OF THE GREAT ANTERIOR VEINS. 
149 
All the varieties of arrangement hitherto observed in the great anterior veins of the 
Mammalia may be classified according to the amount of deviation which they present 
from the type, pointed out by Rathke, as originally common to all the Vertebrata, 
viz. that of four lateral primitive trunks. 
The modifications of this type observed in the cold-blooded Vertebrata are strictly 
of a subordinate kind, affecting merely the relative size which particular vessels ulti- 
mately acquire. Even in birds there is no fundamental deviation from the original 
type. The four primitive lateral veins persist. No transverse branch is formed across 
the root of the neck, though a free communication exists between the jugular veins 
just beneath the skull. The right and left superior cavae remain independent of each 
other as originally laid down, and each receives its own azygos vein. 
Amongst the Mammalia, however, there appears to be no instance in which some 
change from the common primitive type does not take place. Throughout the whole 
class, so far as is known, there is the addition of a communicating branch across the 
root of the neck, between the two anterior primitive venous trunks. It is found even 
in the low bird-like Monotremes ; and, should it prove to be universal, it will con- 
stitute one characteristic mark of the mammalian venous system. 
The formation of this transverse communicating branch, which of necessity jore- 
cedes the occlusion of the left primitive vein in the higher mammalian embryo, ap- 
pears as the first, the simplest and the only change in the lowest forms of the mam- 
malian series, Superadded to this preliminary step in the foetal development, and 
superadded also in the highest forms of adult Mammalia and in Man, is found an- 
other change, depending on one of two modes of partial occlusion of the left anterior 
primitive venous trunk. 
In this way three different permanent conditions arise. In all of them the trans- 
verse communication in the neck exists. The right venous trunk always constitutes 
the vena cava superior of that side ; but the left vein either forms, — A, a similar large 
venous trunk on that side, named a left vena cava superior, which receives the left 
jugular and subclavian veins, the left intercostals and certain cardiac veins ; or B, it 
is reduced to a smaller left venous trunk, which receives merely the left intercostal 
veins and some cardiac veins ; or, C, it remains as a still smaller vessel, receiving 
only a few cardiac veins from the substance of the heart. These three conditions 
accordingly are distinguished by severally presenting — 
A. A right and a left vena cava superior. 
B. A right vena cava superior and a left azygos venous trunk. 
C. A right vena cava superior and a left cardiac venous trunk or coronary sinus. 
the veins from the substance of the heart terminate, and virhich will then be recognized as the great coronary 
vein. This is the condition in Man and in most Mammalia. In fact, in no mammalian does the left canal of 
Cuvier entirely disappear. Even in cases where by far the largest portion of it is obliterated, that part which 
runs along the posterior transverse furrow of the heart remains as the trunk of the cardiac veins.” I have 
availed myself of Dr. Bardeleben’s memoir, to introduce some additional examples of varieties of the great 
veins in animals. 
