The most superficial muscles of the back are the trapezius and the latissimus dorsi. They form two broad muscular sheets which cover over the whole of the back and the posterior part of the neck, and extend also to the shoulder and arm.

Trapezius muscle.—The trapezius muscle has the following attachments. Its origin is from the middle line of the back, from the inner third of the superior curved line of the occipital bone (13, Fig. 73), the fibrous band of the ligamentum nuchæ between the occipital protuberance and the spinous process of the seventh cervical vertebra, and from the spinous processes of the seventh cervical and of all the thoracic vertebræ. From these origins the muscular fibres are carried outward and converge towards the shoulder, the intermediate fibres passing transversely, the superior obliquely downwards (9, Fig. 74), and the inferior fibres obliquely upwards. The muscle is inserted into the shoulder girdle, to the superior border of the spine of the scapula (Fig. 73), the inner edge of the acromion process, and the outer third of the posterior border of the clavicle (19, Fig. 70; 7–13, Fig. 73).

With regard to external form the trapezius muscle presents this remarkable fact, that in certain regions the muscular fibres are replaced by aponeurotic fibres or tendons, so that in these regions the muscle is thinner and in contraction shows slightly depressed surfaces. These regions are three in number: 1. At the lower part of the neck and the upper part of the back (10, Fig. 73), opposite the level of the seventh cervical spine, the fibres of origin of the muscle are aponeurotic, and form with those of the opposite side a lozenge-shaped or elliptical surface with its long diameter vertical, in the centre of which the spinous processes of the sixth and seventh cervical and first thoracic vertebræ form more or less well-marked projections. 2. At the lower part of the back, at the level of the last thoracic vertebræ, the fibres of origin of the trapezius are also aponeurotic, in a small triangular space; and in the living model, when the trapezius is strongly contracted, its lower angle seems hollowed out, because at that level the muscular fibres are wanting, and in consequence do not swell up when the muscle is in action. 3. At the level of the root of the spine of the scapula, the inferior fibres of the trapezius form at their insertion a small triangular aponeurosis which glides on the bone, and gives rise to a distinct depression at that spot during the contraction of the muscle.

Action of the trapezius.—When the shoulder is strongly drawn backwards the whole of the trapezius contracts, and in this case it is the middle portion, of which the fibres are directed horizontally, which shows most prominently beneath the skin. More frequently, however, different parts of the muscle contract separately; thus the superior fibres act, either by taking the shoulder as a fixed point, and bending the head to the corresponding side (as when the face is slightly turned to the opposite side), or by taking their fixed point at the occipital and the cervical origin, and thus raising and supporting the scapula, as when a burden is carried on the shoulder. Under those circumstances the cervical border of the trapezius, that which proceeds from the occipital bone to the clavicle (Fig. 74), becomes prominent in a line nearly parallel to that of the outer border of the sterno-cleido-mastoid (26, Fig. 74); and between these two prominent lines appears a groove (the posterior triangle of the neck), to which we will return in dealing with the deep muscles of this region. Lastly, if the inferior fibres of the trapezius contract alone, they draw the shoulder downwards, and thus we see them become prominent whenever the model causes a dragging with his upper limbs from above downward, e.g., in the case of a bell-ringer who pulls violently and with all his weight on a rope.

The two trapezius muscles (right and left) form together (Fig. 73) a rhomboidal figure with its apex below, which resembles the contour of a monk’s hood. Hence the trapezius has been called by anatomists and artists the cucullary muscle (cucullus, a hood).

Latissimus dorsi muscle (1, 2, 3, 5, Fig. 73).—This forms a large, somewhat triangular muscular sheet extending from the region of the loins to the upper part of the arm. It arises by means of a broad triangular aponeurosis (vertebral aponeurosis, 1, Fig. 73) from the spinous processes of the lower six thoracic vertebræ, from the spinous processes of the lumbar and sacral vertebræ and the posterior third of the crest of the ilium (3, Fig. 73); the muscular fibres converge from this origin towards the armpit, and the muscle obtains additional origins at its borders. At the outer border of the muscle, three or four fleshy bands join it which arise from the outer surfaces of the last three or four ribs by slips which interdigitate with the fibres of the external oblique muscle of the abdomen (4, 4, Fig. 74). At its upper border, as it crosses the inferior angle of the scapula, the muscle receives a narrow slip of origin from this part of the shoulder-blade. From these origins the muscular fibres are directed upwards and outwards, and converge so as to form a fleshy layer (5, Fig. 73) which, passing over the inferior angle of the scapula and along its axillary border, winds round the teres major muscle (see later), and over the posterior wall of the armpit to reach the upper part of the shaft of the humerus, in which it is inserted by a broad tendon, attached to the floor of the bicipital groove of the humerus (page 61).

This muscle acts first of all like the lower part of the trapezius, but with more energy. It, further, not only depresses the scapula, but also acts on the humerus. It is the contraction of the latissimus dorsi which enables us to bring the arm with force to the side, carrying the upper limb slightly backwards, so that if the contraction is carried very far the arms become crossed behind the back. Indeed, the latissimus dorsi may be called the swimmer’s or golfer’s muscle. It draws the humerus backwards behind the trunk, and at the same time rotates the arm-bone and the whole limb inwards. The prominence formed by the outer border of the latissimus dorsi during contraction (Fig. 74) is principally shown when the muscle accomplishes a powerful effort, such as dragging, or pulling from above downwards, as in pulling on a rope hanging vertically, or in hanging by the arms from a horizontal bar. If in this situation—in the exercise of the trapeze, for example—the model raises himself, and brings the trunk near the bar, the latissimi dorsi muscles become very prominent, for then they take their fixed points at the arms, and act on the trunk by carrying it upwards and forwards.

Among the numerous deep muscles of the back there are not any which are visible on the surface throughout their entire extent, but there are many which appear in part in the spaces which limit the borders of the trapezius, latissimus dorsi, and superficial muscles of the shoulder and neck. These spaces are three in number, one at the side of the neck, one at the level of the lower half of the scapula, and a third over the iliac crest.

The posterior triangle is the name given to the space on the lateral surface of the neck, bounded (Figs. 73 and 74) behind by the supero-anterior border of the trapezius, and in front by the posterior border of the sterno-cleido-mastoid. This space forms a long and superficial groove, extending from the occipital region to the middle of the clavicle. It has a lower part (25, Fig. 74), covered over by the platysma myoides muscle of the neck (to which we will return later), and a superior part, in which parts of one or two powerful muscles of the neck are found. 1st. The muscular fibres which we see (17, Fig. 73) directed obliquely from below upwards and outwards towards the mastoid process belong to the splenius capitis muscle, which arises from the spinous processes of the last cervical and upper four or five thoracic vertebræ, and ascends obliquely outwards to be attached to the mastoid process of the temporal bone beneath the sterno-cleido-mastoid (16, Fig. 73). 2nd. A small fleshy triangle, which may appear above the splenius, at the apex of the posterior triangle, corresponds to a part of a powerful muscle of the neck, called the complexus, which may or may not be completely concealed by the trapezius muscle. It is always responsible for the surface form of the neck, and, along with the other deep muscles, appears as a prominent ridge on each side of the groove formed by the edge of the ligamentum nuchæ.

The space situated in the back at the level of the lower part of the scapula is triangular in form (Figs. 73 and 74). When the arm is hanging beside the trunk the borders of the space are seen to be as follows: Externally, the vertebral border of the scapula; above and internally, the lower edge of the trapezius; and below, the upper edge of the latissimus dorsi. In the floor of the space the greater rhomboid muscle is seen; while external to the triangle are the prominences of the muscles covering the lower part of the dorsal surface of the scapula, the infraspinatus, teres minor, and teres major muscles.

The greater rhomboid muscle (rhomboideus major) arises from the spinous processes of the seventh cervical and upper four or five thoracic vertebræ; its fibres are directed obliquely downwards and outwards, to be inserted into the vertebral border of the scapula. It is only the lower fibres which become superficial, behind the vertebral border of the scapula.

The infraspinatus muscle (19, Fig. 73) rises from the infra-spinous fossa of the scapula. From this origin its fibres ascend and, converging beneath the deltoid (Fig. 74), are inserted by a short tendon into the great tuberosity of the humerus.

The teres minor muscle (20, Fig. 73) rises from the upper part of the thick border of the infra-spinous fossa on the axillary border of the scapula, and ascends along the edge of the infraspinatus and beneath the deltoid, to be inserted into the lowest of three facets on the great tuberosity of the humerus.

The teres major (6, Fig. 73, and 8, Fig. 74) arises from the lower part of the broad axillary border of the infra-spinous fossa; it extends upwards and outwards like the preceding muscles, but it soon leaves the teres minor (5, Fig. 75). Instead of remaining at the posterior part of the shoulder it passes beneath the deltoid, and accompanies the latissimus dorsi muscle (Figs. 17, 18; Fig. 68) to the front of the long head of the triceps, and is finally inserted into the inner lip of the bicipital groove of the humerus. The long head of the triceps is found, therefore, passing between the teres minor, behind, and the upper part of the teres major, in front (Fig. 73, between 20 and 6).

These muscles become more visible and prominent when the arm is raised and arrives at the horizontal position (see the right side of Fig. 73). The space between the edge of the trapezius and the border of the scapular spine above, and the latissimus dorsi below, becomes much lengthened from within outwards, and the deltoid leaves uncovered a greater extent of the infraspinatus, teres major and teres minor muscles; at the same time the scapula, by the elevation of the arm (page 64), being moved so that its inferior angle is drawn away from the vertebral column, a greater part of the rhomboid muscle becomes apparent between the outer border of the trapezius and the upper border of the latissimus dorsi.

Although the other deep muscles of the back are not visible on the subject stripped of its skin, we must not leave the region without giving a few of the names of the powerful fleshy masses which occupy the lumbar region, on each side of the spinous processes, and form two powerful muscular columns, causing a prominence beneath the aponeurosis of the latissimus dorsi (Fig. 73). This mass is formed by muscles in the loin which are closely blended together below, and constitute the erector spinæ. At the lower level of the thorax it separates into an external muscle called the ilio-costalis, which is attached by a series of tendons to the angles of the ribs, and an internal muscle called the longissimus dorsi, which, by a series of tendons, is attached to the ribs and to the transverse processes of the thoracic vertebræ. These muscles are continued through the upper part of the back and the neck by means of smaller slips which need not be particularised. The action of the erector spinæ and its several parts is to straighten the trunk, and maintain it when a burden is borne on the shoulders or back. It is for this reason that their common mass in the lower part of the back is so developed in men who usually carry heavy loads on the shoulders, and forms that powerful muscular mass of the loins, of which the prominence is visible beneath the skin and the aponeurosis of the latissimus dorsi (1, Fig. 73). In the neck, as already stated, the complexus is the most important of the deep muscles in giving rise to the form and contour of this region.