triangular flat tendon, beneath which a bursa is placed into a rough tuberosity at the base of the spine of the scapula (Fig. 328, p. 367). The fibres inserted into the clavicle, acromion, and the upper border of the spine of the scapula spread over the adjacent subcutaneous surfaces of those bones for a variable distance. The occipital portion of the muscle may be in the form of a separate slip, or may be entirely absent.

The trapezius is superficial in its whole extent. Its upper lateral border forms the posterior limit of the posterior triangle of the neck. The inferior lateral border, passes over the upper edge of the latissimus dorsi and the vertebral margin of the scapula, and forms a boundary of the so-called triangle of auscultation, which is completed, below, by the latissimus dorsi, and, laterally, by the vertebral margin of the scapula. This space is partly filled up by the rhomboideus major. The muscle overlaps the latissimus dorsi, and covers the levator scapula, rhomboidei, and the deeper axial muscles of the back, along with the ascending and the descending branch of the transverse artery of the neck, the accessory nerve, and muscular branches from the cervical plexus.

Nerve-Supply. The trapezius has a double nerve-supply: (1) from the terminal fibres of the accessory nerve, and (2) from the cervical plexus (C. 3. and 4.). The cervical nerves communicate with the accessory nerve in the posterior triangle of the neck and beneath the trapezius.

Action. The main action of the trapezius is to draw the scapula backwards and upwards. The upper fibres of the muscle elevate the shoulder-girdle, while the lower fibres, pulling on the base of the spine of the scapula, depress the vertebral margin; the two movements result in a rotation of the scapula, by which the glenoid cavity is tilted upwards, as in the movement of raising the arm above the head in a forward direction.

M. Latissimus Dorsi.-The latissimus dorsi is a large triangular muscle occupying the lower part of the back. It has a triple origin. The greater part

FIG. 327.-MUSCLE-ATTACHMENTS TO THE RIGHT CLAVICLE (Upper Surface).

of the muscle arises (1) from the posterior layer of the lumbo-dorsal fascia. This is a thick membrane which conceals the sacrospinalis muscle in the lower part of the back. Through it the latissimus dorsi gains attachment to the spines of the lower six thoracic vertebræ, the spines of the lumbar vertebræ, and the tendon of the sacrospinalis, with which the fascia blends below. It also arises laterally from the posterior part of the lateral lip of the iliac crest.

From its origin the muscle is directed upwards and laterally, its fibres converging to the inferior angle of the scapula. In relation to its lateral and upper borders additional fibres arise. (2) Along the lateral border muscular slips arise from the lower three or four ribs, interdigitating with the slips of origin of the obliquus abdominis externus. (3) As the superior border of the muscle passes, horizontally, over the inferior angle of the scapula, an additional fleshy slip usually takes origin from that part of the bone and joins the muscle on its deep surface (Fig. 329, p. 368).

Beyond the inferior angle of the scapula the latissimus dorsi, greatly narrowed, curves spirally round the teres major muscle, and forms the prominence of the posterior axillary fold. It ends in a ribbon-like tendon, which is closely adherent, at first, to the teres major, and is inserted into the floor of the intertubercular sulcus of the humerus, extending for about three inches distal to the distal and lateral part of the lesser tubercle (Fig. 336, p. 376). It is placed behind the axillary vessels and nerves, and in front of the insertion of the teres major, from which it is separated by a bursa.

In the back the latissimus dorsi is superficial, except in its upper part, which is concealed by the trapezius. It covers the lumbo-dorsal fascia, serratus

FIG. 328.-SUPERFICIAL MUSCLES OF THE BACK, AND VERTEBRO-SCAPULAR MUSCLES.

posterior inferior, the ribs, and inferior angle of the scapula, and at its borders two triangular spaces are formed; at the upper border is the so-called triangle of auscultation; at the lateral border is the lumbar triangle of Petit, a small space

bounded by the iliac crest, the latissimus dorsi, and the obliquus abdominis externus. This space is sometimes the site of a lumbar hernia.

Nerve Supply. The muscle has a single nerve-the thoraco-dorsal nerve-which supplies it in its deep (axillary) surface. It is a branch from the posterior cord of the brachial plexus (C. (6.) 7. 8.).

Action. The main action of the latissimus dorsi muscle is to depress and rotate the humerus medially, so lowering the arm as in swimming. (Acting from the humerus it elevates the lower ribs, and is an extraordinary muscle of inspiration.

M. Levator Scapula.-The levator scapulæ is a strap-like muscle, arising by tendinous slips from the posterior tubercles of the transverse processes of the first three or four cervical vertebræ, between the attachments of the scalenus medius in front and the splenius

RHOME: MINOR

Deltoid (origin)

Triceps brachii
(origin of long head)

Teres minor (origin) with

cervicis behind.

It is directed downwards along the side of the neck, to be inserted into the vertebral margin of the scapula in its upper fourth, from the medial angle to the spine (Fig. 329).

It is concealed in its upper third by the sterno mastoid muscle. Its middle third forms

gap for circumflex scapular part of the floor of the posterior triangle. In its lower third it is again hidden from view by the trapezius. It conceals the splenius cervicis and iliocostalis cervicis.

(Dorsal Surface).

and fourth cervical nerves enter the muscle on its superficial surface near its origin. Action. The levator scapulæ raises the superior angle and vertebral margin of the scapula, as in shrugging the shoulders.

M. Rhomboideus Minor. The rhomboideus minor may be regarded as a separated slip of the rhomboideus major, with which it is often continuous. It arises from the ligamentum nucha and the spines of the seventh cervical and first thoracic vertebræ.

Passing obliquely downwards and laterally it is inserted into the vertebral margin of the scapula below the levator scapula muscle, and opposite to the base of the spine (Fig. 329).

M. Rhomboideus Major. The rhomboideus major arises from the spinous processes of the thoracic vertebræ from the second to the fifth inclusive, and from the corresponding supraspinous ligaments,

It also passes downwards and laterally and is inserted, below the rhomboideus minor, into the vertebral margin of the scapula, between the spine and the inferior angle (Fig. 329). The muscle is only inserted directly into the scapula by means of its inferior fibres. Its superior part is attached to a membranous band, which,

connected to the vertebral margin of the scapula, for the most part, by loose areolar tissue, and is fixed to the bone at its extremities, above near the base of the spine, and below at the inferior angle.

The rhomboid muscles are concealed to a large extent by the trapezius. The lower part of the rhomboideus major is superficial in the triangle of auscultation. The muscles cover the serratus posterior superior and vertebral aponeurosis.

Nerve-Supply.-The rhomboid muscles are supplied by the dorsal scapular nerve from the brachial plexus (C. 5.), which supplies branches in the deep surface of the muscles.

Action. The rhomboid muscles elevate and draw backwards the vertebral margin of the scapula.

THE FASCIA AND MUSCLES OF THE

PECTORAL REGION.

FASCIÆ.

The superficial fascia of the chest usually contains a quantity of fat, in which the mamma is embedded. The origin of the platysma muscle lies beneath its superior part.

The deep fascia is attached above to the clavicle, and in the median plane to the sternum. Below it is continuous with the fascia of the abdominal wall. It gives origin to the platysma and invests the pectoralis major. At the lateral border of the great pectoral muscle it is thickened, and forms the floor of the axillary space (axillary fascia), continued posteriorly on to the posterior fold of the axilla and laterally into connexion with the deep fascia of the arm.

Costo-Coracoid Membrane.-Beneath the pectoralis major a deeper stratum of fascia invests the pectoralis minor muscle. At the superior border of this muscle it forms the costo-coracoid membrane, which passes upwards to the inferior border of the subclavius muscle, where it splits into two layers, attached in front of and behind that muscle to the borders of the inferior surface of the clavicle. The membrane traced medially along the subclavius muscle is attached to the first costal cartilage; passing laterally along the upper border of the pectoralis minor it reaches the coracoid process. The part of the membrane extending directly between the first costal cartilage and the coracoid process is thickened and forms the costo-coracoid ligament. The costo-coracoid membrane is otherwise thin and of comparatively small importance. It is pierced by the cephalic vein, thoraco-acromial artery and vein, and branches of the lateral anterior thoracic nerve. By its deep surface it is connected to the sheath of the axillary vessels.

At the inferior border of the pectoralis minor there is a further extension of the deep fascia beneath the pectoralis major. It passes downwards to join the fascia forming the floor of the axilla, and is continued laterally into the fascia covering the biceps and coracobrachialis muscles.

MUSCLES OF THE PECTORAL REGION.

The anterior muscles connecting the upper limb to the axial skeleton comprise the pectoralis major, pectoralis minor, subclavius, serratus anterior, and sternocleido-mastoid. The last is described in a later section (p. 458).

M. Pectoralis Major. The pectoralis major is a large fan-shaped muscle arising in three parts: (1) a pars clavicularis arising from the anterior aspect of the clavicle in its medial half or two-thirds (Figs. 327, p. 366, and 331, p. 371); (2) a pars sternocostalis, the largest part of the muscle, arising from the anterior surface of the manubrium and body of the sternum by tendinous fibres decussating with those of the opposite muscle (Fig. 330, p. 370), and, more deeply, from the cartilages of the first six ribs; (3) a pars abdominalis, a small and separate slip, arising from the aponeurosis of the obliquus abdominis externus muscle. The abdominal slip, at first separate, soon merges with the sterno-costal portion, but a distinct interval usually remains between the two first-named parts of the muscle.

The fibres converge towards the proximal part of the arm, and are inseparably blended at a point half an inch from their insertion into the humerus. The muscle is inserted into (1) the lateral border of the sulcus intertubercularis of the humerus, extending proximally to the greater tubercle and blending, laterally, with the insertion of the deltoid, and medially, with the insertion of the latissimus dorsi (Fig. 336, p. 376); (2) from the proximal border of the insertion a membranous band extends proximally to the capsule of the shoulder-joint, enveloping at the same time the tendon of the biceps; and (3) from the distal border a band of fascia. passes distally to join the fascia of the arm.

Sterno-cleido-mastoid (origin)

The arrangement of the fibres of the muscle at its insertion is peculiar. The muscle is twisted on itself, so that the lower (sterno-costal) fibres are directed upwards and laterally behind the upper (clavicular) part of the muscle; in consequence the clavicular part is attached to the humerus more distally than the sterno-costal portion, and is inserted also into the fascia of the arm. The twisting of the fibres is specially found in the inferior sterno-costal fibres of the muscle and the abdominal fibres. These curve upwards behind the superior sterno-costal fibres, and have the highest attachment to the shaft of the humerus, helping to form the fascial expansion which extends upwards over the biceps tendon to the capsule of the shoulder-joint. In this way a bilaminar tendon is produced united along its inferior border; consisting of a superficial lamina formed by the superior sternocostal fibres, blending for the most part with the tendon of the clavicular portion; and a deep lamina, composed of the twisted lower sterno-costal and abdominal fibres. The disposition of the muscular fibres at their insertions is the reason for the application of the terms "portio attollens" to the clavicular portion, and "portio deprimens" to the sterno-costal and abdominal portions of the muscle.

ECTORA

Rectus

abdominis
(insertion)

FIG. 330.-MUSCLE-ATTACHMENTS TO
THE FRONT OF THE STERNUM.

Placed superficially, the pectoralis major forms the anterior wall and anterior fold of the axilla. Its superior border is separated from the edge of the deltoid muscle by an interval in which lie the cephalic vein and deltoid branches of the a. thoracoacromialis. Its deep surface is in relation with the ribs and intercostal muscles, the costo-coracoid membrane and the structures piercing it, the pectoralis minor, the axillary vessels, and the nerves of the brachial plexus.

Nerve-Supply.-The pectoralis major has a double nervesupply, from both anterior thoracic nerves. The lateral anterior thoracic nerve, derived from the lateral cord of the brachial plexus (C. 5. 6. 7.), divides into two trunks. One pierces the costo-coracoid membrane, and supplies the clavicular part, and superior portion of the sterno-costal part of the muscle. The other branch communicates over the axillary artery with the medial anterior thoracic nerve, a derivative of the medial cord of the brachial plexus (C. 8. T. 1.). They then supply the pectoralis minor and, piercing that muscle, terminate in the lower part of the pectoralis major.

Action. The pectoralis major draws the arm to the side. The clavicular fibres flex the shoulder-joint and raise the arm besides drawing it forwards. The sterno-costal and abdominal portions, on the other hand, depress the arm, while drawing it forwards.

Sternalis Muscle.-The sternalis is an occasional muscle placed, when present, parallel to the sternum upon the sterno-costal origin of the pectoralis major. It has attachments which are very variable both above and below, to the costal cartilages, sternum, rectus sheath, sternomastoid, and pectoralis major. Its nerve-supply is from one or both of the anterior thoracic nerves. In certain rare cases it has been said to be innervated by intercostal nerves. It is present in 44 cases out of 100, and it is slightly more frequent in the male than in the female. It has been regarded by different observers as (1) a vestige of the panniculus carnosus, (2) a homologue of the sterno-mastoid, or (3) a displaced slip of the pectoralis major.

Chondroepitrochlearis, Dorsoepitrochlearis, Axillary Arches, Costocoracoideus.-One