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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 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.
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.
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.
Rectus abdominis (insertion)
FIG. 330.-MUSCLE-ATTACHMENTS TO
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
or other of the above-named slips is occasionally present, crossing the floor of the axilla in the interval between the latissimus dorsi and the pectoralis major. They take origin from the costal cartilages, ribs, or borders of the pectoralis major (chondroepitrochlearis, axillary arches, costocoracoideus), or from the border of the latissimus dorsi (dorsoepitrochlearis, axillary arches, costocoracoideus). Their insertion is variable. The chondroepitrochlearis and dorsoepitrochlearis are inserted into the fascia of the arm, the medial intermuscular septum, or the medial epicondyle of the humerus. The axillary arches are inserted into the border of the pectoralis major, the fascia of the arm, or the coracobrachialis or biceps muscle. The costocoracoideus, arising from the ribs or the aponeurosis of the obliquus externus, or detaching itself from the border of the pectoralis major or latissimus dorsi, is attached to the coracoid process, alone or along with one of the muscles attached to that bone. These variable slips of muscle are supplied by the medial anterior thoracic nerve, the medial cutaneous nerve of the arm, or the intercostobrachial.
M. Pectoralis Minor.-The pectoralis minor is a narrow, flat, triangular muscle. It arises, under cover of the pectoralis major, from (1) the surfaces and superior borders of the third, fourth, and fifth ribs near their anterior ends, and (2) from the fascia covering the third and fourth intercostal spaces between those ribs. It may have an additional origin from the second rib (Fig. 414, p. 468); and that from the fifth rib is often absent.
Directed obliquely upwards and laterally, it is inserted by a short, flat tendon into the lateral half of the anterior border and upper surface of the coracoid process (Fig. 333, p. 372), and usually also into the conjoint origin of the biceps brachii and coracobrachialis.
It enters into the formation of the anterior wall of the axilla, and gives attachment along its superior border to the costo-coracoid membrane. It crosses the axillary vessels and the cords of the brachial plexus, and is pierced by the medial anterior thoracic nerve.
Either in part or wholly the pectoralis minor may pass over the coracoid process of the scapula, separated from it by a bursa, to be inserted into the coraco-acromial ligament, or the acromion process; or piercing the coraco-acromial ligament, it may be attached to the capsule of the shoulder-joint (coraco-humeral ligament).
Pectoralis Minimus.-This is a slender slip, rarely present, which extends between the first costal cartilage and the coracoid process.
Nerve-Supply. The pectoralis minor is innervated like the pectoralis major by both anterior thoracic nerves. The lower division of the lateral nerve (C. 5. 6. 7.) communicates with the medial anterior thoracic nerve (C. 8. T. 1.) over the axillary artery. Both nerves pierce and supply the pectoralis minor, and end in the pectoralis major.
Action.-The main use of the pectoralis minor is to draw the shoulder forwards. It is thus a chief assistant of the serratus anterior muscle.
M. Subclavius.-The subclavius muscle arises from the superior surface of the
Pectoralis major (origin)
Coraco-clavicular ligament (trapezoid part)
FIG. 331.-MUSCLE-ATTACHMENTS TO THE RIGHT CLAVICLE (Inferior Surface).
first costal cartilage in front of the costo-clavicular ligament, and from the upper surface of the sternal end of the first rib (Fig. 414, p. 468).
It is inserted into a groove in the middle third of the inferior surface of the clavicle (Fig. 331).
The muscle is invested by the fascia which forms the costo-coracoid membrane, and is concealed by the clavicle and the clavicular origin of the pectoralis major.
Nerve-Supply. The nerve to the subclavius is a fine branch of the brachial plexus (C. 5. 6.), which arises above the clavicle, and passes anterior to the subclavian artery to reach the muscle.
Action. It acts as a depressor of the clavicle; or, the shoulder girdle being fixed, it is capable of raising and fixing the first rib, in inspiration.
The sternoclavicularis is a small separate slip, rarely present, extending beneath the pectoralis major from the upper part of the sternum to the clavicle.
M. Serratus Anterior.-The serratus anterior (O.T. serratus magnus) is a large curved quadrilateral muscle occupying the side of the chest and medial wall of the axilla. It arises by fleshy slips from the lateral aspect of the upper eight and occasionally (as in the figure) from nine ribs. The first slip is a double one, arising from the first two ribs and the fascia covering the intervening space (Fig. 332).
The insertion of the muscle is threefold. (1) The first portion of the muscle (from the first and second ribs) is directed posteriorly to be inserted into the costal aspect of the medial angle of the scapula. (2) The next three slips of the muscle (from the second, third, and fourth ribs) are inserted into the vertebral margin of the scapula. (3) The last four slips (from the fifth, sixth, seventh, and eighth ribs) are directed obliquely upwards and posteriorly, to be inserted on the costal aspect of the inferior angle of the scapula (Fig. 333).
The lateral surface of the muscle is partly superficial below the axillary space, on the side wall of the chest, where its slips of origin are seen inter-digitating with those of the obliquus externus abdominis. Higher up it forms the medial wall of the axilla, and is in contact with the pectoral muscles anteriorly and the subscapularis posteriorly. Its superior border appears in the floor of the posterior triangle, and over it the axillary artery and the cords of the brachial plexus pass in their course through the axilla. The inferior border is oblique, and is in contact with the latissimus dorsi muscle. The muscle may extend higher than usual, so as to be continuous in the neck with the levator scapulæ.
Nerve Supply. The serratus anterior muscle receives its nerve from the long thoracic nerve, a branch from the anterior trunks of the fifth, sixth, and seventh cervical nerves. After piercing the scalenus medius, the nerve enters the axilla, and supplies branches to the several digitations of the muscle on their superficial surface. The highest fibres of the muscle are supplied by the fifth, the lowest fibres by the seventh, and the intermediate part of the muscle by the sixth cervical
Action. The primary action of the muscle is to draw the base of the scapula forwards. This causes
FIG. 333.-MUSCLE-ATTACHMENTS TO THE
the whole shoulder to be brought forward by a movement at the sterno-clavicular joint. The movement of stretching forward the arm as in fencing is due to this action of the muscle. Further, by its relation to the inferior angle of the scapula, the serratus anterior causes (along with the trapezius) a rotation of the scapula, resulting in a tilting upwards of the glenoid cavity, and so facilitating the upward movement of the arm above the head. Acting from the shoulder on the ribs the serratus becomes a powerful muscle of inspiration.
Action of Muscles on the Sterno-Clavicular and Acromio-Clavicular Joints. - The muscles just considered (along with the sterno-cleido-mastoid and omo-hyoid muscles) act for the most part in the sterno-clavicular and acromio-clavicular joints.
A. Sterno-Clavicular Joint. The movements at this articulation are vertical, horizontal, and rotatory, and the muscles concerned may be tabulated as follows:
a. Movement in a Vertical Plane.
b. Movement in a Horizontal Plane.
c. Rotation-a combination of these
B. Acromio-Clavicular Joint.-Movements at this joint are associated with rotation of the scapula. By the combined action of such muscles or the trapezius and serratus anterior (inferior fibres), the inferior angle of the scapula is drawn or thrust forwards, the body of the scapula is rotated, and the glenoid cavity is tilted upwards, so facilitating the upward movement of the arm above the horizontal level.
In forced inspiration, the sterno-mastoid, trapezius, levator scapula, rhomboidei, subclavius, omo-hyoid, serratus anterior, pectoral muscles, and latissimus dorsi, acting together, raise and fix the shoulder girdle; while those of them which have costal attachments-subclavius, pectoral muscles, serratus anterior, and latissimus dorsi-simultaneously elevate the ribs and expand the thorax.
Lateral flexion and rotation of the vertebral column in the neck is effected partly by the action of the trapezius, levator scapulæ, and rhomboid muscles (with the shoulder fixed). The latissimus dorsi and pectoralis major act in climbing in a similar way, raising up the , trunk towards the shoulder.
Action on the Upper Limb.-By reason of their insertion into the humerus the pectoralis major and latissimus dorsi muscles assist the movements of the upper limb. Acting together, the two muscles depress the shoulder, and draw the arm to the side of the body, at the same time rotating the humerus medially. The two parts of the pectoralis major have slightly different actions on the humerus. The clavicular part of the muscle (portio attollens) draws the arm medially and upwards; the sterno-costal part of the muscle (portio deprimens) draws it medially and downwards. The latissimus dorsi acting alone, besides rotating the limb, draws it medially and backwards, as in the act of swimming.
FASCIÆ AND MUSCLES OF THE SHOULDER.
The deep fascia covering the scapular muscles presents no feature of special importance. Attached to the clavicle, acromion, and scapular spine, it is thin over the deltoid muscle. Below the deltoid it is thicker; it encases and gives origin to the infraspinatus muscle, and is continuous with the fascia of the axilla and the back.
The muscles proper to the shoulder comprise the deltoid, supraspinatus, infraspinatus, teres minor, teres major, and subscapularis.
M. Deltoideus.-The deltoid, a coarsely fasciculated multipennate muscle, has an extensive origin from (1) the front of the clavicle in its lateral third (Figs. 327, p. 366, and 331, p. 371); (2) the lateral border of the acromion; (3) the inferior edge of the free border of the spine of the scapula (Figs. 329, p. 368, and 333, p. 372); and (4) from the deep fascia covering the infraspinatus muscle. Its origin embraces the insertion of the trapezius.
The fibres of the muscle converge to the lateral aspect of the body of the
humerus, to be inserted into a well-marked V-shaped impression above the radial groove (Fig. 336, p. 376). The insertion is partly united with the tendon of the pectoralis major.
The most anterior part of the deltoid muscle is formed of parallel fibres, not
uncommonly separate from the rest of the muscle at their origin from the clavicle. These fibres be continuous may EUS MAJOR With the trapezius over the clavicle. The most posterior part arises by a fascial origin from the spine of the scapula and the fascia over the infraspinatus muscle. These portions are attached respectively to the front and back of the main tendon of insertion. The intermediate fibres are multi-pennate, attached above and below to three or four septal tendons, which extend for a variable distance downwards and upwards from the origin and insertion of the muscle.
The deltoid is superficial in its whole extent, and forms the prominence of the shoulder. Its anterior border is separated from the pectoralis major by a narrow interval, in which the cephalic vein and deltoid branch of the thoracoacromial artery are placed. The deep surface of the muscle, separated from the capsule of the shoulder-joint by a large bursa, is related to (1) the coracoid process, associated with which are the coraco-acromial liga(tendon of insertion) ment, and the attachments of the
pectoralis minor, the coracobrachialis, and the short head of the biceps brachii; (2) the capsule of the shoulder-joint covering the head of the humerus, associated with which are the long head of the biceps, and the attachments of the subscapularis, supraspinatus, infraspinatus, and teres minor; and (3) the proximal part of the lateral surface of the body of the humerus, associated with which are the posterior circumflex vessels of the humerus and the axillary nerve.
FIG. 334.-LEFT SCAPULAR MUSCLES AND TRICEPS.
Nerve-Supply. The deltoid muscle is supplied by the terminal branches of the axillary (O.T. circumflex) nerve from the fifth and sixth cervical nerves.
Action.-The main action of the deltoid is to abduct the arm, and bring the humerus into