OF THE GREAT ANTERIOR VEINS. 
1.37 
cardinal vein and the left canal of Cuvier remain open to form the left azygos vein 
present in that animal; whilst that part of the left primitive venous trunk which is 
situated between the canal of Cuvier and the junction of the left subclavian and 
jugular veins, disappears, and only a subordinate twig is subsequently found in its 
place, which ends in a small left superior intercostal vein. 
Lastly, in those animals which in the adult condition have a right and left superior 
vena cava, the left primitive jugular vein, together with the corresponding canal of 
Cuvier, remains pervious throughout life. This knowledge of the common type of 
formation of the veins in the Vertebrata, and of their metamorphosis in certain species 
by partial occlusion, suggested to Rathke the explanation of the occasional occurrence 
of double vena cava superior in the human subject as the result of an arrest of deve- 
lopment. Nevertheless, the details of this metamorphosis have not been fully indi- 
cated, nor, as far as I am aware, have any persistent remnants of the foetal structure 
been recognized in the adult. By Rathke himself the left primitive vein is said, in 
Man, to diminish and entirely disappear from opposite the left end of the transverse vein 
of the neck, down to the heart-, but this seems to have been an inference from the 
known adult condition, and I do not knoyi^ that either he or others have given any 
description or delineation of the development of these veins, as actually traced in the 
human embryo. 
The metamorphosis of the primitive lateral and symmetrical venous trunks in the 
higher Mammalia and in Man may be said to consist of two fundamental changes, 
viz. a. the formation of the cross branch or communication in the neck, and, h. the 
occlusion of a greater or less portion of the left primitive venous trunk. Besides 
this, however, there are, during embryonic life, c. certain concurrent and subsequent 
alterations in the size, position and direction of the venous trunks which finally re- 
main pervious. Lastly, d. there are the changes which take place after birth. 
Development in the Sheep. 
a. Formation of the Transverse Communication in the Neck. 
In embryos measuring -^^ths of an inch, well-preserved in spirit, and in which the 
blood was hardened in the veins, no appearance of a cross branch was discernible. 
The earliest satisfactory indications of its commencement were met with in embryos 
from -Ifths to -g^ths of an inch in length. In these (Plate 11. figs. 1, 2, 3), two little 
spur-shaped points, filled with hardened blood, projected towards each other from the 
inner borders of the jugular trunks (a a') itnmediately above the commencing peri- 
cardium, on a level with the subdivision of the ascending aorta. In some cases, no 
intermediate portion of vein between these points could be detected ; but in others 
the connection was evidently completed by a material, more opake than that around, 
which could be often raised as an indistinct narrow cord, containing hovvever no 
MDCCCL. 
T 
