4. The postero-superior triangle is a large space of singularly little interest, having its inferior boundary fixed by the omo-hyoid mus cle, its anterior by the diagonal which inter sects this, its posterior by the edge of the trape zius, and its apex by the mastoid process. It contains, below, a part of the brachial plexus (the anterior branches, namely, of the fifth and sixth cervical nerves, which directly pass be neath the omo-hyoid muscle into the adjoining inferior triangle,) the whole of the cervical plexus and many of its branches, the spinal accessory nerve, obliquely crossing from the sterno-mastoid to the trapezius, which it enters near its clavicular insertion, and some rami fications from the artcria transversalis colli, which, under the name of superficial cervical, ascend in the space, supply its cellular mem brane and lymphatic glands, and ultimately inosculate with descending twigs from the occipital. The pre-vertebral fascia covers its deep parts; the common cervical extends be tween its borders ; the platysma myoides exists as a covering for it only in its lower part.
5. The postero-iVerior triangle, (that (Stile subelavian artery,) is one of manifold impor tance. The well-known lines of the omo-hyoid and clavicle limit its area above and below, the former dividing it from the space last con sidered, the latter from the pectoral region ; intersecting the omo-hyoid, our imaginary di agonal, as it stretches from the centre of the sterno-clavicular joint upward and outward, bounds it internally, and constitutes an arbitrary but most useful separation between the space, exclusively appropriated to the subclavian artery with its branches and that internally adjoining it, (the antero-inferior,) which is the proper ter ritory of the carotid. The parts forming its deep or posterior wall are, the transverse pro cesses of the lower cervical vertebra arid head of the first rib, the outer edge of the longus colli and the broad lower part of the scalenus posticus: its inferior wall presents the upper snrface of the first rib, and within the curve of this bone a part of the upper inlet of the ‘") thomx, at which during life the pleum bul gingly rises, deriving considemble support from the horizontal infixion of the cervico thoracic fascial septum. Externally to the curve of the nb, (with the coracoid process bounding it outwardly, the clavicle in front, and the su perior costa of the scapula behind,) is the space through which vessels and nerves connect the cervical and axillary regions; to the borders of which, deep layers of aponeuroses are so fixed that. the regions only communicate in the line of the vessels, within the infundibulum of pre vertebral fascia. Its anterior or covering wall presents, in addition to the platysma and sub cutaneous areolar tissue, which in all direc tions extend beyond its margins, the cervical fascia, as a single layer (except where it splits at the trapezius and sterno-mastoid) fixed to the clavicle below, and enclosing the omo-hyoid above. From the higher part of its posterior wall, originating at the anterior tubercles of the transverse processes, descends the scalenus anticus to fix itself in the floor of the space, on the upper surface of the rib, anteriorly. It intercepts, like a flying buttress, a space be tween itself and the posterior wall, occupied by the brachial plexus and subclavian artery, round all which, as also round the subclavian vein, which lies in front of the scalenus, the prevertebral aponeurosis is folded and prolongs itself as a funnel ; it is from this, that the slip of fascia is derived, which passes to the cla vicle, in the manner described above, as a ho rizontal process, dividing the axilla from the neck.
As the distributive anatomy of the vessels and nerves will be detailed in a future article, (vide SPINAL NERVES, SUBCLAVIAN ARTERY), their arrangement will now be only sketched, in its regard to surgical relations. The many important points of distinction between the right and lett sides of the body in this region will presently be considered, the description meanwhile applying to both indifferently. The subclavian artery, from the stemo-clavicular joint outward, over-arches the floor of this region, presenting upwards a convexity in the interspace of the scaleni, downwards a con cavity, which adapts itself to the pleura and to the rib. It gives off, as from an axis, branches from the four cardinal points of its circum ference: 1. downwards the internal mammary, which, crossed at its origin by the phrenic nerve, descends within the cartilages of the ribs ; 2. upwards the vertebral, which, after a
course of an inch between the scalenus anticus and long,us colli, enters the canal of the trans verse processes, usually at the sixth ; 3. for wards the thyroid axis, a short trunk giving origin to the inferior thyroid branch (already seen obliquely ascending behind the carotid sheath), the ascending cervical, which mounts beside the phrenic nerve, along the scalenus anticus, and two transverse branches, which direct themselves outwardly, crossing that mus cle,—the transversal is humeri along the clavicle, the tmnsversalis colli higher, amid the branches of the bmchial plexus and winding round the sealenus posticus to gain the inner edge of the scapula; lastly, 4. backwards an artery, which, directing itself to the neck of the rib, sub divides there into two branches, one of which descends across the rib to the thorax, the supe rior intercostal, while the other continues, be tween the neck of the rib and the seventh cer vical transverse process, the backward direc tion of the common trunk, and then ascends among the deep muscles of the dorsal region— the arteria cervicalis profunda. The course of the subclavian artery is conveniently divided into three stages; a last or distal one, in which after having pas.sed behind the scalenus anticus, it has, behind it, the scalenus posticus, below it the groove of the rib, above it (extending likewise a little behind) the brachial plexus of nerves, in front of it the coverings of the space we are considering, a familiar knowledge which is here especially needed, since it is i this portion of its course that the artery i usually tied for axillary aneurism : a secon stage, in which it lies between the scaleni, i convexity toward their origin from which th brachial plexus divides it, its concavity re posing, on the pleum; and a first or trachea portion of its course, differently related on th two sides of the body, but thus far alike i both, that from it the bmnches originate, th its concavity is to the pleura and its convexit almost at right angles to the direction of carotid, looks upward ; that it is related, b hind, to the sympathetic and to the last cervic transverse process,—in front, to the vagus an phrenic nerves and to the jugular and su clavian veins,—inwardly to the carotid arter The circumstances of difference are mainly d to the fact, that, while on the right side a co mon bmchio-cephalic trunk exists—the arte innominata,—which lies at no great depth fro the sternum, so that its branches diverge their respective destinations from a compa tively superficial and single point, behind t sterno-clavicular joint ; on the left side, co trarily, the carotid and subclavian arise sep rately from the arch, the latter, at a vast dep from the surface, actually beside the vertehrx ; with the exception or having a thoracic com mencement (nearly corresponding to the tra cheal .icrif of the arteria innominata), the left carotid can scarcely be said to differ impor ' tantly from the right, at least in virtue of i own course; it is somewhat deeper, lies front of the cesophagus from the inclinati of that tube, has the thoracic duct ascendin at its outer side, and is, as will be explained directly, overlapped by the jugular vein in the lower part of the neck. The subclavian artery on the rig,ht side passes from its origin almost transversely to the scalene space, covered by the muscles which have been enumerated, crossed at right angles by the phrenic and pneumo-gastric nerves and by the jugular vein the left subclavian, on the other hand, reachee the groove on the rib after a very deep and I very oblique course; it can scarcely be said ti have any transverse direction, but gradually by an inclination outwards and forwards, ap proaches the rib during its ascent, so that,5 traced toward its origin from the trachml edg of the scaleni, it would appear, instead of having, as its fellow has, a certain length of transverse course, to bend abruptly toward the arch of the aorta, becoming deeper and deeper; or, in other words, while the right subclavian has a considerable extent at its highest level, from the sterno-clavicular joint to the scalene space, the left has comparatively but a cul minating point, to which it suddenly rises and from which it quickly sinks. Thus the nerves, which cross the course of the right, are nearly parallel to that of the left : and the relation of the jugular vein is similarly changed, while the subclavian vein, having a longer course than on the right side, obliquely crosses the thoracic portion of its artery.