II. MUSCLES CONNECTING THE UPPER LIMB WITH THE ANTERIOR AND LATERAL THORACIC WALLS
| Pectoralis major | Subclavius |
| Pectoralis minor | Serratus anterior |
The superficial fascia of the anterior thoracic region is continuous with that of the neck and upper limb above, and of the abdomen below. It encloses the mammary gland and gives of numerous septa which pass into it to support its various lobes. From the fascia over the front of the gland, fibrous processes pass forwards to the integument and nipple; these were called by Sir Astley Cooper the ligamenta suspensoria.
The pectoral fascia is a thin lamina, covering the surface of the Pectoralis major and sending numerous prolongations between its fasciculi; it is attached in the median plane to the front of the sternum; above, to the clavicle; laterally and below, it is continuous with the fascia of the shoulder, axilla and thorax. It is very thin over the upper part of the Pectoralis major; but thicker in the interval between it and the Latissimus dorsi, where it forms the floor of the axillary space and is named the axillary fascia; this divides at the lateral margin of the Latissimus dorsi into two layers, which ensheathe this muscle and are attached behind to the spines of the thoracic vertebrae. As the fascia leaves the lower edge of the Pectoralis major to cross the floor of the axilla, it sends a layer upwards under cover of the muscle : this lamina splits to envelop the Pectoralis minor; and at the upper edge of this muscle is continuous with the clavipectoral fascia, (coracoclavicular fascia). The hollow of the armpit seen when the arm is abducted, is produced mainly by the traction of this fascia on the axillary floor, and hence the lamina is sometimes named the suspensory ligament of the axilla. At the lower part of the thoracic region the deep fascia is well developed, and is continuous with the fibrous sheath of the Rectus abdominis.
Applied Anatomy. -In cases of suppuration in the axilla, the pus is prevented from extending downwards by the axillary fascia, and therefore tends to spread upwards, behind the pectoral muscles, towards the root of the neck. Early evacuation of the pus is therefore necessary. The incision should be made midway between the anterior and posterior axillary folds; so as to avoid the lateral thoracic and subscapular vessels, and the edge of the knife should be directed away from the axillary vessels.
The Pectoralis major (fig. 619) is a thick, triangular muscle situated at the upper and front part of the chest. It arises from the anterior surface of the sternal half of the clavicle : from half the breadth of the anterior surface of the sternum, as low down as the attachment of the cartilage of the sixth or seventh rib : from the cartilages of all the true ribs, with the exception, frequently, of the first, or seventh, or both, and from the aponeurosis of the Obliquus externus abdominis. From this extensive origin the fibers converge towards their insertion; those arising from the clavicle pass obliquely downwards and laterally, and are usually separated from the rest by a slight interval; those from the lower part of the sternum, and the cartilages of the lower true ribs, run upwards and laterally; while the middle fibers pass horizontally. They all end in a flat tendon, about 5 cm. broad, which is inserted into the lateral lip of the bicipital groove (intertubercular sulcus) of the humerus. This tendon consists of two laminae, placed one in front of the other, and usually blended together below. The anterior lamina, the thicker, receives the clavicular and the uppermost sternal fibers; they are inserted in the same order as that in which they arise: that is to say, the most lateral of the clavicular fibers are inserted at the upper part of the anterior lamina, and the uppermost sternal fibers to the lower part of the lamina, which extends as low as the tendon of the Deltoid muscle and joins with it. The posterior lamina of the tendon receives the attachment of the greater part of the sternal portion and the deep fibers, i.e. those from the costal cartilages. These deep fibers, and particularly those from the lower costal cartilages, turn backwards successively behind, and reach a higher level than, the superficial and upper ones, so that the tendon appears to be twisted. The posterior lamina of the tendon reaches higher on the humerus than the anterior, and gives off an expansion which covers the bicipital groove and blends with the capsule of the shoulder-joint. From the deepest fibers of this lamina at its insertion an expansion is given off which lines the bicipital groove, while from the lower border of the tendon a third expansion passes downwards to the fascia of the upper area.
Relations.-In front, the Pectoralis major is related to the skin, superficial fascia, Platysma, medial and intermediate supraclavicular nerves, mammary gland, and deep posterior surface is in contact with the sternum, ribs and costal cartilages, clavipectoral fascia, Subclavius, Pectoralis minor, Serratus anterior, and Intercostal muscles; it forms the superficial stratum of the anterior wall of the axillary space, and so covers the axillary vessels and nerves and the upper parts of the Biceps and Coracobrachialis. Its upper border is separated from the Deltoid muscle by a slight interspace (the infraclavicudar fossa), in which the cephalic vein and deltoid branch of the acromiothoracic (thoracoacromial) artery lie. Its lower border forms the anterior fold of the axilla; it is separated from the Latissimus dorsi by a considerable interval at the medial wall of the axilla, but the two muscles gradually converge towards the lateral wall of the space.
Nerve-supply.-The Pectoralis major is supplied by the lateral and medial pectoral nerves (lateral and medial anterior thoracic nerves); through these it receives filaments from all the nerves entering into the formation of the brachial plexus; the fibers for the clavicular part of the muscle are derived from C. 5 and 6.
Actions.-The Pectoralis major adducts the arm and. rotates it medially. When the arm is extended, i.e. drawn backwards and laterally, the Pectoralis major draws it forwards and medially. The two heads of the muscle work together in effecting these movements. When the arm is flexed, the sternocostal fibers take no part in the movement, which is carried out by the clavicular head (portio attollens) acting with the anterior fibers of the Deltoid muscle and the Coracobrachialis. When the opposite movement, usually carried out with the assistance of the force of gravitation, is resisted, the sternocostal head (portion deprimens) alone helps the Latissimus dorsi and Teres major to depress the arm. When the arms are fixed, the sternocostal fibers draw the trunk upwards and forwards as in climbing.
The Pectoralis minor (fig. 620) is a thin; triangular muscle, situated at the upper part of the thorax, deep to the Pectoralis major. It arises from the tipper margins and outer surfaces of the third, fourth and fifth ribs (frequently the second; third and fourth ribs) near their cartilages, and from the aponeuroses covering the External intercostal muscles; the fibers pass upwards and laterally, and converge to form a flat tendon, which is inserted into the medial border and upper surface of the coracoid process of the scapula. Sometimes a part or the whole of the tendon is continued over the coracoid process and through the coraco-acromial ligament; when this occurs the tendon blends with the coracohumeral ligament and thus gains an attachment to the humerus.
Relations.-Its anterior surface is in relation with the Pectoralis major, the lateral pectoral (anterior thoracic) nerve and the pectoral branches of the acromiothoracic artery; its posterior surface, with the ribs, External intercostal muscles, Serratus anterior, the axillary space, and the axillary vessels and brachial plexus of nerves. Its upper border is separated from the clavicle by a narrow triangular interval occupied by the clavipectoral fascia; behind which are the axillary vessels and nerves. Running parallel with the lower border of the muscle is the lateral thoracic artery; piercing and partly supplying the muscle is the medial pectoral (anterior thoracic) nerve..
Nerve-supply.—-The Pectoralis minor is supplied by both the pectoral (anterior thoracic) nerves (C. 7 and 8, and T. 1).
Actions.–The Pectoralis minor assists the Serratus anterior to draw the scapula forwards round the chest wall. Acting with the Levator scapulae and the Rhomboids, the Pectoralis minor rotates the scapula backwards so as to depress the point of the shoulder. When the arm is fixed, it assists in elevating the ribs in forced inspiration.
The Subclavius (fig. 620) is a small, triangular muscle, placed between the clavicle and first rib. It arises by a short, thick tendon from the junction of the first rib and first costal cartilage, in front of the costoclavicular liga ment; the fleshy fibers proceed obliquely upwards and laterally, to be inserted into the groove on the under surface of the intermediate third of the clavicle.
Relations.-Its posterior surface is separated from the first rib by the subclavian vessels and brachial plexus of nerves. Its anterior surface is separated from the Pectoralis major by the clavipectoral fascia, which, with the clavicle, forms an osseofibrous sheath for the muscle.
Nerve-supply.—The Subclavius is supplied by a branch which derives its fibers from C. 5 and 6.
Action.-The Subclavius pulls the point of the shoulder downwards and forwards and steadies the clavicle, during movements of the shoulder, by bracing it against the articular disc of the sternoclavicular joint.
The Serratus anterior (fig. 620) is a muscular sheet, situated between the ribs and scapula at the upper and lateral parts of the chest. It arises by fleshy slips or digitations from the outer surfaces and superior borders of the upper eight or nine ribs, and from the aponeuroses covering the intervening Intercostal muscles. Each digitation arises from the corresponding rib, but the first springs in addition from the second rib, and from the fascia covering the first intercostal space. The lower four slips interdigitate with the upper five slips of the Obliquus externus abdominis. From this extensive attachment the fibers pass backwards, closely applied to the chest-wall, and are inserted into the costal surface of the medial (vertebral) border of the scapula in the following manner. The first digitation is inserted into a triangular area on the costal surface of the superior angle. The next two or three digitations spread out to form a thin, triangular sheet; the base of which is directed backwards and is inserted into nearly the whole length of the costal surface of the medial border. The lower four or five digitations converge to form a fan-shaped mass, the apex of which is inserted, by muscular and tendinous fibers, into a triangular impression on the costal surface of the inferior angle.
Actions.-The Serratus anterior, acting with the Pectoralis minor, draws the scapula, forwards, and is the chief muscle concerned in all pushing and punching movements. Its lower and stronger fibers move the lower angle forwards and assist the Trapezius in rotating the bone at the acromioclavicular joint, and thus aid this muscle in raising the arm above the head. It is also an assistant to the Deltoid in raising the arm, in as much as during the action of this latter muscle it helps the other muscles inserted into the scapula to steady the bone and so enables the Deltoid to exert its action on the humerus only. While the Deltoid is raising the arm to a right angle with the scapula, the Serratus anterior and the Trapezius are rotating the scapula, and the arm can be raised above the head as the result of this combination of movements. When the scapula is fixed, the lower part of the muscle will pull upon the ribs and act as a muscle of inspiration.
Applied Anatomy.–When the Serratus anterior is paralyzed, the medial (vertebral) border. and especially the lower angle of the scapula, leave the ribs and stand out prominently on the surface, giving a peculiar ‘winged’ appearance to the back. The patient is unable to raise the arm or to carry out pushing movements, and attempts to do so are followed by a further projection of the lower angle of the scapula from the back of the thorax.
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