OF THE GREAT ANTERIOR VEINS. 
139 
b. Occlusion of a 'portion of the Left Primitive Vein. 
Before the commencement of this stage of the metamorphosis, the jugular trunks 
are closely approximated and open freely into each other by means of the wide and 
very short communicating branch (fig. 7, d). Previously to this change also, that por- 
tion of both primitive jugulars which is situated below the cross branch, becomes 
elongated in accordance with the growth of the thorax, and now, inclining outwards, 
descends through a much longer course upon the pericardium, beneath the pleura, to 
join the cardinal vein of its own side and end in the corresponding canal of Cuvier. 
It is just this portion of the left primitive jugular vein, viz. from the transverse branch 
in the neck to the commencement of the left canal of Cuvier, which becomes closed 
in the Sheep. In embryos measuring 1 inch and 1 line (fig. 7)? the vessel («') has 
either not begun, or is only just beginning to shrink; but so rapidly does the pro- 
cess take place, that in others scarcely 1 line longer (fig. 8), the vessel is already 
closed, and now appears as a semitransparent cord (o) extending from the point of 
junction of the primitive jugular trunks in the neck, to the arch formed by the left 
cardinal vein {V) and left canal of Cuvier (c'). This cord is finer at its lower end ; 
it lies at first in front of the aorta, and then passes down close to the pericardium, 
on the left side of the ductus arteriosus ; it is in contact, below, with the left pleura ; 
the par vagum descends behind it, and it is crossed by the phrenic nerve. Close 
behind the upper end of this cord, but not actually connected with it, a considerable 
vein, formed by the left vertebral and a large perforating intercostal from the back, 
joins the under side of the left innominate vein. 
In later embryos the above-mentioned cord becomes less and less distinct ; in an 
embryo measuring 1^ inch long (fig. 10) it cannot be continuously traced, and in a 
foetal sheep, 4 inches in length (from the vertex to the tail), it is not perceptible. 
In its place, however, there is found a long ridge or elevation of the pericardium, 
containing fibrous tissue, which may be followed from below upwards, close to the 
left side of the large ductus arteriosus, in a direction towards the middle of the root 
of the neck. Above the pericardial sac, the traces of it are almost entirely lost. 
c. Concurrent and subsequent Changes in the Pervious V essels. 
In size. As the left primitive vein is undergoing occlusion, the lower part of the 
right jugular vein (figs. 8, 10, a), and the right canal of Cuvier, simply enlarge, as 
Rathke states, to form the superior vena cava. The adjoining part of the right car- 
dinal vein, having first shrunk in consequence of the wasting of the Woltfian body, 
remains for a considerable time as a right azygos vein, equaling in size the left 
azygos ; but about the middle of foetal life it is smaller than the vein on the left 
side, and afterwards, its connection with the right intercostal veins being gradually 
cut off, it slowly diminishes until it exists only as a very small vessel, or sometimes 
wholly disappears. The left cardinal vein (6'), on the contrary, forms, after the dis- 
appearance of the corresponding Wolffian body, a left vena azygos, which, though for 
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