140 
Ml?. MARSHALL ON THE DEVELOPMENT 
a time of the same size as the right azygos, afterwards surpasses it, in consequence 
of receiving the lower right intercostal veins, in addition to almost all those of its own 
side. The left canal of Cuvier (c'), reduced greatly in width after the occlusion of the 
primitive jugular vein, constitutes the intrapericardial part of the left azygos vein, and 
with the rest of that vessel forms a venous arch, which turns over the root of the left 
lung, and is connected above with the cord-like vestige of the occluded vein (o). 
In position and direction. The cardinal veins, in order to unite with the jugular 
veins in the corresponding canals of Cuvier, bend forwards above the commencing 
lungs (figs. 1 and 2, — 4, 4'), from which they are for a time separated by the upper 
end of the Wolffian bodies (5, 5'). The rudimentary right lung being, almost from the 
commencement, larger and somewhat higher up in the thorax than the left, the right 
cardinal vein {h), even at a very early period (see figs. 2 and 9), reaches higher than 
the vein of the left side (&') ; a peculiarity in position which subsequently increases, 
so that in a foetus of 4 inches in length the arch of the right azygos rises three 
lines higher up in the thorax than that of the left vein. 
The cardinal veins, like the jugulars, are altogether outside the pericardium ; but 
as soon as that sac is formed, the canals of Cuvier are found almost entirely within it. 
At first (figs. 1, 2, 3, c, o') these canals pass horizontally forwards and inwards to the 
back of the auricular portion of the heart, into which they open on the same level, 
one on either side, in front of the inferior cava; but as the lungs enlarge and occupy 
more of the thoracic cavity, the Cuvierian canals have to descend more and more 
obliquely in front of the roots of those organs to reach the heart (figs. 7 to 10). In 
accordance with the higher position of the root of the right lung, this change in direc- 
tion is more marked in the right canal of Cuvier, or future vena cava superior. 
Iffie heart itself, in the progress of growth, becomes slightly twisted, especially at 
its base or auricular part, its right border being turned somewhat upwards, supposing 
the thorax to be placed vertically (fig. 9, embryo 1 inch and 2 lines long; the parts 
being seen from behind). Hence the enlarged right canal of Cuvier (c) or upper 
vena cava reaches the future right auricle sooner and more directly than the smaller 
left canal (o') or left azygos venous trunk ; the former having a comparatively short 
and almost vertical course, whilst the latter, after having descended in front of the 
root of the left lung, has to turn beneath the base of the heart to gain its destined 
end in the right auricle. At the same time the position of the openings of the meta- 
morphosed canals of Cuvier in the future right auricle undergo a change ; for instead 
of both of them being placed, as at first, on the same level, one on each side of the 
inferior cava, the orifice of the right canal or upper cava opens widely into the 
highest part of the auricle, nearly in a line with the lower cava (e), whilst the left canal 
or left azygos ends at the lower and back part of the auricle close to the commencing 
septum. The serous layer of the pericardium is at first equally reflected over both 
the Cuvierian canals, but in time, whilst it covers only a smaller and smaller part of 
the anterior surface of the right canal, or upper cava, it forms a more and more 
