display the external respiratory nerve, or the nerve of Bell, upon the axillary surface of the serratus magnus. These structures being secured, the dissector may proceed with his work more boldly, as the remaining contents of the space are not so liable to injury. The axillary artery and vein and the great brachial nerves may next be exposed. Note the close manner in which they cling to the outer wall of the axilla in the various movements of the limb, and then isolate them thoroughly by removing their areolar sheaths, and establish their individual identity. In dissecting these structures care must be taken to secure the small internal cutaneous branch of the musculo-spiral nerve. This nerve is generally given off within the axilla in common with a muscular branch to the long head of the triceps, and it crosses the latissimus dorsi and teres major tendons on a deeper plane than the branches of the intercosto-humeral nerve. The lowest subscapular nerve must now be looked for upon the surface of the subscapularis muscle.

Axillary Artery (arteria axillaris).-It is the third part of the axillary artery which is now exposed, and the vein will be seen to lie upon its inner side, and also partly in front of it. It is important to note the position of the large nerves, with reference to the artery, before they are much disturbed by the dissection. The ulnar nerve lies in direct contact with its inner side. The nerve of Wrisberg (the lesser internal cutaneous nerve) is also internal to the artery, but is separated from it by the axillary vein, to which it is closely applied. The internal cutaneous nerve, and the inner head of the median, lie in front of the artery; the musculo-spiral and circumflex nerves are directly behind it; while the median and musculo-cutaneous nerves are placed upon its outer side. The latter nerve soon leaves the artery, by deviating outwards and entering the substance of the coraco-brachialis muscle. Its branch of supply to that muscle should be secured at this stage.

In this part of its course the axillary artery gives off three branches-the subscapular, which has already been found; the posterior circumflex, which arises from its posterior aspect; and the anterior circumflex, a small vessel which runs outwards under cover of the coraco-brachialis, and is apt to be injured in cleaning the nerves.

Dissection. The axillary space must now be dissected from the This is done by reflecting the clavicular part of the pectoralis

front.

major. The sternal portion of the muscle is not to be disturbed at present. Divide the clavicular part close to its origin from the clavicle, and throw it downwards and outwards. This must be done with care, because some twigs from the external anterior thoracic nerve, and also some of the pectoral thoracic branches of the thoracic axis artery, enter its deep surface. These must be cleaned and preserved.

FIG. 9.-Diagram of the manner in which the main branches of the brachial plexus are arranged around the third part of the axillary artery.

E.C. External cord.

P.C. Posterior cord.

I.C. Internal cord.

A. Axillary artery.

V. Axillary vein.

C.B.

Coraco-brachialis muscle.

Costo-Coracoid Membrane.A space or gap between the clavicle and pectoralis minor is now exposed. This gap, however, is closed by the costo-coracoid membrane, the connections of which must be studied. Trace it outwards to its attachment to the coracoid process, and inwards to its attachment to the first rib. Above it constitutes the sheath of the subclavius muscle by splitting into an anterior and a posterior layer. These, passing upwards, enclose the muscle, and are attached, the one to the anterior border of the clavicle, and the other to the posterior border of the bone.

But how can the two layers be demonstrated? Divide the anterior lamina transversely close to the clavicle, and throwing it downwards, pass the handle of the scalpel upwards behind the muscle. The posterior attachment can in this manner be verified, and at the same time the nerve to the subclavius will be seen sinking into the deep surface of the muscle.

Observe that the density of the membrane diminishes almost immediately below the subclavius, and this so abruptly that a crescentic margin is formed, which, on account of its being thicker and stronger than the rest of the membrane, is

sometimes called the costo-coracoid, or bicornuate ligament. The lower connections of the membrane are somewhat indefinite, and difficult to establish with precision. In a good subject, however, it will be seen to join the sheath of the axillary vessels, and also to give a process of fascia to the sheath of the pectoralis minor.

Four structures pierce the costo-coracoid membrane, and these should now be cleaned. They are:-(1) the thoracic axis artery, breaking up into pectoral, clavicular, acromial, and humeral branches; (2) the thoracic axis vein; (3) the cephalic vein; (4) and lastly, the external anterior thoracic

nerve.

Dissection.-The costo-coracoid membrane should be removed, and the axillary space entered from above. With a little dissection the con

VOL. 1.-3

tents of the upper part of the space may be exposed. These are the axillary artery, with the axillary vein on its inner side, and partly overlapping it. To the outer side, and to some extent above the vessels, are placed the great brachial nerves. All these important structures are enveloped in a loose, funnel-shaped sheath, which is prolonged into the axilla from the deep cervical fascia. Crossing behind the artery, and therefore lying very deeply, is the external respiratory nerve, or nerve of Bell. A small branch, called the superior thoracic, takes origin from this part of the axillary artery, and must be followed out. Lastly, the cephalic and thoracic axis veins must be traced to their junction with the axillary vein.

The sternal part of the pectoralis major muscle may now be divided about its middle, and the two portions thrown outwards and inwards. Several nerves will be observed entering its deep surface, and these must be preserved. They come from the external and internal anterior thoracic nerves. One or more from the latter pierce the pectoralis minor, and are now seen emerging from its anterior surface. pectoralis minor must be cleaned and its attachments defined.

The

The Pectoralis Minor is a fan-shaped muscle, which extends from the thoracic wall to the scapula. It arises by three flat, tendinous slips from the third, fourth, and fifth ribs, close to their cartilages. Between the ribs these slips are prolonged into the anterior intercostal aponeuroses. From this origin the fibres proceed outwards and upwards, and converge upon a stout tendon, which is inserted into the front part of the inner border and upper surface of the coracoid process. The pectoralis minor is supplied by the internal anterior thoracic nerve.

Dissection. The axillary vessels and the brachial nerves can now be cleaned throughout their entire extent, but the pectoralis minor muscle should not be reflected until the relations of these important structures have been thoroughly studied.

The Axillary Artery (arteria axillaris) is a portion of the great arterial trunk which carries blood for the supply of the upper limb. It begins above at the outer border of the first rib, where it is continuous with the subclavian artery, and it ends below at the lower border of the teres major muscle, where it becomes continuous with the brachial artery. Its course through the axilla varies with the position of the limb. When the arm is abducted from

the trunk (as it is when the axilla is being dissected), a straight line, drawn from the centre of the clavicle to a point below the anterior fold of the axilla, and immediately to the inner side of the slight prominence caused by the coraco-brachialis muscle, will, with tolerable accuracy, indicate the course pursued by the vessel.

The relations of the axillary artery vary very much as it traverses the armpit; and with the view of obtaining a greater precision of description, anatomists are in the habit of arbitrarily dividing the vessel into three parts. The first part extends from the outer border of the first rib to the upper border of the pectoralis minor; the second part lies under cover of that muscle; the third part extends from the lower border of the pectoralis minor to the lower border of the teres major.

The first part of the axillary artery lies very deeply. It is covered by the skin, superficial fascia, deep fascia, clavicular part of the pectoralis major, and the costo-coracoid membrane. But, even when these are removed, the vessel is not completely exposed, because it is enveloped, along with the axillary vein and great nerves, by a funnel-shaped sheath, which is prolonged upon them from the deep cervical fascia. Further, it is crossed by the cephalic and thoracic axis veins, and the loop of communication between the two anterior thoracic nerves likewise lies in front of it. Posteriorly this part of the vessel is supported by the first intercostal space and the first digitation of the serratus magnus muscle, and the nerve of Bell crosses behind it. To its inner side, and somewhat overlapping it, is the axillary vein, whilst above and to its outer side are the large brachial nerve-trunks.

The second part of the axillary artery is placed behind the two pectoral muscles, and has the three cords of the brachial plexus disposed around it. Thus the inner cord lies upon its inner side, the outer cord upon its outer side, and the posterior cord behind it. The axillary vein is still upon its inner side, but is separated from the artery by the inner nerve-cord. Strictly speaking, it is not in apposition