Part 15

_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_).

_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.