1060 THE VASCULAR SYSTEM. 



THE VEINS OF THE INFERIOR EXTREMITY. 



The great saphenous vein is not subject to much variation, but the small saphenpus vein 

 may terminate by joining the great saphenous, or, after piercing the deep fascia in the distal part 

 of the thigh, it may ascend and join the inferior glutseal vein or one of the tributaries of the 

 profunda vein. 



The venas comites are generally described as terminating in the lower extremity, at the 

 distal part of the popliteal fossa, but they may ascend as far as the femoral trigone ; as a matter 

 of fact, one or more small additional veins usually accompany the popliteal and femoral arteries, 

 although as a rule there is only one large popliteal and one large femoral vein. 



In a few cases the popliteal vein does not pierce the distal part of the adductor magnus, but 

 ascends behind that muscle and becomes continuous with the profunda vein, the femoral artery 

 being unaccompanied by any large vein during its passage through the adductor canal. 



ABNORMALITIES OF THE LYMPH VESSELS. 



Variations of the glands and smaller vessels of the lymphatic system are so common that 

 they can hardly be regarded as abnormalities ; variations of the larger vessels, however, are 

 comparatively rare. This is especially the case with respect to the two terminal trunks, the thoracic 

 duct and the right lymph duct, the abnormalities of which are interesting and important. 



When the arch of the aorta is on the right side instead of on the left side, the thoracic duct 

 terminates usually in the right innominate vein, in which case it receives the tributaries which 

 usually open into the right lymph duct, whilst the corresponding area on the left side is drained 

 by lymph vessels terminating in a left lymph duct which opens into the commencement of 

 the left innominate vein. A similar arrangement of the terminal lymph trunks sometimes occurs 

 even when the arch of the aorta is in its normal position on the left side. In either case the 

 thoracic duct may commence in the usual way, and after reaching the level of the fifth thoracic 

 vertebra continue upwards on the right side, instead of crossing to the left side of the vertebral 

 column ; more rarely it commences on the left side and crosses over to the right at a higher level. 



In one case in which the thoracic duct opened into the right innominate vein, instead of the 

 left, no trace of a lymph duct was discovered on the left side. 



Occasionally the thoracic duct commences and terminates in the usual manner, but crosses the j 

 vertebral column immediately after its origin and ascends on the left side. 



Not uncommonly there is no distinct cisterna chyli, in which case the terminal lymph 

 vessels of the abdomen merely unite to form a larger vessel which does not present any obvious 

 dilatation, and from which the thoracic duct is continued. The terminal lymph trunk may open 

 into the internal jugular vein, previous to its junction with the subclavian, instead of into the | 

 commencement of the innominate vein. 



Occasionally the thoracic duct is double, either in the whole or in part of its extent, and 

 sometimes it breaks up into a plexus of vessels which may reunite into a single trunk in the 

 upper part of the thorax. Both the thoracic duct and the right lymph duct may, before 

 terminating, divide into branches which, though sometimes reuniting on each side into a single 

 trunk, not infrequently open separately into the great veins at the root of the neck. 



As a rule the thoracic duct joins the commencement of the left innominate vein, but it maj ; 

 end in the internal jugular, vertebral, or subclavian veins of the left side ; whilst very rarely, i j 

 opens into the vena azygos. 



NOTES. 



1 (see p. 995). It is stated by H. Downey (Anat. Record, 1915)"~that there are no endotlielia 

 cells covering the trabeculae of lymph glands. He asserts that the cells described as enelothelu 

 are connected with the fibrils of the reticulum. 



2 (see p. 1025). More recent evidence throws doubt on this statement ; it appears probabl 

 that blood and blood-vessels may be formed in situ in the embryonic region. 



