"As gestation advances, the head becomes more distinct, and the neck begins to be formed after the second month, which, as it lengthens, has the effect of removing the brain upwards to a greater distance, and of drawing out the larynx from the chest, in consequence of which the nerves of the par vagum and their recurrents become elongated, and hence the circuitous route the latter are found to take afterwards, forming loops in which the aorta and right subclavian artery are, as it were, suspended."* 2. The left suhclavian artery is much less liable to vary in the mode of its origin than the right ; the varieties observed are also fewer in number. a. The origin of the left sub clavian artery is sometimes more approximated than usual to the origin of the left carotid, and in a few instances, b. its origin is fused into that of the latter, so that these two vessels arise by a common trunk, which is a left arteria innominata.
It has been observed in those instances where the arch of the aorta is reversed, so as to pass from left to right, that the same irre gularities affect the transposed branches which then arise from it, as have been stated to occur when the arch holds its more usual po sition.
Branches of the subclavian arteries.—In the number and arrangement of the branches of the subclavian artery so much variety occurs, that no general description could, perhaps, be given, which would accurately describe their arrangement in any one instance, and hence, in works on anatomy, much discrepancy exists on this subject ; the following description agrees with that of the best authorities in this country, and is strictly in accordance with the accurate statistical details of Professor Quain.
In its first stage the subclavian artery usually gives origin to the following branches—the vertebral, internal mammary, and thyroid axis ; the last, after a very short course, divides in a radiating manner into the inferior thyroid, the supra-scapular (transversalis humeri), and the transverse cervical (transversalis colli) arteries ; besides, in the majority of instances, the left subclavian artery, in its first stage, furnishes (in addition to the branches already enumerated) the superior intercostal artery, a branch which, on the right side, more fre quently arises under cover of the scalenus anticus muscle. All these branches arise nearer to the scaleni muscles, and are conse quently more immediately covered by the internal jugular vein on the left side of the neck than on the right.
The position of these branches of the sub clavian artery, especially on the right side, is of much interest in a surgical point of view. " The situation in which the branches arise fi•oni any large artery is an important con sideration in its history, because of the influ ence which their presence has on the result of an operation for the cure of aneurism. And, considering the shortness of the trunk, the size of the offsets, and the manner of their arrangement on the parent vessel, it may be confidently stated that there is no artery in which the influence alluded to is more considerable than in the subclavian."* The following statistics, derived from the author just quoted, are useful to show the average length of the subclavian artery, from its origin to the point where its first brands arises; in other words, the extent of the artery in its first stage, suitable for the appli cation of a ligature.
In sixty-five bodies examined, this distance measured— inch and under- - - in 8 More than I inch and not exceed ing 1 inch - - Of More than 1 inch and not exceed ing 11 inch - - „ 23 q inch (the extreme length) - „ 1Occasionally, the vertebral, or inferior thy roid artery, has been seen to arise very close to the arteria innominata.
In the second stage the subclavian artery most frequently gives off but a single branch, which soon subdivides into the cervicalis pro funda and superior intercostal arteries. It is less usual for these two arteries to arise separately; thus, in 285 examinations, this occurred in the proportion of but 1 to 20,1, whereas the former arrangement existed in 266 out of the entire number.
In the majority of instances no branches arise from the subclavian artery external to the scaleni; but the posterior scapular artery (in general the continued trunk of the transver salis colli) frequently arises from the third stage of the subclavian, and so frequently, indeed, that Cruveilhier regards it, not as a variety, but as the normal arrangement. It is computed by Professor Quain that this occurs in the proportion of 1 in 2* cases.
I. Vertebral artery.— This artery, which is the first and largest, is at the same time one of the most regular of the branches of the subclavian artery, from the upper and posterior aspect of which it usually arises. There is no example on record where this branch arose either between or beneath the scaleni muscles.