1058 . THE VASCULAR SYSTEM. 



indeed, it replaces the whole of the dorsalis pedis continuation of the latter vessel ; in others, 

 however, only the lateral tarsal and arcuate branches are so replaced. The peroneal sometimes 

 arises from a stem common to it and the anterior tibial artery. 



The anterior tibial artery may be absent, its place being taken by branches of the posterior 

 tibial and peroneal arteries. It is longer than normal when the popliteal artery divides at a 

 more proximal level than usual, and in those cases it may pass either posterior or anterior to 

 the popliteus muscle. Occasionally the anterior tibial artery and its dorsalis pedis continuation 

 are larger than normal, and the terminal part of the dorsalis pedis takes the place, more or less 

 completely, of the lateral plantar artery. 



The medial plantar artery is sometimes very small, and it may be absent ; its place is 

 taken by branches of the dorsalis pedis or lateral plantar arteries. The lateral plantar 

 artery also may be small or absent, the plantar arch being formed entirely by the dorsalis pedis. 



ABNORMALITIES OF VEINS. 



Abnormalities or variations of veins are as frequently met with as those of arteries, and they 

 are due to similar causes. 



THE SUPERIOR VENA CAVA. 



The superior vena caya may develop on the left side instead of on the right. This peculiarity 

 is due to the persistence of the left duct of Cuvier instead of that on 'the right side, and it is 

 associated with absence of the coronary sinus, which is replaced by the lower part of the left 

 superior vena cava. An exceptional case is recorded in which the opening of the coronary sinus 

 into the heart was obliterated, and the cardiac veins terminated in a trunk which passed upwards 

 to the left innominate vein. This trunk was obviously formed by enlargement of the left duct of 

 Cuvier and the lower part of the left anterior cardinal vein. Not very uncommonly, as the 

 result of the persistence of both ducts of Cuvier, there are two superior venae cavse, the transverse 

 anastomosis which usually forms the left innominate vein being small or entirely absent. In 

 such cases the left innominate vein descends in the left part of the superior mediastinum, crosses 

 the aortic arch, is joined by the left superior intercostal vein, and becomes the left superior 

 vena cava ; which descends anterior to the root of the left lung, and terminates in the lower and 

 back part of the right atrium. It receives the great cardiac vein, and, turning to the back 

 of the heart, replaces the coronary sinus. This arrangement is normal in many mammals. 

 Occasionally^in man the left superior vena cava terminates in the left atrium, and the coronary 

 sinus, which represents a pa/rt of the sinus venosus, has been seen to have a similar ending ; both 

 these abnormal endings must be the result of malposition of the interatrial septum. 



The vena azygos may be formed on the left side ; it then arches over the root of the left 

 lung, and terminates in the left end of the coronary sinus. This is the normal arrangement in 

 some mammals, and it is due to the persistence of the left posterior cardinal vein and the left 

 duct of Cuvier. 



Occasionally the azygos vein is the only vessel by which blood is returned to the heart from 

 the lower limbs and the lower parts of the body walls. In such cases that portion of the inferior 

 vena cava which usually extends from the right renal vein to the heart is absent and the azygos 

 vein is the direct continuation of the inferior vena cava. This condition probably results from 

 the absence of those parts of the inferior vena cava which are usually formed from the right 

 vitelline and the right subcardinal veins, and to the enlargement of the whole of the supra- 

 pelvic portion of the right posterior cardinal vein. 



The hemiazygos and the accessory hemiazygos veins may be absent. In such cases each 

 left intercostal vein opens separately into the vena azygos. On the other hand the hemiazygot- 

 and the accessory hemiazygos veins may form a continuous trunk which may open \>y a transverse 

 anastomosis into the azygos vein, or it may join the left innominate vein. When the hemiazygos 

 and the accessory hemiazygos veins form a single trunk, which receives the left intercostal vein:- 

 and opens into the left innominate vein, the condition is due to the persistence of the whole of 

 the thoracic part of the left posterior cardinal vein and of the lower part of the left anterior 

 cardinal vein. 



Cases also occur in which the thoracic part of the posterior cardinal vein is represented 

 by three instead of two stems, either the hemiazygos or the accessory hemiazygos vein being 

 represented by two vessels. 



The internal jugular vein may be either smaller or larger than normal. In. either case com- 

 pensatory changes in size occur in the transverse sinus and internal jugular vein of the opposite 

 side, or in the external and anterior jugular veins of the same side. 



The external jugular vein is sometimes absent, or it may be smaller than usual ; in botli 

 cases either the anterior or the internal jugular vein is enlarged. In some of the cases in whicl 

 the external jugular vein is small it receives no communication from the posterior facial vein 

 but is merely the continuation of the posterior auricular vein. On the other hand, it may b( 

 enlarged, and receive the whole of the posterior facial vein. 



The anterior jugular vein may be absent, or it may be unusually large, especially in tin 

 lower part of its extent, and after it has received an occasional tributary from the common facia ; 

 vein. 



