occurs at the termination of a vein, where it enters either a larger 

 stem or the heart. These terminal valves are present in certain veins which other- 

 wise are quite destitute of valves, as, for instance, in the internal jugular, the internal 

 maxillary, and the vertebral veins. 



It has already been noted that valves are entirely wantipg in certain veins. Among these 

 are the sinuses of the cranium, the cerebral, ophthalmic, periosteal, pulmonary, bronchial, 

 portal, renal, uterine, ovarian, and innominate (brachio-cephalic ) veins, and the superior and 

 inferior vense cavae. Furthermore, they are usually absent in the internal iliac and facial veins, 

 although occasionally they occur in both. 



In their position and arrangement also the veins differ noticeably from the 

 arteries. While veins are usually to be found accompanying the arteries, enclosed with 

 them in a common fibrous sheath, additional veins of considerable size are abundant 

 immediately beneath the skin — a condition which is almost entirely foreign to the 

 arteries. Furthermore, although in a general way a vein may pursue the same 

 course as an artery, it may lie at some little distance from the latter and fail to follow 

 its course exactly. This is true, for instance, of the facial and the lingual veins 

 and also of the subclavian vein, which is separated from the corresponding artery by 

 the scalenus anticus muscle ; this likewise applies to the veins at the root of the neck 

 which accompany in a general way the branches of the subclavian artery, but open into 

 the innominate vein instead of the subclavian. In many cases the veins which accom- 

 pany arteries are double, one lying on either side of the artery and forming what are 

 generically known as venae comites (venae comitantes ). The causes which determine 

 this double condition are obscure. The arrangement is not found in the larger 

 venous trunks, occurring, for instance, in the leg only below the knee and in the 

 arm only as far up as the middle of the brachium ; size alone, however, does not 

 seem to be the determining factor, since the internal mammary and epigastric veins 

 are double, while the intercostal and lumbar veins, almost of the same size as the 

 former, are single. Nor does the quality of the tissue in which the veins occur 

 determine their duplication, for those which are embedded within the muscles of the 

 tongue are doubled, while those within the heart musculature are single ; again, 

 while, as a rule, the veins which occur in fibrous tissue — as, for mstance, the menin- 

 geal veins — are double, yet those of the skin are single. Finally, it may be noted 

 that there are exceptions to the rule that the veins which occur in the cavities of the 

 body are single, since a duplication is found in the spermatic veins and also in those 

 of the gall-bladder. 



Not only doubling of many of the veins occurs, but a prevailing tendency exists 

 towards extensiv^e anastomoses far surpassing that displayed by any of the arteries. 

 Even in the cases of the larger proximal trunks communications exist, those between 

 the pulmonary and bronchial veins and that between the superior and inferior venae 

 cavae by way of the azygos being examples. In the smaller vessels the anastomoses 

 are often so numerous as to result in the formation of plexuses. Venae comites are 

 united by frequent cross-connections, sometimes so numerous as to present the ap- 

 pearance of a plexus surrounding the artery. Complicated venous plexuses also 

 accompany the various ducts of the body, as, for example, the parotid ducts, the 

 ureters, and the vasa deferentia. In addition, extensive venous plexuses occur in 

 various regions of the body, as in the neighborhood of its orifices, in the terminal 

 phalanges of the fingers and toes, in the diploe of the skull, in the spinal canal, in 

 the pelvis, and in connection with the genito-urinary organs. Since the larger 

 trunks usually arise at several points both from these and from the wider-meshed 

 plexuses occurring elsewhere, opportunity is thus afforded for the return of the 

 blood to the heart by different paths — an arrangement explaining the frequent ineffi- 

 ciency of a ligation of even large trunks to prevent venous hemorrhage. 



Special mention should be made of one set of the venous channels — namely, 

 the sinuses of the dura mater — which establish communication between the cerebral 

 and ophthalmic veins and the internal jugular. They are channels contained within 



