On carefully removing the dense cellular membrane of the floor, and that more loose tissue which it conceals, the edges of the axillary folds will be seen. Close to the an terior of them we observe the thoracica longior artery, with its accompanying veins and several lymphatic glands, and, under cover of the posterior, the subscapular vessels and nerves ; whilst a great bundle of arteries, veins, and nerves, with the biceps and coraco-brachialis muscles, stretch along the humerus. To this view of the parts the operating surgeon will look with peculiar interest. It is from below that we generally operate on the axilla, and the three sets of vessels just now mentioned con stitute the most important subjects for consi deration when the scalpel is to be used. It is obvious that free incisions may be practised in the centre of this space or upon its thoracic side, but that all its other boundaries are beset with dangers.
To follow up the anatomy of this region with advantage, each of its walls must be examined in detail. The anterior wall consists of the pectoralis major and minor muscles. The pectoralis major is a large flat muscle, of a tri angular shape, extending over the front of the thorax, from the clavicle and sternum to the humerus.
The origin of this muscle is curved, its con vexity being directed upwards and inwards; this may be called its base. The insertion or apex is outwards and downwards. One surface looks outwards and forwards, the other back wards and inwards. The inferior margin extends from the seventh rib to the humerus and is nearly horizontal, folded on itself and free. The outer edge is nearly vertical, at first about an inch dis tant from the deltoid, but soon coming into con tact with it, and so continuing to its insertion.
The triangular space between the deltoid and pectoral may be seen even in the living person when the shoulders are shrugged up, especially if the individual be thin. It is in this situation that the axillary artery commences, and might be cut down upon without dividing any muscular fibres except those of the platysma ; it is however protected by the costocoracoid ligament, and by the edge of the pectoral mus cle. In this interval we see the cephalic vein and a small artery, the thoracica humeraria, which is the descending branch of the thoracica acromialis. The cephalic vein is derived from a plexus on the outer and back part of the hand. After various communications in its superficial course it gets between the deltoid and pectoral muscles, and on arriving at the triangular interval above mentioned, it dips in under the edge of the great pectoral and just above the lesser, to empty itself into the axillary vein.
When the pectoralis major has been raised, we bring into view a stratum of cellular tissue, in which several branches of the thoracica suprema artery and some nervous filaments ramify before they enter the muscles. Under neath this tissue lies the pectoralis minor, still concealing the cavity of the axilla.
The posterior surface of the great pectoral is not nearly so extensive as the anterior; its fibres arise from the cartilages of the ribs, and, there fore, the extreme limit of the axilla in front is not to be estimated by the superficial dimensions of the muscle. A line drawn oblique'y downwards and outwards, beginning one inch outside the sterno-clavicular articulation, and ending an inch outside the nipple, will nearly mark the junction of the anterior and internal walls.
This muscle is sometimes torn across by ex ternal violence. We have seen this occasioned by the passage of a railway carriage over it, and marked by a deep depression, but without any laceration of the integuments.
The pectoralis minor is shaped like the major, but it is considerably smaller. Its base is ap plied to the ribs, its apex to the coracoid pro cess of the scapula. One surface is turned outwards and forwards to the greater pectoral, the other back to the axilla. Attached on the one hand to the upper edge and the external surface of the third, fourth, and fifth, and some tunes the second, true ribs, near their cartilages, by•so many distinct slips, (hence its occasional name serratus minor anticus,) and an aponeu rosis which covers the intercostal muscles, it terminates in a flat tendon which is inserted into the inner border of the coracoid process near its apex. In this situation it is intimately con nected with the coraco-brachialis and short head of the biceps, sometimes sending fibres to be continuous with the triangular or coraco-acromial ligament, and in some rare instances the entire tendon runs across the coracoid process, and through this ligament to join the capsular liga ment of the shoulder-joint. The tendon is about an inch broad ; very short on the posterior surface, longer on the anterior, and longer still at the lower edge. The surface now exposed was covered by cellular tissue, and concealed by the pectoralis major every where except a small part of its lowest digitation, which is generally to be seen below it, in contact with the integuments.