A fleshy slip (Scalenus minimus, Summering) is often found to pass from the scalenus an ticus to the lower, or costal extremity, of the scalenus posticus ; in this course it runs be tween the roots of the brachial plexus, and consequently subdivides into two the scalene space. The lower compartment contains the subelavian artery, the cone of the pleura, and the inferior portion of the brachial plexus, constituted by the seventh cervical nerve, and the cord resulting from the union of the eighth cervical, with the first dorsal nerve; whilst in the upper compartment (corresponding to the apex of the triangle), the fifth and sixth nerves of the plexus are seen to unite into a single trunk.
On the front of the scalenus anticus, and separated by that muscle from the subclavian artery, are found the following parts :—Most inferiorly the subclavian vein, lying on the tendinous insertion of the muscle, and under cover of the clavicle ; above the vein the transverse branches of the thyroid axis, viz. the supra-scapular, and the transversalis colli arteries, of which the former is the more inferior, whilst the phrenic nerve descends obliquely inwards towards the tracheal edge of the muscle, and intersects these two trans verse arteries by passing behind them.
Superficial to all these important structures is the clavicular origin of the sterno-cleido mastoid muscle. In size, shape, and direction, this muscle accurately corresponds to the sca lenus anticus, which lies deeper than it, and from which it is separated by the parts just now stated, to lie on the front of that muscle.
The separation of the subclavian vein from the subclavian artery in their second stage, " constitutes one of the most remarkable fea tures in its (anatomical) history ;" this con dition is not, however, constant, for the vein has been found to lie with the artery between the scaleni (Blandin), and, in a few other equally rare instances, the artery has accom panied the vein superficial to both muscles (Manec, Quain). Lastly, the artery has been seen to pass through the anterior scalenus, and hence to lie in part behind and in part in front of that muscle (Quain). It is to be understood that these deviations from the normal arrangement are remarkably infre quent.
The anterior relations of the subclavian ar tery in its second stage, may be arranged in four orders of parts: I. Skin, platysma, fascia.
2. Sterno-cleido-mastoid muscle (its clavi cular origin).
3. Subclavian vein, supra-scapular and transverse cervical arteries, phrenic nerve.
4. Scalenus anticus muscle.
Subclavian artery in its third stage.— After the subclavian artery emerges from beneath the scalenus anticus muscle, it inclines down wards and outwards, and thus completes the arch which its entire course represents. In this stage its position is marked by precise limits, which are always recognisable, even in the living subject. The artery, with the vein and brachial plexus of nerves, which still ac company it, is here contained in the postero inferior triangle of the neck (Ono-clavicular space, Velpeau), the several boundaries of which are constituted by the clavicle, and by the sterno-mastoid and omo-hyoid muscles.
Whilst traversing this region the subclavian artery lies at first on the scalenus posticus, and then passes on the upper surface of the first rib. The subclavian vein lies inferior and in ternal to the artery, and on a more superficial plane than it, throughout the entire of this portion of its course; it also passes with the artery over the first rib. Two superficial grooves in the adult bone mark the trajet and relative position of these great vessels ; a tu bercle, which gives insertion to the scalenus anticus muscle, separates these grooves from each other (that for the artery is the more posterior of the two) ; it is constantly present, and serves as a guide to point out with pre cision the situation of the artery, and so assist the operator to distinguish that vessel from the parts which surround it. Such a guide to the artery is peculiarly valuable, because.
in operations upon the larger blood-vessels, the touch often fails to discriminate the pro per object, the characteristic pulsation of a large artery being, under such circumstances, often wanting.
In the postero-inferior triangle of the neck the artery is covered by the integument, su perficial fascia and platysma, descending su perficial (supra-clavicular) twigs of the cervical plexus, and by the external jugular vein. The situation of this vein in the supra-clavicular space, is subject to much variety ; it most fre quently runs near to the inner boundary of the triangle and parallel to the outer edge of the sterno•mastoid muscle, but frequently de scends in the very centre of the space ; in the latter case it much embarrasses the operator in attempting to expose the subclavian artery.