The upper edge of this muscle is nearly hori zontal, and placed about an inch below the clavicle. In the space between, when some fat is carefully removed, and some absorbent glands, we see the axillary artery running down wards and outwards, internal and anterior to which is the axillary vein, and behind it the nerves. The cephalic vein is observed passing upwards and inwards from the edge of the del toid muscle to the axillary vein, and the tho racica suprema artery standing forwards from the axillary artery and resting on this edge of the pectoral. The thoracica acromialis artery runs in this space out towards the acromion process, and is often a branch of the suprema. Here, too, we see the lower surface of the sub clavius muscle, turned forwards, and covered by a pretty strong fascia which terminates in the costo-coracoid ligament.
The costo-coracoid ligament is very thin, but strong. It extends from the cartilage of the first rib, just below the origin of the subclavius muscle, to the coracoid process of the scapula, in an arch across the vessels and nerves. It is concave inferiorly, and appears to be only the thickened edge of the fascia which covers the subclavius and descends a little below that muscle. This view of its true mode of forma tion is favoured by the fact that it has an at tachment also to the clavicle, and consequently may be called costo-cleido-coracoid. The name ligamentum bicorne is sometimes applied to in dicate its horn-shaped extremities ; Blandin denominates it .fascut clavicularis, and Gerdy, ,ligament suspenseur de l'aisselle. As a ligament it has little power, but as an aponeurosis it pro tects the vessels, and sends down a thin process upon them.
Below the lesser pectoral the vessels and nerves again come into view, and the thoracica longior or external mammary artery is seen passing downwards and forwards along its lower edge. For a fuller description of the pre ceding muscles, see Tito nAx , MUSCLES OF THE.
The inner wall of the axilla exhibits the ribs_, intercostal muscles, and serratus magnus, with some vessels and nerves. One of these last is remarkable for its length and vertical direction; it lies on the serratus magnus, and appears as if flattened against the side of the thorax. It arises generally by two branches from the back of the anterior division of the fifth and sixth cervical nerves (counting eight in the neck). It communicates with the phrenic, descends be hind the brachial plexus, under the clavicle and trapezius, appears upon the serratus magnus, on which it runs a great distance and enters its lowest division by many filaments. It is classed
among the respiratory nerves by Sir Charles Bell, by whom it has been named the inferior ex ternal respiratory nerve of the trunk, its function being, according to his views, to associate the muscle to which it is distributed with the ge neral respiratory movements. It was known to antecedent anatomists as the posterior thoracic branch of the brachial plexus."' Crossing the axilla from the thorax to the arm, we see two nerves, frequently called nerves of Wrisberg. They are the external branches, or costo-humeral, of the second and third inter costal nerves. They pierce the external layer of intercostal muscles opposite the origin of the serratus magnus, between the second and third and the third and fourth ribs, and pass out ob liquely to the arm, where they are lost in the in teguments on the inner and back part of the arm and elbow. The superior of them is the larger.
The great vessels and nerves are seen pass ing from the first rib to the lower border of the teres major muscle, forming an arch whose con cavity is downwards. By raising the arm to the horizontal position we obliterate the arch, and by supinating the hand strongly we bring them more into view. In the upper third of this curve the order of the parts, proceeding outwards, is, the axillary vein, axillary artery, and plexus of nerves. In the middle the vein is situated as before, and then the nerves sur rounding and hiding the artery ; and in the inferior third we first meet the vein, then the nerves, and lastly the artery.
The axillary vein is about three inches in length, commencing a little above the edge of the teres major ; thence it runs upwards, in wards, and forwards to the second rib, which it touches, as also some fibres of the serratus magnus there arising ; next it gets on the first intercostal muscles, after which it becomes the subclavian vein, crosses over the first rib, under the clavicle, before the scalenus anticus muscle, and then enters the thorax. It is formed by the confluence of three veins, viz. the basilic and the two venom comites which convey their fluid from the fore-arm, and it is afterwards en larged by the accession of those veins which accompany, usually in pairs, the subscapular, the thoracic, and the circumflex arteries. It also receives the cephalic a little higher up, as before described.