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

Insertion on the first rib and the lateral portion of its costal cartilage.

Relations.—Outer surface with the pectoralis minor (Fig. 65, o). Inner surface with the scalenus (Fig. 73, f‴), the rectus abdominis (Fig. 73, k), and the intercostales externi (Fig. 73, m). At its insertion it is united with a portion of the scalenus (Fig. 73, f‴).

Action.—Draws the sternum forward.

Mm. levatores costarum.—Small muscles having origin on the transverse processes of the thoracic vertebræ, passing caudoventrad, and becoming inserted on the angle of the rib lying immediately caudad of the origin. They are continuous with the external intercostals.

Relations.—Outer surface with the longissimus dorsi (Fig. 69, f), and craniad with the iliocostal (Fig. 69, h). Inner surface with the internal intercostals (Fig. 69, k).

Action.—Pull the ribs dorsocraniad.

Mm. intercostales externi (Fig. 73, m; Fig. 69, i).—The external intercostals are placed in the outer portion of the intercostal spaces. They are composed of bundles of fibres attached by their ends to the adjacent borders of the ribs and having in general the direction of the external oblique muscle, i.e., they pass from their cranial ends caudoventrad. They occupy the intercostal spaces between the true ribs and extend even caudad into the spaces between the false ribs. They are lacking between the ventral ends of the costal cartilages of the first six to eight ribs, so that the internal intercostals (Fig. 69, k) are here exposed. The more caudal external intercostals are more nearly craniocaudal in direction.

Relations.—Outer surface with obliquus abdominis externus (Fig. 68, p), latissimus dorsi (Fig. 68, m), serratus posterior inferior (Fig. 73, n) and superior (Fig. 73, l), serratus anterior (Fig. 73, i), scalenus (Fig. 73, f), and iliocostal (Fig. 69, h). Inner surface with the internal intercostals (Fig. 69, k).

Action.—Protractors of the ribs.

Mm. intercostales interni (Fig. 69, k).—The internal intercostals are similar to the external intercostals, beneath which they lie. Their fibres pass between the ribs at nearly right angles to those of the external intercostals and have nearly the direction of the fibres of the internal oblique. They occupy all the intercostal spaces from the first to the thirteenth ribs.

Relations.—Outer surface with the external intercostals (Fig. 69, i), and ventrad with the scalenus (Fig. 73, f), transversus costarum (Fig. 73, j), and rectus abdominis (Fig. 73, k). Inner surface with the pleura and the transversus thoracis.

Action.—Retractors of the ribs.

M. transversus thoracis (triangularis sterni; sternocostalis internus).—This represents a thoracic portion of the transversus abdominis. It consists of five or six flat muscular bands lying on the inner surface of the thoracic wall.

Origin on the lateral borders of the dorsal face of the sternum, opposite the attachments of the cartilages of the third to the eighth ribs. The six bands thus formed, each about one centimeter wide, pass laterad and are inserted into the cartilages of the ribs near their junction with the ribs, and into the fascia which covers the inner surface of the internal intercostals in this region.

Relations.—Outer surface with the internal intercostals and the cartilages of the ribs. Inner surface with the pleura.

Diaphragma (Fig. 74).—The diaphragm consists of a central so-called semilunar tendon (e) and of muscular fibres which pass radially from the body wall to the tendon. It forms a complete oblique partition between the abdominal and thoracic cavities. The dorsal end is farther caudad than the ventral.

The central tendon (e) is thin and irregularly crescent-shaped, with the convexity ventrad and the horns of the crescent prolonged as two tendinous bands (e′) which end in two triangular membranous portions (d) of the diaphragm, one on each side of the spinal column. It is pierced by an opening for the vena cava (f).

The muscular portion is in two parts: (a) The vertebral portion (a, b) arises by a single tendon from the ventral surface of the second, third, and fourth lumbar centra. The tendon diverges into two, the right one (a) of which is much stronger, and from each of the two arise muscle-fibres. Each mass of fibres is one of the two crura (sing. crus) of the diaphragm. The aorta (h) enters the abdomen between the crura dorsally. The fibres of each crus diverge to be inserted into the central tendon and the dorsal continuation of its horn. The two sets of fibres unite ventrad of the opening of the aorta. Between this opening and the central tendon is another opening for the œsophagus (g); this lies entirely in that part of the diaphragm which arises from the right crus (a). The fibres are again united ventrad of the œsophageal opening.

(b) The sternocostal part (c, c′) takes origin from the xiphoid process and the last five ribs, by fleshy bundles which interdigitate with those of the transversus abdominis (i). The fibres converge to the central tendon (e). Between the crus of each side and the most dorsal of the costal fibres is the membranous interval (d) mentioned above.

4. Abdominal Muscles.

M. obliquus abdominis externus (Fig. 68, p).—A large, thin sheet of muscle covering the whole abdomen and part of the thorax ventrally.

Origin.—(a) From the last nine or ten ribs by means of as many tendons, which are interconnected to form arches that span the slips of the serratus anterior. The muscle-fibres arise from these tendons and from their intervening arches. (b) From the lumbodorsal aponeurosis common to it and the internal oblique. The cranial fibres pass nearly ventrad, the caudal fibres caudad, and the intervening fibres take an intermediate course. The fibres end in a thin aponeurosis of insertion along a curved line which passes at first caudad and then laterodorsad. The aponeurosis fibres continue in the direction of the muscle-fibres to the

Insertion into the median raphe ventrad of the sternum from the insertion of the seventh costal cartilage to the xiphoid process, into the linea alba from the sternum to the pubic tubercle, and into the tubercle and the cranial border of the pubis. Caudad of the xiphoid process the aponeurosis is closely united to the superficial layer of the internal oblique, where it forms the outer layer of the sheath of the rectus abdominis. Laterad of the pubic tubercle the tendon is perforated by the inguinal canal. In the cat neither the caudal part of the muscle nor its tendon is attached to the ilium, as it is in man and the dog, so that no Poupart’s ligament, or inguinal ligament, is formed.

Relations.—Outer surface with the cutaneus maximus, the integument, and near the origin with the latissimus dorsi (Fig. 68, m). Inner surface with the obliquus internus (Fig. 73, o), the rectus abdominis (Fig. 73, k), the intercostales externi (Fig. 73, m), a small part of the serratus posterior inferior (Fig. 73, n), and by its dorsal tendon with the longissimus dorsi (Fig. 69, f).

Action.—Constrictor of the abdomen.

M. obliquus abdominis internus (Fig. 73, o).—A thin sheet similar to the preceding but of less extent. Its fibres cross those of the external oblique nearly at right angles and lie beneath them.

Origin.—(1) Between the fourth and seventh lumbar vertebræ from the lumbar aponeurosis which is common to it and the external oblique. The lumbar aponeurosis takes origin from the lumbar spinous processes and the interspinous ligaments, and is continuous craniad with the aponeurosis of the serratus posterior inferior. Laterad it splits into three sheets, two for the above-named muscles and a third which passes to the vertebral transverse processes and forms the fascia covering the supraspinous muscles of the lumbar region. (2) By a similar aponeurosis from the ventral half of the iliac crest. (3) By fleshy fibres from the three crural arches. These are three delicate ligamentous arches which stretch from the crest of the ilium to the pubic spine. The dorsal one gives exit to the iliopsoas muscle, the middle to the femoral vessels, and the ventral to the spermatic cord. In the female the middle and ventral arches may fuse. The pillar between the dorsal and middle arches is attached to the iliopectineal eminence.

Insertion.—The fibres pass cranioventrad and end along a longitudinal line in a thin aponeurosis of insertion, which is united in the linea alba to those of the external oblique and transversus. At the caudal end of the aponeurosis all its fibres pass outside of the rectus abdominis (Fig. 73, k). At its cranial end the fibres divide into two sheets or laminæ, one of which passes outside of the rectus and the other inside. The outer lamina unites with the aponeurosis of the external oblique, while the inner lamina unites with that of the transversus. There is thus formed a sheath for the cranial part of the rectus muscle.

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.)

Origin.—Its superficial fibres are continuations of the clavotrapezius (Fig. 68, d); other fibres have origin from the clavicle and from a raphe laterad of the clavicle which is common to this muscle and the clavotrapezius. It passes distad along the cranial surface of the arm, growing narrower as it approaches the convexity of the elbow.

Insertion.—Eight to ten millimeters from the ulna it joins the brachialis muscle (Fig. 79, i) to be inserted with it by a flat tendon (Fig. 79, i; Fig. 87, c) upon a rough area on the medial surface of the ulna just distad of the semilunar notch, and about midway between the dorsal and ventral borders.

Relations.—Outer surface with the integument. Inner surface with the pectoralis major (Fig. 65, l), biceps (Fig. 77, g), lateral head of the triceps (Fig. 75, h), acromiodeltoid (Fig. 75, f), and brachialis (Fig. 80, h). Medial border with the pectoantibrachialis (Fig. 65, m). Lateral border free except near the insertion, where it is in relation with the brachialis.

Action.—Flexor of the antibrachium.

M. supraspinatus (Fig. 75, a) occupies the whole of the supraspinatus fossa of the scapula. It is covered by strong fascia which stretches from the free edge of the spine to the coracoid border of the scapula and to the coracoid half of its vertebral border.

Origin (Fig. 76, b) by fleshy fibres from the whole surface of the supraspinatus fossa, from the above-mentioned fascia, and from the subscapularis (Fig. 77, a) craniad of the coracoid border of the scapula.

Insertion (Fig. 83, a).—It passes over the capsule of the shoulder-joint, to which it is closely attached, and is inserted into the free border of the great tuberosity ventrad (or proximad) of the fossa for the infraspinatus (Fig. 83, c).

Relations.—Outer surface with the spinotrapezius (Fig. 68, j), acromiotrapezius (Fig. 68, h), levator scapulæ ventralis (Fig. 68, f), clavotrapezius (Fig. 68, d), and cleidomastoid (Fig. 65, h). Inner surface with the scapula. The distal end of the coracoid border is closely related to the pectoralis minor (Fig. 79, f). The glenoid border is related to the origin of the deltoidei.

Action.—Extends the humerus after it has been flexed on the scapula.

M. infraspinatus (Fig. 75, c).—This fills the infraspinatus fossa, its fibres converging to the insertion on the great tuberosity of the humerus.

Origin (Fig. 76, a).—By fleshy fibres from the whole infraspinatus fossa, and by a raphe between it and the teres major (Fig. 75, d), and sometimes by fibres from the teres minor (Fig. 80, c), triceps, spinodeltoideus (Fig. 75, e), spinotrapezius (Fig. 68, j), and subscapularis (Fig. 77, a).

Insertion (Fig. 83, c).—By a flat tendon which passes over the capsule of the joint, into the ventral half of the infraspinatus fossa on the great tuberosity of the humerus. (The dorsal half of the fossa is covered by a synovial bursa.)

Relations.—Outer surface with the spinotrapezius (Fig. 68, j), spinodeltoid (Fig. 68, i), acromiodeltoid (Fig. 68, g), teres major (Fig. 75, d), and levator scapulæ ventralis (Fig. 68, f). Inner surface with the scapula. Glenoid border with the teres minor (Fig. 80, c), the teres major (Fig. 75, d), and the long head of the triceps (Fig. 75, g).

Action.—Rotates the humerus outward.

M. teres minor (Fig. 80, c).—A small muscle from the glenoid border of the scapula to the proximal end of the humerus.

Origin (Fig. 76, e) by a sheet of tendinous fibres from the glenoid border of the scapula beginning about one-fifth the length of the border from the glenoid fossa and extending to its middle. It is often attached to the infraspinatus (Fig. 80, b) and the caput longum of the triceps (Fig. 75, g).

Insertion (Fig. 83, d) by a short tendon into the tubercle just distad of the infraspinatus fossa on the great tuberosity of the humerus.

Relations.—Outer surface with the spinodeltoideus (Fig. 75, e), acromiodeltoideus (Fig. 75, f), and the infraspinatus (Fig. 80, b). Inner surface with the lateral (Fig. 75, h) and long (Fig. 75, g) heads of the triceps and the capsule of the joint.

Action.—Assists the infraspinatus to rotate the humerus outward.

B. Muscles on the Medial Surface of the Shoulder.

(Fig. 77.)

M. subscapularis (Fig. 77, a).—A triangular mass from the subscapular fossa to the lesser tuberosity of the humerus.

Origin (Fig. 78, a).—From the whole subscapular fossa except along the fusiform area for the attachment of the levator scapulæ and the serratus anterior near the vertebral border, and except over a quadrangular area about one centimeter long near the glenoid angle. The origin is by fleshy fibres directly from the periosteum except along two or three lines marked by oblique ridges. To these lines are attached tendinous fibres. At the glenoid border the area of origin sometimes occupies part or the whole of the surface of the fossa for the origin of the teres major, so that the teres arises from the fascia on the surface of the subscapularis. At the coracoid border some of the fibres may take origin from the adjacent fascia of the supraspinatus (Fig. 77, d).

Insertion (Fig. 82, b).—The fibres converge to the glenoid border, and the insertion is by a strong, flat tendon into the dorsal border of the lesser tuberosity of the humerus.

Relations.—Lateral surface with the scapula and the capsule of the shoulder-joint. Medial surface with the levator scapulæ (Fig. 73, h), serratus anterior (Fig. 73, i), part of the scalenus (Fig. 73, f), the transversus costarum (Fig. 73, j), and the coracobrachialis (Fig. 77, f). Cranial border with the supraspinatus (Fig. 77, d). Caudal border with the teres major (Fig. 77, c) and infraspinatus (Fig. 75, c).

Action.—Pulls the humerus inward (mediad).

M. teres major (Fig. 75, d; Fig. 77, c).—A thick muscle, triangular in cross-section, lying parallel with the glenoid border of the scapula.

Origin (Fig. 76, j; Fig. 78, b) from the vertebral one-third of the glenoid border of the scapula, and from fascia covering the subscapularis (Fig. 77, a) and the infraspinatus (Fig. 75, c) over a small area near the glenovertebral angle of the scapula. It may touch the insertion of the rhomboideus.

Insertion (Fig. 82, f) by a tendon common to it and the latissimus dorsi (Fig. 79, d′), as already described.

Relations.—Outer surface with the caput longum (Fig. 75, g) and the long portion (Fig. 80, f) of the caput mediale of the triceps, the latissimus dorsi (Fig. 77, e), and the cutaneus maximus. Inner surface with the serratus anterior (Fig. 73, i), the scalenus (Fig. 73, f), the transversus costarum (Fig. 73, j), and the biceps (Fig. 77, g). Dorsal border with the subscapularis (Fig. 77, a) and infraspinatus (Fig. 75, c).

Action.—Rotates the humerus inward and flexes it in opposition to the infraspinatus, teres minor, and the deltoidei.