Part 35

The =deep portion= (l) is a flat band of parallel fibres about three times as broad as the superficial portion.

Origin.--The ventral surface of the manubrium and of the first three divisions of the sternum, and the median raphe for about one centimeter craniad of the manubrium. It passes directly laterad.

Insertion (Fig. 81, e) along a line which begins at the infraspinatus fossa of the great tuberosity, and runs parallel to the deltoid ridge until it reaches the pectoral ridge at the junction of the second and third fourths of the humerus and then continues in the direction of the pectoral ridge as far as the junction of the third and fourth fourths of the humerus. Some of its fibres may pass caudad of the biceps and be inserted with the epitrochlearis. Into the part of the line which is parallel to the deltoid ridge the muscle is inserted by a flat tendon; into the remainder of the line it is usually inserted directly by muscle-fibres, though at its caudal end its insertion may again be tendinous.

Relations.--Outer surface with the cutaneus maximus, the outer layer of the pectoralis major (Fig. 65, l′), the pectoantibrachialis (m), and the clavobrachial (k). Inner surface with the pectoralis minor (o), the proximal end of the humerus, the coracobrachialis (Fig. 77, f), and the supraspinatus (Fig. 77, d) at the insertion of the latter. At the cranial part of its origin it touches the sternomastoid (Fig. 65, g) and sternohyoid (Fig. 65, e).

Action.--Draws the arm mediad and turns the foot forward.

=M. pectoralis minor=, or =entopectoralis= (Fig. 65, o).--This is a fan-shaped mass of fibres, flat but thicker than the pectoralis major.

Origin from the lateral half of the first six divisions of the body of the sternum and sometimes from the xiphoid process. The fibres are divisible into several bundles which vary in extent and distinctness and are sometimes described as separate muscles. They pass craniolaterad and converge so that the line of insertion is about one-half as long as that of the origin.

Insertion (Fig. 81, d) into the humerus along a line which forms the ventral border of the bicipital groove at the proximal end of the bone, passes thence onto the pectoral ridge and continues in the direction of the pectoral ridge nearly as far as the middle of the length of the humerus. The cranial and caudal fibres are inserted by means of a thin tendon; the middle fibres directly. The cranial border of the tendon of insertion is continued as a thin tendon to the coracoid process; it is closely united with the insertion of the supraspinatus.

Relations.--By its outer surface with the pectoralis major (Fig. 65, l) over its cranial half; with the cutaneus maximus, latissimus dorsi (Fig. 65, q), and integument over its caudal half. By its inner surface with the xiphihumeralis (Fig. 65, p), rectus abdominis (Fig. 73, k), and a mass of fat in the axilla. The tendon of the xiphihumeralis is also connected with its inner surface near its insertion. Its cranial border is free. Its caudal border is in relation at its medial end with the xiphihumeralis, at its lateral end with the tendon of the latissimus dorsi (Fig. 65, q).

Action.--Draws the arm toward the middle line.

=M. xiphihumeralis= (Fig. 65, q).--A long, thin, narrow muscle which may be considered as part of the pectoralis minor.

Origin a median raphe along the xiphoid process or at an angle to the median line on the rectus abdominis muscle. It passes craniad, becoming gradually smaller, and about two centimeters from its insertion ends in a thin tendon.

Insertion.--It ends in a flat tendon which in passing is connected by a strong fascia with the tendon of the latissimus (Fig. 65, q). It passes along the inner surface of the pectoralis minor (o) to be inserted with its cranial fibres near the ventral border of the bicipital groove.

Relations.--Outer surface with the cutaneus maximus at its medial end, with the pectoralis minor (o) at its distal end. Inner surface with the external oblique (Fig. 68, p), the rectus abdominis (Fig. 73, k), and a mass of fat in the axilla. Borders free. Some of the fibres of the cutaneus maximus are attached to its outer surface.

Action.--Assists the pectoralis minor.

B. MUSCLES OF THE WALL OF THE THORAX.

=M. serratus posterior superior= (Fig. 73, l).--A thin sheet of muscle and tendon beneath the serratus anterior (i) on the dorsal part of the thorax and neck.

Origin by fleshy slips from the outer surfaces of the first nine ribs just ventrad of their angles. The origin may extend as far as the tenth or eleventh ribs. The first slip is usually delicate. The fibres are directed dorsad and craniad and unite to form a continuous sheet. This ends along a longitudinal line opposite the vertebral transverse processes in the aponeurosis which covers the longissimus dorsi and other muscles in this region. Its insertion is into the median dorsal raphe between the axial spinous process and the tenth thoracic spinous process.

Relations.--Outer surface with the serratus anterior (i), the latissimus dorsi (Fig. 68, m), and the rhomboideus. Inner surface with the external intercostals (Fig. 73, m), the iliocostal (Fig. 69, h), the longissimus dorsi (Fig. 69, f), the spinalis dorsi (Fig. 69, g), and the splenius (Fig. 73, b).

Action.--Draws the ribs craniad.

=M. serratus posterior inferior= (Fig. 73, n).--The serratus posterior inferior is a thin muscle lying caudad of the serratus posterior superior (l) and sometimes overlying the caudal end of the latter.

Origin by four or five heads from the last four or five ribs. In some specimens only three heads may be present. The separate bundles pass dorsocaudad and unite to form a continuous sheet which ends in a continuation of the aponeurosis of the serratus posterior superior (l).

Insertion into the lumbar spinous processes and the intervening interspinous ligaments.

Relations.--Outer surface with the latissimus dorsi (Fig. 68, p), and at the origins with the intercostales externi (Fig. 73, m). Inner surface with the intercostales interni (Fig. 69, k) at the origin and with the intercostales externi (Fig. 69, i), iliocostal (Fig. 69, h), longissimus dorsi (Fig. 69, f), and the caudal end of the spinalis dorsi (Fig. 69, g).

a, M. occipitoscapularis (cut); b, M. splenius; c, M. levator scapulæ ventralis (cut); d, M. cleidomastoideus (cut); e, M. longus capitis; f, M. scalenus (f′, middle division; f″, dorsal division; f‴, ventral division; f, cervical portion); g, M. longissimus capitis; h, M. levator scapulæ (cut); i, M. serratus anterior (cut); j, M. transversus costarum; k, M. rectus abdominis (k′, its tendon); l, M. serratus posterior superior; m, M. intercostales externi; n, M. serratus posterior inferior; o, M. obliquus abdominis internus; p, lumbodorsal fascia.]

Action.--Assists the serratus posterior superior.

=M. transversus costarum=, or =sternocostalis externus= (Fig. 73, j).--This is a thin muscle applied to the cranial part of the side of the thorax, covering the cranial end of the rectus abdominis (k).

Origin by tendon from the side of the sternum between the attachments of the third and sixth ribs. The muscle passes dorsocraniad to its