OF THE GREAT ANTERIOR VEINS. 
143 
side of the left innominate vein down to the hack of the auricle. This cord is covered 
by the pleura, and crossed by the left phrenic nerve ; it descends in front of the par 
vagum, upon the side of the aorta and ductus arteriosus to the left pulmonary artery, 
opposite to which it penetrates the pericardium. Within this sac it crosses the 
small interval between the left pulmonary artery and veins, enclosed in a minute fold 
of the serous membrane ; and then, becoming applied to the back of the left auricle, 
expands into a small conical pouch {c'),w\\\c,\\ is narrow and pointed above, but wider 
below, where it opens into the right part of the yet undivided auricular cavity. No 
left superior intercostal vein could be found in this embryo. 
In no other fetus examined did the occlusion of the primitive left jugular vein ex- 
tend quite up to the cross branch or left innominate vein ; for there always remained 
a pervious but shrunken portion of the primitive vessel, which, as will immediately 
be shown, forms the trunk of the left superior intercostal vein. This fact is illustrated 
in fig. 5, i. In a somewhat later embryo (fig. 6), the cord of the occluded vein may 
be traced distinctly from the trunk of the left superior intercostal vein (z, which is here 
larger than usual), passing down through the fibrous layer of the pericardium, and 
entering* its now narrow fold of the serous membrane (c"). Still later (Plate IV. fig*. 9), 
the traces of a continuous cord above the pericardium are difficult to follow, and at 
the full period of fetal life it is generally impossible to find in that situation more 
than a few fine vertical fibrous bands lying close beneath the pleura. The little fold 
of the pericardium, so often alluded to, soon after the closure of its contained vessel, 
sinks, as it were, into the interval between the left pulmonary artery and veins 
(Plate III. figs. 5, 6, c"), where it is found in all future stages, and increases in size with 
the other parts (Plate IV. fig. 9, v). Below the fold, the remnant of the closed vein 
descends upon the left auricle, and may be traced for a time, as a slight ridge, but after- 
wards, when the auricle enlarges, as an opake line or streak, passing close beneath the 
lowermost pulmonary vein, down to the apex of the pouch-like pervious portion of the 
primitive vein (Plate III. figs. 4, 5, 6 ; Plate IV. figs. 7, 8, 10, c", and 9, 1). By the period 
of birth (fig. 11), this opake streak (/) has usually become interrupted or obscured. 
c. Concurrent and subsequent Changes in the Pervious V essels. 
Contemporaneously with the shutting off of the left primitive venous channels, the 
cross branch in the neck (Plate III. figs. 1 to 6, d) enlarges and, together with a short 
portion of the left primitive vein below the entrance of the left subclavian, forms the 
left innominate vein ; a small piece of the right primitive jugular, included between 
the entrance of the right subclavian and the junction with the cross branch, becomes 
the right vena innominata \ whilst the remainder of the right primitive jugular, below 
the cross branch, together with the corresponding canal of Cuvier, becomes widened 
and forms the superior vena cava {h). At the junction of its two constituent parts, 
the vena cava receives the metamorphosed right cardinal vein, now the vena azygos, 
in which the right superior intercostal vein generally ends. 
