The serratus anterior muscle (Latin: serratus magnus; French: muscle dentelé antérieur) - named from its serrated or saw-like anterior border and large size - is an irregular quadrilateral sheet curved to the shape of the side of the thorax. Its anterior attached border has a somewhat sinuous curve and arises from the side of the thorax by nine or ten digitations or teeth, which, by their saw-like appearance, give the muscle its name. The muscle may be divided into an upper, middle, and lower part.

Origin of the Serratus anterior

First part, by two teeth from the middle of the outer surface of the first and second ribs, and from the fascia covering the first intercostal space. The second part, by two or three heads from the second, third, and sometimes the fourth ribs upon their outer surface. Third part, by far the largest and strongest portion of the muscle, arises from the fourth or fifth to the eighth or ninth ribs by t series of teeth, which are attached in front near the upper border of each rib, and behind to a line running backwards across the outer surface of the rib from its upper to its lower border. These attachments form a curved line with the convexity forwards, the attachment to the sixth rib being the most anterior and prominent.

Insertion

The first part is attached to an oval space upon the venter of the scapula close to the posterior superior angle. The second part, to the whole of the vertebral border of the scapula upon its ventral aspect. The third part of the large oval space on the venter of the scapula close to its inferior angle.

Structure

The origin of the muscle is by fleshy or short aponeurotic fibers, and in the first part, these fibers converge very slightly towards their fleshy insertion. In the second part, they diverge and form a thin sheet attached to the vertebral border of the scapula. In the third part of the muscle, the fibers converge fanwise and form a very thick and strong fleshy mass which is inserted directly into the inferior angle of the scapula. At their origin the teeth of this part of the muscle interdigitate, that is, meet and fit in between, those of the origin of the abdominal external oblique muscle. All the fibers are curved to fit on the convex wall of the chest.

Nerve-supply

From the brachial plexus by the posterior thoracic nerve, which is derived from the fifth, sixth, and seventh cervical nerves. After running down the side of the chest upon the outer surface of the muscle, the nerve is distributed by many branches to the various digitations.

Action of the serratus anterior muscle

By its contraction this muscle draws forwards the vertebral border of the scapula and, as the third part of it is much the strongest, it will act especially upon the inferior angle, and will rotate the scapula so as to raise the point of the shoulder. It will, therefore, help the trapezius muscle in lifting up the shoulder, and it will be brought powerfully into play whenever the shoulder is used in pushing in a forward direction. It is most important, however, in relation to the movements of the arm. In order that the deltoid muscle may raise the humerus, it is necessary that the fulcrum formed by the glenoid portion of the scapula should be held steady.

For this reason, the scapula extends so far downwards in order that the leverage given to the third and most powerful portion of the serratus may be as great as possible. When the serratus anterior muscle (serratus magnus) is paralyzed, all the efforts of the deltoid to elevate the arm are unsuccessful and only cause the lower angle of the scapula to project from the back of the thorax. "When the arm has been elevated to its full extent by the deltoid, that is, through about a right angle, the rotation of the scapula due to the serratus anterior (serratus magnus) and trapezius will produce a further elevation through another right angle, and so place the arm in a vertical position. One of the results of the action of this muscle is to keep the lower angle of the scapula in close contact with the wall of the thorax. When the muscle is paralyzed, the posterior borders and the inferior angles of the scapulae project backwards from the thorax like small wings (scapulae alatae).

Acting from its scapular insertion, the muscle tends to draw the front of the chest towards the scapula, e.g. where it supports the thorax in crawling on the hands and knees. Judging from the direction in which its fibers are inserted into the ribs, most of them can have little if any action in elevating the ribs, as in forced inspiration.

Relations

Superficially: the pectoralis major and minor, the subscapularis and latissimus dorsi, the subclavian and axillary vessels and the brachial plexus;

Deeply: the external intercostal muscles, and serratus posterior superior (serratus posticus superior).

Variations

It may arise as low as the tenth rib; and above, it may receive slips from the cervical transverse processes, or the Levator scapulae (levator anguli scapulae). Part or the whole of the muscle may be deficient.