OF THE GREAT ANTERIOR VEINS. 
141 
distinct fold or duplicature (fig. 10 c'), in which the left canal or left azygos trunk is 
lodged as it passes down in front of the left pulmonary vessels to reach the side of 
the left auricle immediately behind the appendix. Beyond that point, the vein, as it 
lies on the back of the left auricle, and runs along the auriculo-ventricular groove 
to reach the right auricle, is also covered by the serous layer of the pericardium. 
In the heart of a foetus, 4 inches in length, several small veins from the substance of 
the left ventricle may be seen ending in the lower part of the left azygos trunk ; and, 
amongst them, one, which joins it at an acute angle, is the future coronary vein. 
d. Changes at Birth. 
When, after birth, the short and wide ductus arteriosus shrinks, the long ridge of 
the pericardium with its contained fibrous tissue, already described as resting upon 
that vessel in the position of the occluded left vein, becomes closely applied to the 
left side of the aortic arch, and may be traced in tlie direction of a line drawn from 
the point of junction of the innominate veins at the root of the neck, down to the 
highest point of the arch of the left azygos vein. 
Within the pericardium, the trunk of the left azygos occupies its proper/b/c? of the 
serous membrane, and receives, shortly before its termination in the right auricle 
(as seen in Plate I. fig. 2), the coronary vein of the heart (g) and two posterior cardiac 
branches ip,m), besides a third smaller one (g), which might almost be said to end at 
once in the right auricle. The mouth (t) of this left azygos venous trunk is situated 
to the left of the orifice of the inferior cava (e), close to the interauricular septum, and 
below and behind the fossa ovalis, like that of the left superior cava in the lower 
Mammalia and in Birds. It has no Thebesian valve, which is represented, in the 
Sheep, merely by a slight ridge of the auricular parietes. 
At the entrance of the coronary vein into the left ^zygos, there is, however, a large 
distinct valve {x), consisting of one strong segment. The two cardiac veins succeeding 
it are each guarded by finer valves of two segments, and the third vein generally by 
a single segment. Along the course of the coronary vein, there are from one to four 
other valves, consisting usually of one segment, but sometimes of two. 
Lastly, the right azygos is now a much less important vein than that of the left 
side; it reaches from 3 to 5 inehes higher in the chest, and is so sniall as to have been 
said by some anatomists, including Rathke, to be always wanting. Occasionally, 
(once in five observations) it was found to be so trivial a vessel, that it was difficult 
to distinguish it as the actual persistent representative of the right cardinal vein. 
Development in Man. 
a. Formation of the Transverse Branch in the Neck. 
No opportunity has offered itself of observing the time and mode of origin of the 
transverse branch in the human embryo, though it is probably originally formed in 
the same way as in the Sheep and Guinea Pig. In an embryo of ^ths of an inch in 
