Henry Gray (18251861). Anatomy of the Human Body. 1918.
the chest. It arises by fleshy digitations from the outer surfaces and superior borders of the upper eight or nine ribs, and from the aponeuroses covering the intervening Intercostales. Each digitation (except the first) arises from the corresponding rib; the first springs from the first and second ribs; and from the fascia covering the first intercostal space. From this extensive attachment the fibers pass backward, closely applied to the chest-wall, and reach the vertebral border of the scapula, and are inserted into its ventral surface in the following manner. The first digitation is inserted into a triangular area on the ventral surface of the medial angle. The next two digitations spread out to form a thin, triangular sheet, the base of which is directed backward and is inserted into nearly the whole length of the ventral surface of the vertebral border. The lower five or six 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 ventral surface of the inferior angle. The lower four slips interdigitate at their origins with the upper five slips of the Obliquus externus abdominis.
Variations.Attachment to tenth rib. Absence of attachments to first rib, to one or more of the lower ribs. Division into three parts; absence or defect of middle part. Union with Levator scapulæ, External intercostals or External oblique.
Nerves.The Pectoralis major is supplied by the medial and lateral anterior thoracic nerves; through these nerves the muscle receives filaments from all the spinal nerves entering into the formation of the brachial plexus; the Pectoralis minor receives its fibers from the eighth cervical and first thoracic nerves through the medial anterior thoracic nerve. The Subclavius is suplied by a filament from the fifth and sixth cervical nerves; the Serratus anterior is supplied by the long thoracic, which is derived from the fifth, sixth, and seventh cervical nerves.
Actions.If the arm has been raised by the Deltoideus, the Pectoralis major will, conjointly with the Latissimus dorsi and Teres major, depress it to the side of the chest. If acting alone, it adducts and draws forward the arm, bringing it across the front of the chest, and at the same time rotates it inward. The Pectoralis minor depresses the point of the shoulder, drawing the scapula downward and medialward toward the thorax, and throwing the inferior angle backward. The Subclavius depresses the shoulder, carrying it downward and forward. When the arms are fixed, all three of these muscles act upon the ribs; drawing them upward and expanding the chest, and thus becoming very important agents in forced inspiration. The Serratus anterior, as a whole, carries the scapula forward, and at the same time raises the vertebral border of the bone. It is therefore concerned in the action of pushing. Its lower and stronger fibers move forward the lower angle and assist the Trapezius in rotating the bone at the sternoclavicular joint, and thus assist this muscle in raising the acromion and supporting weights upon the shoulder. It is also an assistant to the Deltoideus in raising the arm, inasmuch as during the action of this latter muscle it fixes the scapula and so steadies the glenoid cavity on which the head of the humerus rotates. After the Deltoideus has raised the arm to a right angle with the trunk, the Serratus anterior and the Trapezius, by rotating the scapula, raise the arm into an almost vertical position. It is possible that when the shoulders are fixed the lower fibers of the Serratus anterior may assist in raising and everting the ribs; but it is not the important inspiratory muscle it was formerly believed to be.
7c. The Muscles and Fasciæ of the Shoulder
In this group are included:
Deep Fascia.The deep fascia covering the Deltoideus invests the muscle, and sends numerous septa between its fasciculi. In front it is continuous with the fascia covering the Pectoralis major; behind, where it is thick and strong, with that covering the Infraspinatus; above, it is attached to the clavicle, the acromion, and the spine of the scapula; below, it is continuous with the deep fascia of the arm.
The Deltoideus (Deltoid muscle) (Fig. 410) is a large, thick, triangular muscle, which covers the shoulder-joint in front, behind, and laterally. It arises from the anterior