The upper and outer part of the prominence of the shoulder is formed by a single powerful muscle, called the deltoid. Beneath this are several deep muscles which fill up the fossæ of the scapula (supraspinatus, infraspinatus, and subscapularis). But when the arm is raised and maintained in the horizontal position, the surface below the root of the arm dips into a cavity or pit, corresponding to the external prominence of the shoulder; this cavity, called the armpit, or axilla, has for a roof the skeleton of the shoulder covered by the deltoid, and for its walls—in front, the pectoral muscles, which adjoin the anterior border of the deltoid, and behind, the subscapularis and the latissimus dorsi, separated from the posterior border of the deltoid by the teres major muscle. On the inner side is a muscle applied to the wall of the thorax, the serratus magnus, and on the outer side is the humerus, covered by the biceps and coraco-brachialis muscles. Of the muscles which we have named, some have already been studied in relation to the muscular structure of the trunk (great pectoral and latissimus dorsi); of the others, we will study the deltoid and serratus magnus, in relation to the region of the shoulder and axilla.

Deltoid muscle.—This muscle is so called because it resembles the Greek letter delta, and is triangular in form; it is short, broad, thick, and shaped like half a cone to clasp the shoulder-joint. It arises above from the outer third of the anterior border of the clavicle (12, Fig. 68), from the acromio-clavicular articulation, from the convex border of the acromion, and from the entire extent of the posterior border of the spine of the scapula (18, Fig. 73). From this origin its fibres are directed downwards, the middle fibres vertically, the anterior or clavicular fibres downwards and backwards, and the posterior fibres obliquely forwards, to be inserted into the external surface of the humerus on a rough -shaped groove, called the deltoid impression (page 68).

This muscle is very thick, and is intersected by a vertical series of strong fibrous bands or septa, which may be seen as depressions beneath the skin in the contracted muscle. The action of the deltoid is to elevate the arm, separating it from the trunk and supporting it in the horizontal position; but while the middle fibres raise the arm directly outwards, the anterior fibres elevate it and carry it forwards, and the posterior fibres carry it backwards. It is necessary to remark that this muscle is never at right angles to the lever which it moves, but it always acts obliquely on the humerus. Hence, although very thick, the deltoid cannot act with great power; therefore the horizontal position of the arm produced by the action of the muscle is one which requires great effort and quickly produces fatigue. In order to understand the unfavourable arrangement of the deltoid with relation to its humeral lever, it is sufficient to compare it with that which the biceps presents relative to the forearm, and to see that the biceps, which acts obliquely on the radius, becomes perpendicular to that bone in proportion as flexion is carried on in the arm; and when the elbow forms a right angle, the biceps muscle is found in the most favourable condition to act with all possible force. In other words, the greatest momentum of a muscle occurs when it is perpendicular to its lever; the deltoid muscle, therefore, does not possess much momentum.

In pointing out the relations which the three borders of the deltoid present, we summarise the various details of the muscular contour of the shoulder:—1st. The superior border of the deltoid, by its origin from the anterior border of the clavicle and the posterior border of the spine of the scapula, repeats the insertions of the trapezius, which is attached to the opposite lip and border of the same bones (Fig. 74). The clavicle, acromion, and spine of the scapula form a species of bony intersection between the trapezius and deltoid, and when the muscles are contracted produce a deep groove between the insertion of the trapezius and the origin of the deltoid. In animals which have no clavicle, and in which the spine of the scapula is not well developed, the fibres of the deltoid and trapezius are directly continuous. We observe an arrangement of this nature in the horse. 2nd. The anterior border of the deltoid is separated from the corresponding border of the great pectoral muscle by a linear interval, very narrow below, but a little broader above, where it forms a small triangle, of which the base corresponds to the middle of the clavicle (Fig. 70). This interval, which becomes visible during the contraction of the two muscles when we endeavour to raise the arm upwards and forwards while it is held behind, as in the act of drawing a load, gives passage to a vein called the cephalic, and under such circumstances this vein becomes prominent and swollen. 3rd. The posterior border of the deltoid forms one of the sides of the triangular space which we have studied in the region of the back, at the level of the infra-spinous fossa (Figs. 73 and 74); and, under its posterior border, pass successively on the one hand the infraspinatus and teres minor muscles, which pass directly beneath the deltoid, and on the other the teres major and latissimus dorsi muscles, which pass more deeply, separated from the deltoid by the long head of the triceps muscle (Fig. 75).

Two muscles of the shoulder remain to be mentioned which are not visible on the model, but must at least be named in order to explain how the fossæ of the shoulder-blade are filled up. These are:—1st. The supraspinatus muscle (11, Fig. 75), which occupies the supra-spinous fossa of the scapula, passes beneath the coraco-acromial arch, and is inserted into the uppermost facet of the great tuberosity of the humerus. 2nd. The infraspinatus muscle occupies the infra-spinous fossa, and is partially concealed by the latissimus dorsi, trapezius, and deltoid. It is also bound down on the back of the scapula by a strong membrane derived from the deep fascia. It is inserted below the supraspinatus into the back of the great tuberosity of the humerus. 3rd. The subscapularis muscle (19, Fig. 68) occupies the subscapular fossa, and is inserted into the lesser tuberosity of the humerus.

The serratus magnus muscle (14, Fig. 68; 5, Fig. 70; 2, Fig. 71; 20, Fig. 73).—This muscle, applied to the lateral part of the thorax, is hidden throughout a great part of its extent by the scapula and the muscles of the chest and shoulder; but it becomes superficial at its lower part, in a series of prominent digitations and muscular bands, which form very characteristic features in the contour of the lateral region of the thorax. At the same time, as the muscle constitutes the inner wall of the armpit, we must describe it here in detail.

The serratus magnus arises by nine fleshy slips from the outer surfaces of the upper eight ribs (an additional slip arising between the first and second ribs). From this origin the muscle, which forms a broad sheet, sweeps backwards round the ribs, to be inserted into the whole length of the vertebral border of the scapula. The five or six upper digitations of the muscle are hidden by the great pectoral muscle (21, Fig. 73), and only its three or four lowest digitations are visible on the side of the thorax between the borders of the great pectoral muscle in front and the latissimus dorsi behind; they interdigitate with the upper slips of origin of the external oblique muscle of the abdomen (20 and 16, Fig. 73). The alternations of origin of the serratus magnus and external oblique are seen when the muscle is contracted, as when the arm is thrust forward. When the arm is hanging loosely, or slightly raised, we see only three digitations of the serratus magnus; but when the arm is strongly elevated the great pectoral frequently leaves another uncovered.

The action of this muscle is to fix the scapula, drawing this bone downwards and forwards, while the rhomboid, on the other hand, draws it upwards and backwards. Fixation of the scapula being necessary to afford a fixed point for the contraction of the muscles of the arm (particularly the biceps), it is easy to perceive that whenever the upper limb accomplishes a powerful effort, the lower digitations of the serratus magnus become clearly visible in the living model, as in a sword-thrust, wrestling, lifting from the ground a heavy body, or in pushing back an adversary, &c.

The serratus magnus muscle forms the inner wall of the armpit or axilla, a cavity of which the anterior wall is represented by the pectoral muscles, and the posterior wall by the subscapularis, teres major, and latissimus dorsi. The cavity forms a triangular pyramid; its summit, directed upwards, corresponds in the skeleton to the interval between the first rib, the clavicle, and the upper border of the scapula. In a dissected subject this cavity is open inferiorly, but in the living model it is closed by the skin which forms the base of the pyramid, and which, in passing from the outer border of the great pectoral muscle to the border of the latissimus dorsi, is hollowed out so as to ascend in the space, into which it is drawn by its attachment to the strong, deep fascia of the axilla.

To complete the description of the axilla we must say a few words concerning its boundaries, and the folds which correspond to the lines of junction of its walls. There is nothing more to add with regard to its anterior limit (formed by the great pectoral muscle) or its posterior limit (formed by the latissimus dorsi and teres major muscles clothing the axillary border of the scapula), but a word must be said of the external limit of the space which corresponds to the root of the arm. This is a comparatively thick surface, and corresponds to the upper part of the shaft of the humerus. The bone is covered by two muscles which descend from the scapula towards the anterior surface of the arm,—the biceps and coraco-brachialis, which we will notice briefly. We will say, first of all, that the form of the coraco-brachialis is clearly visible beneath the skin of the armpit when the arm is strongly raised, as for example in a subject crucified, when it raises the depressed skin in the armpit. The skin is covered with hair more or less abundant, according to the individual, and it is a classic habit to omit this part of the hairy system in every representation of an elevated limb; but the artist should be convinced by the study of anatomy that he should never conform to the habit of tracing on the skin of the hollow of the armpit a fanciful contour, since this skin is smooth and regularly depressed, and only on its external part presents the fusiform muscular prominence of the coraco-brachialis, on the inner side of the anterior surface of the arm. The biceps muscle escapes from under cover of the great pectoral muscle, and lies external to the coraco-brachialis more on the front of the limb.

The triceps muscle of the arm, which, by its long head, takes origin from the scapula, does not pass, like the biceps and coraco-brachialis, through the armpit, but appears behind it on the back of the arm, since, as we have already said, it passes between the teres minor posteriorly and the teres major and latissimus dorsi anteriorly (Figs. 73 and 75).