thorax, at which during life the pleura bulgingly rises, deriving considerable support from the horizontal infixion of the cervico-thoracic fascial septum. Externally to the curve of the rib, (with the coracoid process bounding it outwardly, the clavicle in front, and the superior 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 subcutaneous areolar tissue, which in all directions 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 between 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 clavicle, in the manner described above, as a horizontal 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 left sides of the body in this region will presently be considered, the description meanwhile applying to both indifferently. The subclavian artery, from the sterno-clavicular joint outward, over-arches the floor of this region, presenting upwards a convexity in the interspace of the scaleni, downwards a concavity, 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 circumference: 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 longus colli, enters the canal of the transverse processes, usually at the sixth; 3. forwards 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 muscle,—the transversalis humeri along the clavicle, the transversalis colli higher, amid the branches of the brachial plexus and winding round the scalenus posticus to gain the inner edge of the scapula; lastly, 4. backwards an artery, which, directing itself to the neck of the rib, subdivides there into two branches, one of which descends across the rib to the thorax, the superior intercostal, while the other continues, between the neck of the rib and the seventh cervical transverse process, the backward direction 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 passed 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 of which is here especially needed, since it is in this portion of its course that the artery is usually tied for axillary aneurism: a second stage, in which it lies between the scaleni, its convexity toward their origin from which the brachial plexus divides it, its concavity reposing on the pleura; and a first or tracheal portion of its course, differently related on the two sides of the body, but thus far alike in both, that from it the branches originate, that its concavity is to the pleura and its convexity, almost at right angles to the direction of the carotid, looks upward; that it is related, behind, to the sympathetic and to the last cervical transverse process,—in front, to the vagus and phrenic nerves and to the jugular and subclavian veins,—inwardly to the carotid artery. The circumstances of difference are mainly due to the fact, that, while on the right side a common brachio-cephalic trunk exists—the arteria innominata,—which lies at no great depth from the sternum, so that its branches diverge to their respective destinations from a comparatively superficial and single point, behind the sterno-clavicular joint; on the left side, contrarily, the carotid and subclavian arise separately from the arch, the latter, at a vast depth from the surface, actually beside the vertebræ; with the exception of having a thoracic commencement (nearly corresponding to the tracheal half of the arteria innominata), the left carotid can scarcely be said to differ importantly from the right, at least in virtue of its own course; it is somewhat deeper, lies in front of the œsophagus from the inclination of that tube, has the thoracic duct ascending 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 right 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, reaches the groove on the rib after a very deep and a very oblique course; it can scarcely be said to have any transverse direction, but gradually, by an inclination outwards and forwards, approaches the rib during its ascent, so that, if traced toward its origin from the tracheal edge
