Part 37

Relations.--Outer surface with the obliquus externus (Fig. 68, p). Inner surface with the transversus abdominis (Fig. 69, l), and by its ventral tendon with the rectus.

Action.--Compressor of the abdomen.

=M. transversus abdominis= (Fig. 69, l).--A thin sheet covering the whole surface of the abdomen and lying beneath the internal oblique. Its fibres are nearly transverse.

Origin.--(1) By fleshy fibres or by a thin aponeurosis from the cartilages of all the false and floating ribs, by interdigitation with the fibres of the diaphragm. (2) From the tips of all the lumbar transverse processes. (3) From the ventral border of the ilium. (4) From the dorsal and middle of the three crural arches, where it may blend partly with the internal oblique. The muscle is continuous craniad with the transversus thoracis muscle. Near the lateral border of the rectus abdominis the muscle ends in a thin aponeurosis of insertion which is continued (its fibres having the direction of the muscle-fibres) to the

Insertion in the linea alba.

Relations.--Outer surface with the internal oblique (Fig. 73, o) and the rectus abdominis (Fig. 73, k), dorsad also with the longissimus dorsi (Fig. 69, f). Inner surface with a thin fascia covering the peritoneum and ventrocaudad with the rectus (Fig. 73, k). The dorsal edge of the muscle touches the iliopsoas and longissimus dorsi; the ventral edge touches the muscle of the opposite side.

Action.--Constrictor of the abdomen.

=M. rectus abdominis= (Fig. 73, k).--A rather thick, flat muscle which lies near the median ventral line separated by the linea alba from its fellow of the opposite side and stretching from the pubis to the first costal cartilage. Opposite the first lumbar vertebra it is approximately four centimeters wide. It narrows at both ends.

Origin by a strong tendon from the tubercle of the pubis. The muscle passes craniad at first between the peritoneal fascia and the transversus aponeurosis, then in the sheath formed by the internal and external oblique and the transversus aponeurosis. It emerges from the sheath opposite the xiphoid process and passes ventrad of the costal cartilages. Between the third and fourth costal cartilages it ends in a thin tendon which passes beneath the transversus costarum and is

Inserted into the first costal cartilage near its middle, into the second costal cartilage near its sternal end, and into the sternum between the first and fourth cartilages.

Relations.--Outer surface of the cranial part of the muscle and its tendon with the pectoralis minor (Fig. 68, o) and the transversus costarum (Fig. 73, j); caudad the muscle is covered by the outer layer of the rectus sheath. Lateral edge with the obliquus externus (Fig. 68, p), obliquus internus (Fig. 73, o), and transversus abdominis (Fig. 69, l). Medial edge with the muscle of the opposite side. Inner surface with the internal intercostals (Fig. 69, k) and the rib cartilages; the inner layer of the rectus sheath and the peritoneum.

Action.--Retracts the ribs and sternum and compresses the abdomen.

IV. MUSCLES OF THE THORACIC LIMBS.

The muscles connecting the thoracic limbs with the rest of the body have been described.

1. Muscles of the Shoulder.

A. MUSCLES ON THE LATERAL SURFACE OF THE SHOULDER.

=M. deltoideus.=--The deltoid muscle of the cat is divided into two (or three) portions which are together equivalent to the human deltoid. These are the spinodeltoid, the acromiodeltoid, and possibly the clavobrachial, which is frequently called clavodeltoid.

=M. spinodeltoideus= (Fig. 75, e; Fig. 68, i).--A rather thick, flat muscle between the scapular spine and the deltoid ridge of the humerus. It forms a chord of the angle between the glenoid border of the scapula and the humerus.

Origin by short tendon-fibres from the glenoid border of somewhat more than the middle third of the spine of the scapula (Fig. 76, g), and from a tendinous raphe between the spinotrapezius (Fig. 68, j), acromiotrapezius (Fig. 68, h), and infraspinatus (Fig. 75, c). The origin may pass toward the vertebral border of the scapula onto the infraspinatus muscle (Fig. 75, c).

Insertion (Fig. 81, h) by a flat tendon upon the deltoid ridge of the humerus, nearly parallel to that of the pectoralis major.

Relations.--Outer surface with the integument and at the insertion with the acromiodeltoideus (Fig. 75, f). Inner surface with the infraspinatus (c), teres minor (Fig. 80, c), caput laterale (Fig. 75, h), and caput longum (Fig. 75, g) of the triceps muscle.

Action.--Flexes the humerus and rotates it outward.

=M. acromiodeltoideus= (Fig. 75, f; Fig. 68, g).--A flat muscle which overlies the distal end of the spinodeltoideus (Fig. 75, c). It connects the acromion with the humerus.

Origin (Fig. 76, d).--From the glenoid border of the acromion, and sometimes the adjacent metacromion as far as the tip.

Insertion.--Mostly upon the outer surface of the spinodeltoideus (Fig. 75, c). The outer fibres are continued to the bone, especially at the lateral border of the muscle, and are inserted along a line ventrad of the line of insertion of the spinodeltoideus, and extending farther distally (Fig. 81, g). Some of the outer fibres pass into the brachialis (Fig. 80, h).

Relations.--Outer surface with the integument and the clavobrachial (Fig. 68, e). Inner surface with the infraspinatus (Fig. 75, c), teres minor (Fig. 80, c), spinodeltoid (Fig. 75, e), and caput laterale of the triceps (Fig. 75, h).

Action.--Like that of the spinodeltoid.

=M. clavobrachialis= (Fig. 65, k; Fig. 68, e).--A flat, triangular muscle on the cranial surface of the shoulder, forming a direct continuation of the clavotrapezius (Fig. 68, d). These two are frequently described as constituting a single muscle, the cephalohumeral or cephalobrachial. (The clavobrachial is frequently given the name clavodeltoid; as its homology with the human clavodeltoid appears doubtful, it seems well to use the name clavobrachial, as proposed by Clasen.)