In most parts of the body the description of the artery of one side serves for that of the other likewise ; but this is not the case as regards the subclavian arteries (French : artère sous-clavière), for, as the right subclavian artery commences at the division of the innominate artery, whilst the left subclavian arises at once from the arch of the aorta, it follows that the two vessels must, in the first part of their course, differ materially in their length, direction, and connexions with contiguous parts.
To facilitate the description of these important vessels, each subclavian artery is conveniently divided into three parts, — the first part extending from the origin of the vessel to the inner border of the an- terior scalenus muscle; the second consisting of the portion of the vessel which is situated beneath that muscle; and the third reaching from the outer border of the same muscle to the end of the artery, opposite to the outer border of the first rib. Each of these parts will now be examined in detail. The first part only requires a separate description for the right and the left side, for in it alone is there any material difference in the anatomical history of the two vessels.
1. First part of the right subclavian artery
The first part of the right subclavian artery, [a. subclavia dextra,] commencing at the division of the innominate behind the upper part of the articulation of the sternum with the clavicle, close to the trachea, and ending at the inner margin of the anterior scalenus muscle, arches upwards and outwards away from the carotid artery across the root of the neck, and in doing so ascends above the level of the clavicle, the extent to which it reaches above that bone varying in different cases. The whole of this division of the artery is deeply placed, being covered by the platysma, the sterno-mastoid, the sterno-hyoid and sterno-thyroid, with the fascia separating those muscles. Behind, the artery is opposite to, but separated by an interval from, the transverse processes of the vertebras, which here are covered by the longus colli muscle; and below it (along the concavity of its curve), and likewise somewhat behind it; is the pleura, which is in contact with the vessel till it rests on the first rib.
The subclavian vein is lower than the first part of the right subclavian artery, close under the clavicle. In its course to join this vein, the internal jugular crosses in front of the artery near the scalenus muscle; and a vertebral vein lying along the inner side of the jugular, has the same position with reference to the artery. The anterior jugular vein, running outwards from the fore part of the neck beneath the sterno-mastoid muscle, will likewise be found to cross the course of the subclavian artery.
The vagus nerve crosses in front of the artery, and on the inner side of the internal jugular vein, as would be expected from the relative position maintained by the vein and nerve along the neck : the recurrent laryngeal branch of this nerve turns upwards behind the artery, hooking, as it were, under the vessel, between it and the pleura. Behind the vessel and the vertebral column, but not in contact with it, lies the chain of the sympathetic nerve; and some branches cross before the artery.
2. First part of the left subclavian
The first part of the left subclavian, [a. subclavia sinistra,] differs from the right subclavian in origin, and, as a consequence of this, differs likewise in length, direction, and connexions. It arises from the end of the transverse part of the arch of the aorta, and ascends to the margin of the first rib, behind the insertion of the anterior scalenus muscle to that bone. It is therefore longer than the first part of the right subclavian, and ascends almost vertically out of the chest, instead of arching, like that vessel, outwards across the neck. Commencing then from the deepest part of the aortic arch, the left subclavian is at first behind the left lung, and is covered in front and on the left side by the pleura ; it is placed before the vertebral column (on which is laid the longus colli muscle), and lies, for a short space, in front of the oesophagus, (here deviating to the left side) and the thoracic duct.
To the inner or right side of the vessel are situated the left carotid, the trachea, oesophagus, and thoracic duct.
The pneumogastric nerve is anterior to the left subclavian, and parallel with it, the recurrent branch on this side turning round the aorta. The phrenic nerve descends over the artery along the inner margin of the scalenus muscle, immediately outside the thyroid axis.* The cardiac nerves of the left side descending from the neck, are close to the artery.
The internal jugular vein is immediately before the artery, where it turns outwards from the thorax — close to the scalenus muscle — and the left innominate vein is likewise anterior to it.
3. For the second and third divisions of both subclavian arteries, one description will suffice.
The second division of the subclavian artery, being only that portion which is concealed by the anterior scalenus muscle, is necessarily very short. It forms the highest part of the arch described by the vessel, and is therefore the most distant from the clavicle. Somewhat less deeply placed than the first division, this portion of the vessel is covered by the platysma, the sterno-mastoid, and omo-hyoid, with layers of the cervical fascia. Behind, it rests against the posterior scalenus muscle; and below, it is on the pleura.
3.1. Veins and Nerves
The subclavian vein is lower than the artery, and the anterior scalenus muscle lies between the two vessels. The phrenic nerve, which descends obliquely inwards, is likewise separated from this part of the artery by the same muscle; and beneath the scalenus, immediately above the artery, are the large cervical nerves, which form the brachial plexus.
4. Third division of the subclavian artery
The third division of the subclavian artery, extends outwards and downwards from the border of the anterior scalenus to the upper sur- face of the first rib as far as its outer margin, where the vessel assumes the name axillary. In this part of its course, the artery lies in a small triangular space, the sides of which are formed by the omo-hyoid and anterior scalenus muscles, and its base by the clavicle : the omo-hyoid is in some instances immediately over the artery. The subclavian is here nearer to the surface than elsewhere, being covered only by (besides the common integuments) the platysma, and layers of the cervical fascia; but towards its termination the artery becomes deeper, sinking under the clavicle and the subclavius muscle.
The subclavian vein is anterior to, and lower than the artery. This vein is lower than the artery in its whole course, being close behind the clavicle, while the artery arches above that bone. The external jugular vein lies over the artery, and it receives on the outer side from the shoulder the two veins which accompany the supra-scapular and transverse cervical arteries. The veins in some cases form a sort of plexus over the artery.
Above the vessel, and to its outer side, are placed the large brachial nerves, the lowest cord formed by the union of the last cervical and the first dorsal nerve, being in contact with it. It may be added, that the space which lodges the artery is crossed in front by the superficial descending (clavicular) branches from the cervical plexus, as well as by the little nerve of the subclavius muscle.
In two cases, the phrenic nerve was seen to cross the third part of the artery on the outer side of the scalenus muscle ; but in both instances the thyroid axis arose beyond the scalenus, and the nerve was as usual on its outer side.
5. Peculiarities of the subclavian arteries
Most of the variations of the two subclavian arteries from their ordinary condition require to be separately noticed.
5.1. The right subclavian
The origin of this artery, necessarily varying with the place of bifurcation of the innominate artery, in some cases commences within the thorax, instead of at the ordinary position (the upper margin of the sterno-clavicular joint), and in a smaller number of instances it arises in the neck, at some distance above the level of the clavicle.
The right subclavian artery sometimes springs as a separate vessel from the aorta. And in such cases, it may be the first of the branches from the aortic arch, or, but more rarely, the second or third in order, or, as is most frequently the case, it may be the last of those branches. The course taken by the artery in each of these cases, and its position with regard to other parts, require notice. When the right subclavian is the first branch from the arch, it occupies the ordi- nary position of the innominate artery. In those very rare cases in which this vessel was the second or third of the aortic branches, it gained its usual position after crossing behind the right carotid. Lastly, when the right subclavian is the last branch given off from the arch of the aorta, it springs from the upper, or from the back part of the arch, or it may arise (but this is very rarely met with) much lower down from the descending portion of the aorta.
In any case of late origin of the subclavian, its usual course towards the right side of the neck is to cross obliquely in front of the vertebral column and behind the oesophagus. There would seem to be but one accurately recorded case, in which this artery, arising from the last part of the aortic arch, passed between the oesophagus and the trachea.
5.2. The left subclavian
The left subclavian very seldom arises in common with another vessel; it has, however, been found in a few cases conjoined with the left carotid. When the aorta arches to the right side ; the innominate, if present, exists on the left side, and the left subclavian springs from it. Independently of such cases, the connexion between the left subclavian and left carotid is of extremely rare occurrence.
It is an interesting fact, that although, in cases of transposition of the aorta, the left subclavian artery generally arises, as is usual on the right side, from an innominate trunk, it has also been observed to assume another arrangement occasionally presented as an unusual condition by the artery of the right side, viz., to be given off as the last of the branches derived from the arch of the aorta. In this case it crosses obliquely over the front of the vertebral column towards the left side of the neck.
The left subclavian artery has been observed in the fetus to arise from the ductus arteriosus ; and in the adult from a conical dilatation or pouch, which is connected with the cord formed by the obliterated portion of the "arterial canal." This pouch, which has been met with also in connexion with the origin of the right subclavian when that vessel is detached from the innominate, appears to be formed by a part of the canalis arteriosus, which continues pervious in consequence of the subclavian artery arising from it. A similar pouch, but' of much smaller size, is sometimes seen at the attachment of the ductus arteriosus to the aorta, without having any branch connected with it.
5.3. Peculiarities affecting both subclavian arteries
The height to which these vessels may reach in the neck is liable to variation to some extent. Most commonly the artery crosses the neck a little higher than the clavicle, but it is sometimes placed as high as an inch or even an inch and a half above the level of that bone. The greater extent of elevation above the clavicle, however, is especially seen in the artery of the right side. Now and then the subclavian artery perforates the anterior scalenus muscle, and in a few rare cases it was altogether in front of the muscle, and was at the same time close to the subclavian vein. That vein has also been seen to pass with the artery behind the scalenus muscle.
6. Branches of the subclavian arteries, considered with reference to the trunk
The branches of the subclavian artery are so large and numerous in proportion to the length of the vessel from which they spring, that their number, and the place of their origin, are important considerations in the anatomical history of the artery itself, in consequence of the influence their position would have in determining the point best suited for the application of a ligature in a surgical operation.
Four branches usually arise from each subclavian artery. Of these, three generally spring together from the first division of the artery (namely, the vertebral, the internal mammary, and the thyroid axis,) and one from the second division, viz., the superior intercostal, to which may be added a small spinal branch. On the left side, the second division has usually no branch, the superior intercostal arising most frequently to the inner side of the scalenus muscle. The third part of the artery gives rise to no offset, — at least, it so happens in a majority of cases. But as considerable variations are found to occur in the position and number of the branches, it is necessary to refer more particularly to those given from each part of the artery in different cases, reserving the details respecting each branch for the de- scription which will be given of them individually.
As the first part of the left subclavian artery does not admit of being tied in a surgical operation, the position of its branches has little interest in a practical point of view. It will be enough to say that the branches generally arise close together at the inner side of the anterior scalenus muscle.
On the right side also, the branches occupy usually the same position in regard to the scalenus muscle ; and in consequence of this part being accessible in an operation for aneurism, it is desirable to determine the ordinary distance between the commencement of the artery and the nearest branches.
In a majority of many observations the interval measured from half an inch to an inch; in a smaller number, more than one inch and not exceeding an inch and a half. The space seldom varied from these limits ; but in a few instances it was found to be less than half an inch, and it amounted, in one case only, to an inch and three-quarters.
It occasionally happens that, instead of the three branches arising close together, one is shifted inwards from the accustomed position. In a very few cases the branches were seen to arise at intervals, being, as it were, dispersed over the first part of the artery. Lastly, one or more of the branches have been (very infrequently however) moved outwards to another division of the subclavian.
The second portion of the subclavian artery was found in about two-thirds of a large number of cases (upwards of two hundred and sixty) to give origin to only a single branch, which was usually the common trunk of the superior intercostal and deep cervical arteries. In the remaining third of the total number of cases examined, this part of the subclavian artery was, in most instances, quite destitute of branches, and this occurred more frequently on the left than on the right side. In some few cases, two and even three branches arose from it.
As regards the third part of the artery : in more than half of upwards of two hundred and sixty cases, it furnished no branch whatever ; in less than half, it gave origin to a single branch, which was usually the posterior scapular artery. A few examples occurred of two, and much more rarely of three branches, arising from this part of the vessel.
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