7 
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 primitive jugular vein. Not very uncommonly, as the 
result of the persistence of both ducts of Cuvier, there are two superior ven cavee, the transverse 
anastomosis which usually forms the left innominate vem being small or entirely absent. In 
these 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 ; this latter vessel descends in front of the root of the left lung, and terminates in the 
lower and back part of the right auricle. It receives the great cardiac vein, and, turning to the 
back of the heart, replaces the coronary sinus. This arrangement is normal in many mamunals. 
Occasionally in man the left superior vena cava terminates in the left auricle, and the coronary 
sinus, which represents a part of the sinus venosus, has been seen to have a similar ending ; both 
these abnormal endings must be the result of malposition of the interauricular septum. 
The vena azygos major 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 cardinal vein and the left duct of 
Cuvier. 
The azygos veins may be reduced or increased in number. — In the former case there may be 
only one azygos vein which receives the intercostal veins of both sides, or there may be two 
azygos veins, a right and a left, the left usually being divided into upper and lower sections 
which are connected by a smaller intermediate portion, and united to the right vein by one or 
niore transverse anastomoses, or 1t may terminate by joing either the left innominate vein or 
the left superior vena cava. The small intermediate section on the left side may form a separate 
vessel, and then the number of the azygos veins is increased to four, each of the three left veins 
terminating in the right vein. W lien there is only one azygos vein the portion of the left 
cardinal vein, from which, usually, the vertical portions of the left azygos veins are formed, has 
disappeared, and the left intercostal veins open into the right azygos vein by separate transverse 
anastomoses, as in the case of the left lumbar veims and the inferior vena cava. On the other 
hand, when (Mien is only one left azygos vein the intermediate sections of the ahr acic part of the 
left cardinal vein and one or more of the transverse anastomoses have persisted, whilst, when the 
left azygos terminates in the left innominate vein, the transverse anastomoses have disappeared, 
and the lower part of the left primitive jugular vein has remained patent. 
Occasionally the vena azygos major takes the place of the upper part of the inferior vena 
cava, and the whole of the left cardinal vein is enlarged ; in these cases the upper portion of the 
normal inferior vena cava is absent or exceptionally small. 
The internal jugular vein is sometimes smaller or larger than normal. In either case com- 
pensatory changes in size occur in the lateral 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 both 
cases either the anterior or internal jugular veins are enlarged. In some of the cases in which 
the external jugular vein is small it receives no communication from the temporo-maxillary 
vein, but is merely the continuation of the posterior auricular vein. On the other hand, it may 
be enlarged, and receive the whole of the temporo-maxillary vein. 
The anterior jugular vein may be absent, or it may be unusually large, especially in the 
lower part of its extent, and after it has received an occasional tributary from the common facial 
vein. 
The temporo-maxillary vein may terminate entirely in the common facial vein, or in the 
extermial or the internal jugular vei. It may be very small, and occasionally it is absent. 
Variations of the cranial blood sinuses are not numerous. One ,lateral sinus may be 
absent or very small when, as a rule, that of the opposite side is enlarged. The inferior longi- 
tudinal, the occipital, or the spheno- parietal sinuses may be absent, and there may be an additional 
petro-squamous tributary to the lateral sinus. The petro-squamous sinus, when present, is the 
remains of that portion of the primitive lateral simus which crossed the tempor al bone, passed 
through the post-condyloid foramen and terminated in the primitive jugular vein. Very occasion- 
ally in the human adult it still pierces the skull behind the condyle of the jaw, and terminates 
in the temporo-maxillary vein, and this is the normal arrangement in some mammals. 
904 THE VASCULAR SYSTEM. 
THE VEINS OF THE UPPER EXTREMITY. 
The superficial veins of the forearm are extremely variable ; any of them may be absent, 
but most commonly it is the median or the radial vein which is wanting. The median cephalic 
and the cephalic veins may be small or absent, and on the other hand the cephalic vein may be 
larger than usual. Moreover the cephalic vein may end in the external jugular vein, its original 
termination ; ; or it may be connected with the external jugular vein by an anastomosing channel 
which sometimes passes over the clavicle and sometimes through that ‘bone. 
The basile vem is sometimes larger and sometimes smaller than usual, and it may pierce the 
fascia of the arm at a higher or at a lower level than is customary. 
The vene comites of the arteries of the upper extremity generally terminate at the lower 
border of the subscapularis, where they join the axillary vein, but they may end above or below 
the position of their usual termination. 
The subclavian vein sometimes passes behind instead of in front of the scalenus anticus 
muscle, and it has been seen passing between the clavicle and the subclavius muscle. 
