Part 14

_Origin_ from the lateral border of the transverse process of the atlas on its ventral margin. The fibres pass craniad and diverge.

_Insertion_ into the caudal side of the mastoid process of the temporal bone and into a line parallel with the lambdoidal ridge and ventrad of it, from the mastoid process to within one centimeter of the median line.

_Relations._--Outer surface with the splenius (Fig. 73, _b_), complexus (Fig. 69, _b_), and longissimus capitis (Fig. 73, _g_). Inner surface with the obliquus inferior (Fig. 71, _b_), the wing of the atlas, and the rectus lateralis (Fig. 72, _d_). Ventral border with the cleidomastoid (Fig. 73, _d_) and one head of the levator scapulæ ventralis (Fig. 72, _c′_).

_Action._--Flexes the head laterally.

=Mm. interspinales= and =intertransversarii= are found in the neck region also; they are described with the muscles of the back.

C. MUSCLES OF THE TAIL.--=M. extensor caudæ medialis= (Fig. 70, _e_).--This is a continuation caudad of the multifidus spinæ (Fig. 70, _d_); it lies next to the dorsal median line, the muscles of right and left side touching one another in the middle line.

_Origin_ by numerous fleshy bundles from the spinous processes of the sacral and first caudal vertebræ. The fibres pass caudad and are inserted by tendons into the articular processes and the dorsal surface of the caudal vertebræ.

_Action._--Extends (raises) the tail.

=M. extensor caudæ lateralis= (Fig. 70, _f_).--This is a continuation caudad of the medial portion of the longissimus dorsi (Fig. 70, _a_); it lies just laterad of the extensor caudæ medialis (Fig. 70, _e_).

_Origin_ in many fleshy bundles from the articular processes of the sacral vertebræ, and the transverse processes of the caudal vertebræ. The fibres curve dorsocaudad and are inserted by many long slender tendons on the dorsal surfaces of the caudal vertebræ. The muscle grows continually smaller as it passes caudad.

_Action._--Raises the tail.

=M. abductor caudæ= (coccygis) =externus= (Fig. 68, _w_; Fig. 70, _g_).--A rounded muscle on the lateral surface of the cranial part of the tail.

_Origin_ from the medial side of the dorsal border of the ilium, and from the dorsal surface of the sacrum. The muscle passes caudad, lying just ventrad of the extensor lateralis, and is _inserted_ into the transverse processes and lateral surfaces of the caudal vertebræ, as far back as the eighth or ninth.

_Action._--Bends the tail sideways.

=M. abductor caudæ internus= (or M. coccygeus).--A large flat muscle, having origin on the spine of the ischium. The muscle passes dorsomediad, spreading out, and is inserted into the transverse processes of the second to the fourth caudal vertebræ.

_Relations._--Lateral surface with the caudofemoralis (Fig. 68, _s_), gluteus maximus, and pyriformis. Medial surface with the iliocaudalis and the flexor caudæ longus (Fig. 68, _x_).

=M. iliocaudalis= (Str.-D.) (Fig. 162, 11′, page 398).--This represents a portion of the levator ani of man, and in some specimens is united with that muscle (Fig. 162, 11) in the cat.

_Origin_ along the ventral half of the medial surface of the ilium, caudad of the sacrum. The fibres pass caudad and are _inserted_ by a flat tendon into the ventral surface of the caudal vertebræ, from the second or third to about the seventh.

_Relations._--Lateral surface with the gluteus maximus and abductor caudæ internus (Fig. 68, _w_). Medial surface with the levator ani (Fig. 162, 11), with which this muscle may be partly united.

_Action._--The two muscles together flex the tail.

=M. flexor caudæ longus= (Fig. 162, 12; Fig. 113, _c_, page 270).

_Origin_ on the ventral surface of the last lumbar vertebra, of the sacrum, and of the transverse processes of the caudal vertebræ. Caudad the muscle forms long, strong tendons which are _inserted_ on the ventral surface of the tail.

_Relations._--Lateral surface with the ilium, the gluteus maximus, the abductor caudæ internus, the iliocaudalis, and the skin of the tail. Dorsally it touches in the caudal region the abductor caudæ externus (Fig. 68, _w_), medially the flexor caudæ brevis (Fig. 162, 13).

_Action._--Flexes the tail.

=M. flexor caudæ brevis= (Fig. 162, 13).

_Origin_ on the ventral surface of the caudal vertebræ, from the first to the seventh or eighth. The principal head comes from the first vertebra. The bundles thus formed pass caudad and are inserted each into the ventral surface of a vertebra some distance caudad of the origin. The muscle extends to about the tenth caudal vertebra, and its most caudal parts are closely interconnected with the flexor caudæ longus.

_Relations._--Lateral edge with the flexor caudæ longus. Medial edge in the sacral region with the levator ani (Fig. 162, 11) and the iliocaudalis (Fig. 162, 11′); in the caudal region with the muscle of the opposite side.

_Action._--Flexes the tail.

2. =Muscles on the Ventral Side of the Vertebral Column.=

A. LUMBAR AND THORACIC REGIONS.--The iliopsoas (Fig. 162, 8) belongs in this group, but since it moves the leg it is described with the muscles of the pelvic limbs.

=M. psoas minor= (Fig. 162, 9, page 398).--This muscle lies along the ventral surface of the vertebral column, next to the peritoneum, and extends from the thoracic vertebræ to the ilium.

_Origin_ usually by five heads, from the caudal border of the centra of the last two (or one) thoracic and first three (or four) lumbar vertebræ. These heads also serve as origin for a part of the iliopsoas, and are closely united at their origin with the quadratus lumborum. They unite to form an, at first, rather large flat muscle, which rapidly becomes smaller caudad, finally forming a slender tendon which is _inserted_ on the iliopectineal line, just craniad of the acetabulum.

_Relations._--Dorsolateral surface with the quadratus lumborum and iliopsoas (Fig. 162, 8). Ventral surface with the pleura and diaphragm near the origin; with the peritoneum caudad.

_Action._--Flexes the back in the lumbar region.

=M. quadratus lumborum.=--A flat muscle lying on the ventral surface of the transverse processes of the lumbar vertebræ. Craniad it has origin on the ventral surface of the last two thoracic vertebræ, and by a few fibres from the last rib. The muscle passes caudad, lying on the lateral side of the psoas minor (Fig. 162, 9), and becomes attached to each of the transverse processes of the lumbar vertebræ. Caudad the muscle passes into a strong flat tendon which is inserted into the anterior inferior spine of the ilium.

_Relations._--Dorsal surface with the transverse processes and the intertransverse muscles. Ventral and medial surfaces with the psoas minor (Fig. 162, 9) and iliopsoas (Fig. 162, 8).

_Action._--Bends the vertebral column sideways.

B. MUSCLES ON THE VENTRAL SIDE OF THE NECK (Fig. 65, page 109).--=M. sternomastoideus= (Fig. 65, _g_; Fig. 68, _c_).--The sternomastoid (sternal portion of the human sternocleidomastoid) is a flat band one to three centimeters wide extending from the cranial end of the manubrium and the midventral line craniad of it to the lambdoidal ridge.

The _origin_ extends from the cranial end of the manubrium along the midventral line as far as the caudal border of the cricoid cartilage, and is in two parts. The =caudal= portion arises from the lateral surface of the manubrium along the dorsal half of its cranial end, and from the median raphe for about one centimeter craniad of the manubrium. Its fibres are parallel and tend to diverge into two layers. The =cranial= portion arises by means of fibres which cover the median line and interdigitate with the fibres of the opposite muscle between the cricoid cartilage and a point a few millimeters craniad of the manubrium. Its caudal border is thus overlaid by the caudal portion. Its fibres converge and join those of the caudal portion. The muscle passes dorsocraniad and is

_Inserted_ by means of a flat tendon into the lateral half of the lambdoidal ridge and into a continuation of the ridge onto the mastoid portion of the temporal bone as far as the mastoid process. The thickest part of the tendon is inserted into the mastoid process.

_Relations._--Outer surface at the caudal end with the pectoralis major (Fig. 65, _l_); in the middle part with the platysma, the depressor conchæ (Fig. 64, _b_), and with the external jugular vein (Fig. 65, 5), which crosses it obliquely; at the cranial end with the submaxillary (Fig. 65, 2) and parotid (1) glands. Inner surface with the sternohyoid (Fig. 65, _e_), sternothyroid (_g′_), internal jugular vein, longus capitis muscle, the cleidomastoid (_h_), levator scapulæ ventralis (_i_), a large lymphatic gland ventrad of the ear, and the splenius (Fig. 73, _b_).

_Action._--One muscle turns the head and depresses the snout. Both together depress the snout.

=M. sternohyoideus= (Fig. 65, _e_).--A slender muscle on the midventral line of the neck close to the opposite muscle.

_Origin_ from the cranial border of the first costal cartilage. The muscle passes craniad closely united near its caudal end to the sternothyroid (_g′_).

_Insertion_ (Fig. 67, _m_) into the outer half of the ventral surface of the body of the hyoid bone caudad of the origin of the geniohyoid (Fig. 67, _g_).

_Relations._--Outer surface with the sternomastoid (Fig. 65, _g_). Inner surface with the thyrohyoid (Fig. 65, _f_; Fig. 67, _p_), the trachea (Fig. 67, 4), and the larynx.

_Action._--Draws the hyoid caudad. Raises the ribs and sternum when the hyoid is fixed.

=M. sternothyreoideus= (Fig. 65, _g′_).--The sternothyroid lies beneath the sternohyoid and is connected with it at its caudal end.

_Origin_ from the first costal cartilage beneath the sternohyoid.

_Insertion_ (Fig. 67, _o_) into the lateral part of the caudal border of the thyroid cartilage of the larynx.

_Relations._--Outer surface with the sternohyoid (Fig. 65, _e_) and laterally with the sternomastoid (Fig. 65, _g_). Inner surface with the trachea (Fig. 67, 4), and at the lateral edge with the thyroid gland (Fig. 67, 6).

_Action._--Pulls the larynx caudad.

=M. scalenus= (Fig. 73, _f_-_f‴_).--This is a large and complex muscle lying on the ventral side of the neck and the lateral surface of the thorax. It might equally well be classified with the muscles of the thorax. It is divisible into a considerable number of interconnected bundles which are sometimes described as separate muscles.

The largest, middle portion (=scalenus medius=) (_f′_) takes origin by thin tendons from the sixth, seventh, eighth, and ninth ribs, just dorsad of their junction with the cartilages; these tendons unite near their origins to form a flat band which becomes fleshy at about the fifth rib, passes craniad, and unites just craniad of the first rib with the other parts of the muscle. The dorsal portion (=scalenus posterior=) (_f″_) has origin by a very slender tendon from about the middle of the outer surface of the third or fourth rib; it extends craniad as a narrow band which passes laterad between the heads of the serratus anterior (_i_) that are attached to the second and third (or third and fourth) ribs, and unites with the other heads. The ventral head (part of =scalenus anterior=) (_f‴_) arises by one or two minute tendons from the cartilages of the second and third ribs; they are partly united with the transversus costarum (_j_). This ventral head passes craniad and unites with the other heads.

These three divisions unite just craniad of the first rib with each other and with the cervical portion of the muscle (_f_) (part of =scalenus anterior=). This consists of a number of small bundles of fibres which arise from the first rib and the transverse processes of the first thoracic and the last six cervical vertebræ.

The _insertion_ is onto the transverse processes of all the cervical vertebræ, including the axis and atlas. According to Strauss-Durckheim the fibres from any given transverse process are inserted into the transverse processes of all the vertebræ craniad of it.

The muscle is partly continuous craniad with the longus capitis (_e_). The cervical nerves pass out between the bundles of the cervical portion.

_Relations._--Outer (lateral) surface in the thoracic region with the external oblique (Fig. 68, _p_), the pectoralis minor (Fig. 65, _o_), and in part with the serratus anterior (Fig. 73, _i_); in the cervical region with the sternomastoid (Fig. 65, _g_). Inner (medial) surface with the serratus anterior (Fig. 73, _i_), the levator scapulæ (Fig. 73, _h_), the longus atlantis, the transverse processes of the cervical vertebræ, and the longus capitis (Fig. 73, _e_).

_Action._--Flexes the neck and draws the ribs craniad.

=M. longus capitis= (or =rectus capitis anterior major=) (Fig. 72, _a_; Fig. 73, _e_).--This is a long muscle lying on the ventral aspect of the cervical vertebræ. With the muscle of the opposite side and the underlying longus colli (Fig. 72, _g′_) it forms a trough in which lie the œsophagus, pharynx, and trachea.

_Origin_ by five (or six) heads from the ventral margins of the transverse processes of the cervical vertebræ from the second to the sixth inclusive. The heads unite into a common belly, the lateral border of which is united with the levator scapulæ ventralis (Fig. 72, _c_) near its insertion.

_Insertion_ into the body of the sphenoid bone between the bulla and the middle line. The insertion extends craniad onto the basisphenoid.

_Relations._--Ventral surface with the sternomastoid (Fig. 65, _g_) and the large lymphatic gland ventrad of the external ear. Lateral surface with the scalenus (Fig. 72, _h_) and craniad with the levator scapulæ ventralis (Fig. 72, _c_-_c″_). Medial surface with the carotid artery, the trachea, œsophagus, and pharynx. Dorsal edge with the vertebræ, the longus colli (Fig. 72, _g′_), and the rectus capitis anterior minor (Fig. 72, _b_).

_Action._--Lowers the snout.

=M. rectus capitis anterior minor= (Fig. 72, _b_).--This is a flat band beneath (dorsad of) the longus capitis (_a_).

_Origin_ from the ventral surface of the inferior arch (body) of the atlas (3) for about five millimeters laterad of the middle line.

_Insertion_ into a deep depression on the basioccipital caudad of the insertion of the longus capitis (_a_) and between the caudal end of the bulla (1) and the median line.

_Relations._--Ventral surface with the longus capitis (_a_). Dorsal surface with the anterior (ventral) atlanto-occipital membrane (2).

_Action._--Depresses the snout.

=M. rectus capitis lateralis= (Fig. 72, _d_).--This lies ventrad of the obliquus superior (_e_) on the ventral face of the transverse process of the atlas and at first appears to be a part of the obliquus superior.

_Origin_ from the median half of the ventral surface of the transverse process of the atlas. The fibres form a cylindrical mass which passes craniad and slightly laterad.

_Insertion_ into the fossa laterad of the condyle of the occipital bone.

_Relations._--Ventral surface with the digastric (Fig. 65, _b_) and the lymphatic gland ventrad of the ear. Dorsal surface with the wing of the atlas.

_Action._--Flexes the head laterally.

=M. longus colli= (Fig. 65, _g′_, _g″_).--A long, rather slender muscle lying on the ventral surface of the cervical and first six thoracic vertebræ. It consists of many separate bundles, and is divisible into a thoracic and a cervical portion.

The =thoracic portion= (_g″_) has origin by six heads from the ventral surface of the first six thoracic vertebræ. The separate heads join to form a band which passes craniad and is inserted for the most part into the processus costarius of the sixth cervical vertebra; a portion of it joins the cervical division of the muscle.

The =cervical portion= (_g′_) is a continuation craniad of the thoracic portion. It arises in small bundles from the transverse processes and sides of the ventral surfaces of the centra of the cervical vertebræ. These bundles pass craniomediad, so that those from the muscles of opposite sides meet and are inserted on the centra of the vertebræ in the middle line, each pair of bundles forming a V opening caudad. The most cranial insertion is into the tubercle on the middle of the ventral surface of the ventral arch of the atlas (3).

_Relations._--Dorsal surface with the centra of the vertebræ. Ventral surface in the thorax with the trachea; in the neck with the œsophagus. Medial edge with the muscle of the opposite side. Lateral margin in the cervical region with the longus capitis (_a_).

_Action._--Bends the neck.

3. =Muscles of the Thorax.=

A. BREAST-MUSCLES (connecting the arm and thorax) (Fig. 65, page 109).

The muscles connecting the arm with the sternum, corresponding to the pectoralis muscles of man, fall into a considerable number of not always clearly separated bundles in the cat. We shall distinguish by name four such bundles; these might be, and frequently are, further subdivided.

=M. pectoantibrachialis= (Fig. 65, _m_).--This is a small flat bundle about one or two centimeters wide, forming the most superficial portion of the pectoralis group.

_Origin_ on the lateral surface of the manubrium. The muscle passes laterad onto the arm and is _inserted_ by a flat tendon into the superficial fascia of the dorsal border of the forearm, near the elbow-joint. The tendon is continuous craniad with the border of the clavobrachial (_k_), so that a bundle of fibres may pass from this muscle to be inserted with the clavobrachial, or a bundle from the clavobrachial to be inserted with this. The tendon is continuous caudad with that of the epitrochlearis (_r_). The fascia of insertion may be traced to an attachment into the dorsal border of the ulna. This muscle usually receives near its insertion a slip, sometimes called its caudal division, from the deep layer of the pectoralis major.

_Relations._--Outer surface with the integument. Inner surface with both divisions of the pectoralis major (_l_) on its medial two-thirds, and with the biceps (_t_) and bicipital arch (_t′_) in its lateral one-third. The cranial border is closely applied to that of the clavobrachial (_k_), except at the two ends.

_Action._--Draws the arm mediad.

=M. pectoralis major=, or =ectopectoralis= (Fig. 65, _l_, _l′_).--In this it is usually possible to distinguish a superficial and a deep layer.

The =superficial= (_l′_) layer is a flat band of fibres one or two centimeters wide.

_Origin_ from a raphe in the midventral line, along the cranial half of the manubrium and for five to ten millimeters craniad of it.

_Insertion_ (Fig. 81, _f_).--The pectoral ridge of the middle third of the shaft of the humerus, slightly dorsad of the line of insertion of the deep portion.

_Relations._--Outer surface with the clavobrachial (Fig. 65, _k_) and integument, the medial end caudally with the pectoantibrachialis (_m_), and the distal end with the brachialis (_v_). Inner surface with the deep portion (_l_), the fibres of which it crosses obliquely.

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.