The axillary artery traverses this region from above downwards in a course doubly ob lique, from within outwards, and from before backwards ; at its upper part it rests on the chest separated by the serratus magnus muscle, and lies close under the anterior wall of the axilla, whilst below it rests on the subscapularis muscle (posterior wall), and is very near the arm. Its complicated relations with the nerves, veins, glands, &c. come more properly under consideration in the next article (AXILLARY ARTERY), to which we refer.
It is plain from this view of the parts that a wound in the axilla, near the clavicle, might penetrate both the artery and vein, and be fol lowed by aneurismal varix, but that no such consequence could follow a puncture of these vessels lower down. We see too that there would be much difficulty in compressing the axillary artery through the anterior wall of the axilla, (formed as it is of the two pectorals,) ex cept in the triangular interval between the great pectoral and deltoid muscles close to the clavicle, and that the subclavius muscle and the ligamentum bicorne would bear off pres sure even there to a great extent. In this place the vein and artery lie closer to each other than they do above the clavicle, a circumstance to be remembered in attempting to command the circulation of the limb. Collections of pus are often met with in the cellular tissue under the great pectoral muscle. In children they will frequently be found to have been occasioned by laceration which the tissue has suffered in the act of raising them up by the arm. These abscesses elevate the muscle considerably, and do not always point in the lower part of the axilla as might be expected. They approach the surface directly in front in some cases: But if an early opening were not made, it is pro bable they would oftener extend themselves all through the axilla.
The nerves in the axilla are large, numerous, and complicated. The principal ones are in a bundle, at first behind the axillary artery and then surrounding it. They arise in the cervical region, interlace in a remarkable way to form the axillary or brachial plexus, give off some branches in the neck, and on reaching the axilla separate to supply the arm, forearm, and hand. (For a particular description of this plexus we refer to the article CERVICAL NmivEs.) The nerves we meet with in the axilla, besides the costo-humeral, are, three thoracic branches, three subscapular, and six others of much greater size, viz. the external cutaneous, median, internal cutaneous, ulnar, musculo-spiral, and The thoracic branches are most commonly three in number ; the anterior, arising from the seventh cervical, runs in front of the great sels and is lost in the pectoralis major and pectoralis minor muscles ; the middle, very small, passes under the vessels and is lost in the lesser pectoral ; the posterior, the largest, is the respiratory, and has been already de scribed.
The subscapular branches are also three in number generally ; they come from different points at the upper and back part of the plexus: the smallest quickly enters the subscapular muscle : the other two sometimes arise by a common trunk, or one of them comes from the circumflex, both run along with the sub scapular artery, the larger pierces the teres major and is lost in the latissimus dorsi, the smaller is distributed to the subscapularis, teres major and teres minor.
The external cutaneous, orperfbrans Casscrii, comes from the external part of the plexus, chiefly from the fifth and sixth cervical branches, and leaves the axilla by running downwards and outwards. It is superficial and external to the axillary artery.
The median arises from the front of the plexus by two roots, one of which is placed on each side of the artery ; they soon unite, the nerve then lies on the artery, •and inclining a little outwards escapes from the axilla, being destined principally for the hand.
The internal cutaneous issues from the inter nal and inferior part of the plexus, lies very su perficially along the inner side of the artery, and quits the axilla where the basilic vein is entering.
The ulnar, arising from the internal and pos terior part of the plexus, inclines backwards, separating itself slowly from the inner side of the artery.
The musculo-spiral arises still farther back, and is concealed from view by the other nerves.
The circumflex nerve arises above and be hind all the others, and completely concealed by them ; it descends before the subscapular muscle for a little, then turns backwards and outwards, close to the capsular ligament of the shoulder-joint, and in company with the pos terior circumflex artery ; then it appears on the outside of the neck of the humerus, between the long head of the triceps, the bone, and the teres major and minor muscles, and soon enters the deltoid in two branches. The situation of this nerve accounts for the paralysis of the del toid muscle which sometimes follows dislo cation of the head of the humerus into the axilla.