Part 12

=M. pterygoideus internus= (Fig. 66, _c_).--This lies caudad of the pterygoideus externus, nearly parallel to it, and is connected with it at its lateral end.

_Origin_ from the whole surface of the internal pterygoid fossa (Fig. 40, _s_).

_Insertion_ by fleshy fibres into the ventral surface of the external pterygoid and its tendon, into the medial surface of the angular process of the mandible and caudad of the angular process for about one centimeter into a fascia common to it and the masseter muscle, and into the stylomandibular ligament.

_Relations._--Ventral surface with the styloglossus and the soft palate. Dorsal surface with the bulla tympani (5) and base of the skull. Cranial surface with the pterygoideus externus (_b_), with which it is partly united.

_Action._--Assists the pterygoideus externus.

(_b_) _Muscles of the Hyoid Bone._

=M. stylohyoideus= (Fig. 65, _d_).--The stylohyoid is a ribbon-like muscle about three millimeters wide. It lies on the superficial surface of the digastric, caudad and ventrad of the angle of the jaw.

_Origin_ by fleshy fibres from the outer surface of the stylohyal bone near its middle. The muscle curves over the outer surface of the digastric (_b_), and ends about one centimeter from the middle line in a slender flat tendon.

_Insertion_ into the middle of the ventral surface of the body of the hyoid bone. Some of the fibres of the mylohyoid (_c_) are inserted into its tendon.

_Relations._--Outer surface with the submaxillary gland (2) and the submentalis muscle. Inner surface with the digastric (_b_), the hyoglossus (Fig. 67, _h_), and the geniohyoid (Fig. 67, _g_).

_Action._--Raises the hyoid.

=M. geniohyoideus= (Fig. 67, _g_).--The geniohyoid is a long slender muscle which lies next to the median line between the symphysis menti and the hyoid bone, closely applied to its fellow of the opposite side and within (dorsad of) the mylohyoid (Fig. 65, _c_).

_Origin_ from the ventral half of the inner surface of the mandibula for about one millimeter from the symphysis.

_Insertion_ by muscle-fibres into the lateral half of the ventral surface of the body of the hyoid bone.

_Relations._--Outer (ventral) surface with the stylohyoid (Fig. 65, _d_) and the mylohyoid (Fig. 65, _c_); medial border with the muscle of the opposite side. Lateral border with the hyoglossus (Fig. 67, _h_) and genioglossus (Fig. 67, _f_). Inner (dorsal) surface with the genioglossus.

_Action._--Draws the hyoid forward.

=M. jugulohyoideus= (Fig. 67, _b_).--The jugulohyoid is a small flat quadrangular muscle covering the outer part of the ventral surface of the tympanic bulla.

_Origin_ from the ventral border of the jugular process.

_Insertion_ into the caudal side of the cartilaginous part of the stylohyoid opposite the origin of the styloglossus.

_Relations._--Ventral surface with the digastric. Dorsal surface with the tympanic bulla (3).

_Action._--Draws the hyoid backward and thus gives a firmer surface of origin for the styloglossus.

=M. mylohyoideus= (Fig. 65, _c_).--The mylohyoid is a large thin muscle seen after the reflection of the digastric, filling with its fellow of the opposite side the whole angle between the rami of the lower jaw. Its fibres are transverse.

_Origin_ from the middle of the medial surface of the body of the mandible between the caudal opening of the mandibular canal and the symphysis of the jaw.

_Insertion_ with the opposite muscle into a median raphe which extends from the symphysis of the jaw to the hyoid bone and is closely united to the external surface of the geniohyoid. Some of the fibres gain insertion into the body of the hyoid bone through their insertion into the tendon of the stylohyoid (_d_).

_Relations_.--Outer surface with the submentalis (Fig. 64, _c_) and the digastric (Fig. 65, _b_). Lateral edge with the masseter (_a_), the digastric (_b_), and the mandibula. Caudal border with the stylohyoid (_d_). Inner (dorsal) surface with the geniohyoid (Fig. 67, _g_), the hyoglossus (Fig. 67, _h_), and the genioglossus (Fig. 67, _f_).

_Action_.--Raises the floor of the mouth and thus brings the hyoid forward.

=M. ceratohyoideus.=

_Origin_, the two proximal pieces of the cranial horn of the hyoid. The fibres pass caudad parallel to one another and within the middle constrictor of the pharynx.

_Insertion_ into the whole length of the caudal cornu of the hyoid bone.

_Relations_.--Outer surface with the middle constrictor of the pharynx. Inner surface with the pharynx.

_Action_.--Draws craniad the body of the hyoid by bringing its cornua together and bending the joint between the two pieces of the cranial cornua.

The muscles connecting the hyoid and the tongue are described in connection with the description of the tongue.

III. MUSCLES OF THE BODY.

=1. Muscles of the Back.=

A. MUSCLES OF THE SHOULDER (connecting the forelimb with the back) (Fig. 68).

=M. trapezius=.--The trapezius muscle is divided in the cat into three portions, the spinotrapezius, the acromiotrapezius, and the clavotrapezius.

=M. spinotrapezius=, or trapezius inferior (Fig. 68, _j_).--The spinotrapezius (caudal part of the human trapezius) is a flat triangular muscle.

_Origin_ from the tips of the spinous processes of all or nearly all the thoracic vertebræ and from the intervening supraspinous ligament. The origin may extend from a point anywhere between the first and fourth spines to any point between the eleventh and thirteenth.

_Insertion_ along an S-shaped line (Fig. 76, _k_-_l_) which crosses the tuberosity of the scapular spine and forms an angle with the spine of about forty-five degrees. The line extends on one side onto the fascia covering the supraspinatus muscle, and on the other side onto the fascia covering the infraspinatus, but does not reach the borders of the scapula. The cranial two-thirds of the insertion is by a flat tendon which is broader craniad.

_Relations._--Outer surface with the cutaneus maximus, and craniad with the acromiotrapezius (_h_). Inner surface with the latissimus dorsi (_m_) caudad and with the rhomboideus, supraspinatus, and infraspinatus craniad. Caudal border free; cranial border in contact with the acromiotrapezius (_h_).

_Action._--Draws the scapula dorsocaudad.

=M. acromiotrapezius=, or trapezius superior (Fig. 68, _h_).--The acromiotrapezius (middle part of the human trapezius) is a flat four-sided muscle just craniad of the spinotrapezius, from the median dorsal line to the scapular spine.

_Origin_ along the median dorsal line from the spinous process of the axis to a point anywhere between the spinous processes of the first and fourth thoracic vertebræ. Its caudal two-thirds is from a flat tendon which is directly continuous with the muscle of the opposite side and thus bridges the depression between the vertebral borders of the scapulæ. This tendon narrows craniad, and its cranial fourth is attached by a fascia to the spinous processes of the cervical vertebræ. At the cranial border of the muscle the fibres sometimes reach the middle line. The fibres of the muscle run nearly transversely.

_Insertion_ (Fig. 76, _h_).--Into the outer surface of the metacromion, the glenoid border of the scapular spine from the metacromion to the tuberosity, and along a line continued for about one centimeter from the tuberosity onto the surface of the spinotrapezius (_j_) at the junction of its muscular and tendinous portions.

_Relations._--Outer surface with the cutaneus maximus, the integument and platysma. Inner surface with a mass of fat partially covering the rhomboideus; with the occipitoscapularis, spinotrapezius, supraspinatus, levator scapulæ, and splenius. Near its origin its caudal border touches the outer surface of the spinotrapezius (_j_). Its cranial border is overlaid near its insertion by the levator scapulæ ventralis (_f_) and is closely related to the caudal border of the clavotrapezius (_d_) near its origin.

_Action._--The two muscles hold the scapulæ together.

=M. clavotrapezius= (Fig. 68, _d_).--The clavotrapezius (cranial part of the human trapezius) is a flat muscle passing from the lambdoidal crest and middorsal line to the clavicle and covering the side of the neck.

_Origin._--The medial half of the lambdoidal crest and the middorsal line between the crest and the caudal end of the spine of the axis. The fibres are parallel and pass caudoventrad so that the muscle covers the ventral surface of the neck caudally and fills the concavity at the front of the shoulder.

_Insertion_ into the clavicle and into a raphe between the clavotrapezius and the clavobrachial (_e_) muscles. This raphe extends along the clavicle and for some distance laterad of it. The muscle is partly continuous with the clavobrachial (_e_).

_Relations._--Outer surface with the platysma, and at its craniomedial angle with the levator auris longus. Inner surface with the rhomboideus, splenius, occipitoscapularis, levator scapulæ, cleidomastoid, and supraspinatus. The cranial edge touches the sternomastoid. The caudal border touches the acromiotrapezius near the origin.

_Action._--Draws the scapula craniodorsad.

This muscle and the clavobrachial (_e_) are sometimes considered as forming a single muscle, the cephalohumeral, or cephalobrachial.

=M. occipitoscapularis= (or levator scapulæ dorsalis or rhomboideus capitis) (Fig. 73, _a_).--The occipitoscapularis (equivalent to a part of the human rhomboideus) is a slender flat muscle from the lambdoidal ridge to the coracovertebral angle of the scapula.

_Origin_, the medial half of the lambdoidal ridge beneath the clavotrapezius (Fig. 68, _d_). The origin does not extend quite to the middle line. The muscle passes almost directly caudad. Its caudal end is thicker and narrower than the cranial end and is wedged between the rhomboideus dorsally and the levator scapulæ ventrally.

_Insertion_ either into the inner surface of the levator scapulæ near its dorsocranial border at its insertion or into the scapula near the coracovertebral angle (Fig. 78, _f_) or into both. Its fibres are very closely related to those of the rhomboideus.

_Relations._--By its outer surface with the clavotrapezius (Fig. 68, _d_) and acromiotrapezius (_h_). At its insertion it is often overlaid by either the rhomboideus or the levator scapulæ or both. By its inner surface with the splenius (Fig. 73, _b_). Its caudal third is wedged between the rhomboideus and the levator scapulæ.

_Action._--Draws forward and rotates the scapula.

=M. rhomboideus= (probably equivalent to the human rhomboideus major and rhomboideus minor).--A rather thick trapezoid muscle connecting the vertebral border of the scapula with the spinous processes of the adjacent vertebræ.

_Origin_ from the caudal two or three fifths of the cervical supraspinous ligament and from the sides and tips of the first four thoracic vertebral spines and the interspinous ligaments caudad of these four spines. It passes lateroventrad, and the fibres converge somewhat so that the line of insertion is shorter than the line of origin.

_Insertion_ by a short tendon (1 millimeter long) into the vertebral border of the scapula, the line of insertion passing gradually from the inner (Fig. 78, _e_) to the outer (Fig. 76, _i_) surface; and by fleshy fibres into the outer surface of the glenovertebral angle of the scapula (Fig. 76, _i_) in close connection with the origin of the teres major (_j_). A bundle of fibres at the cranial border may be inserted into the cranial border of the levator scapulæ at its insertion.

_Relations._--By its outer surface with the spinotrapezius (Fig. 68, _j_), acromiotrapezius (_h_), latissimus dorsi (_m_), and clavotrapezius (_d_); it is usually partially separated from these muscles by a mass of fat. By its inner surface near its insertion with the serratus anterior (Fig. 73, _i_) and levator scapulæ (Fig. 73, _h_), and craniad with the splenius (Fig. 73, _b_), and the tendon of the serratus posterior superior (Fig. 73, _l_). At its lateral end it touches the occipitoscapularis (Fig. 73, _a_), serratus anterior (_i_), levator scapulæ (_h_), supraspinatus (Fig. 75, _a_), and infraspinatus (Fig. 75, _c_), and the teres major (Fig. 75, _d_).

_Action._--Draws the scapula toward the vertebral column.

=M. cleidomastoideus= (clavicular portion of the human sternocleidomastoid) (Fig. 65, _h_).--A narrow flat muscle between the mastoid process and the clavicle.

_Origin_ from the apex and caudal margin of the mastoid process of the temporal bone, by tendon and muscle fibres. The muscle passes caudad, becoming broader and thinner.

_Insertion_ into the lateral four-fifths of the clavicle within the insertion of the clavotrapezius, and into the raphe which continues laterad of the clavicle.

_Relations._--Outer or lateral surface at the caudal end with the clavotrapezius (Fig. 65, _j_); further craniad with the depressor conchæ (Fig. 64, _b_), the platysma, and the sternomastoid (Fig. 65, _g_). Inner or medial surface with the supraspinatus (Fig. 75, _a_) caudad, with the mass of fat in the axilla at its middle, with the levator claviculæ ventralis (Fig. 65, _i_) and obliquus capitis superior (Fig. 71, _e_) craniad.

_Action._--Pulls the clavicle craniad when the head is fixed. Turns the head and depresses the snout when the clavicle is fixed.

=M. levator scapulæ ventralis=, or levator claviculæ (Fig. 68, _f_; Fig. 65, _i_; Fig. 72, _c_, _c′_, _c″_) (not usually found in man).

_Origin_ (Fig. 72, _c′_, _c″_) in two parts. (1) By fleshy fibres from the ventral surface of the atlantal transverse process near its caudal border (_c′_). (2) By a small tendon from the ventral surface of the basioccipital opposite the middle of the length of the bulla tympani (_c″_) in common with the longus capitis (_a_). The two heads unite to form a flat band which passes caudoventrad and is inserted (Fig. 76, _f_) by a flat short tendon, about eight millimeters broad, into the outer surface of the metacromion along its ventral border, and into the infraspinatus fossa for one or two millimeters distad of that border.

_Relations._--Near the insertion the dorsal border overlaps the border of the acromiotrapezius (Fig. 68, _h_), and the ventral border is attached by strong fascia to the border of the clavotrapezius (Fig. 68, _d_). The outer surface is in relation with the longissimus capitis (Fig. 73, _g_), clavotrapezius (Fig. 68, _d_), cleidomastoid (Fig. 73, _d_), platysma, and the skin. The inner surface is in relation caudad with the supraspinatus and with a mass of fat and a lymphatic gland which occupy the hollow of the shoulder. Craniad it is in relation with the cleidomastoid (Fig. 73, _d_), the scalenus (Fig. 73, _f_), the longus capitis (Fig. 73, _e_), and the obliquus superior (Fig. 71, _e_).

_Action._--Pulls the scapula craniad.

=M. latissimus dorsi= (Fig. 68, _m_; Fig. 65, _q_; Fig. 77, _e_).--A large triangular sheet covering rather more than the dorsocranial half of the abdomen and thorax. It arises from the middorsal line, from the pelvis to the fifth thoracic spine, and is inserted into the humerus.

_Origin_ from the tips of the neural spines of the vertebræ from the fourth or fifth thoracic to about the sixth lumbar. As far as the tenth or eleventh thoracic vertebra the origin is directly by muscle-fibres. Caudad of this is a broad triangular tendon which may be so closely united to the tendons of the underlying muscles that it cannot be separated. The muscle passes cranioventrad, the fibres converging to the axilla. In the axilla a part of the cutaneus maximus and the epitrochlearis (Fig. 65, _r_) take origin from the outer surface.

_Insertion._--The muscle then ends in a flat tendon, to the cranial surface of which the fibres of the teres major (Fig. 77, _c_) are attached. Five to ten millimeters from the humerus the muscle-fibres of the teres give place to the tendon-fibres, and the conjoined tendon of the teres and latissimus which is thus formed (Fig. 79, _d′_) is inserted into a rough elongated area (Fig. 82, _f_) which lies parallel with the pectoral ridge on the medial surface of the shaft of the humerus (Fig. 48, _g_). The teres tendon forms the proximal part of the conjoined tendon. A part of the tendon of the latissimus may join the pectoralis minor at its distal or caudal border (Fig. 65), and thus it may contribute to the formation of both pillars of the bicipital arch (Fig. 65, _t′_).

_Relations._--Its outer surface is covered caudad by the cutaneus maximus, and craniad by the spinotrapezius (Fig. 68, _j_) and the long head of the triceps (Fig. 68, _k_). The cranial border comes into relation with the spinotrapezius (j) and teres major (Fig. 77, _c_). The caudal border is in contact at the ventral end with the xiphihumeralis (Fig. 65, _p_) and pectoralis minor (Fig. 65, _o_). The inner surface of the latissimus covers near its cranioventral end parts of the xiphihumeralis (Fig. 68, _n_), pectoralis minor (Fig. 68, _o_), teres major (Fig. 77, _c_), and serratus anterior (Fig. 73, _i_). The caudal border covers a portion of the obliquus externus abdominis (Fig. 68, _p_); the dorsal and middle portions cover parts of the longissimus dorsi (Fig. 69, _f″_), the spinalis dorsi (Fig. 69, _g_), the serratus posterior superior (Fig. 73, _l_), and serratus posterior inferior (Fig. 73, _n_).

_Action._--Pulls the arm caudodorsad.

=M. serratus anterior= and =levator scapulæ= (Fig. 73, _i_ and _h_) in the cat are continuous at both origin and insertion, and it is barely possible to separate their contiguous borders without cutting the fibres. The muscle extends from the transverse processes of the last five cervical vertebræ and from the first nine or ten ribs to the medial surface of the scapula near its vertebral margin (Fig. 78, _c_ and _d_).

=M. serratus anterior= (or serratus magnus) (Fig. 73, _i_).

_Origin._--From the first nine or ten ribs in as many subdivisions. The first subdivision arises from the whole or nearly the whole of the caudal border of the first rib. The next four or five arise either from the ribs or from the costal cartilages near or at their junction with the ribs. The last four subdivisions arise from the ribs at increasing distances from the cartilages. The fibres converge and the subdivisions join one another.

_Insertion_ (Fig. 78, _c_) into a narrow area on the medial surface of the scapula near the vertebral border. The glenoid half of the insertion is by a short tendon, the remainder by muscle-fibres.

_Relations._--Outer (lateral) surface with the latissimus dorsi (Fig. 68, _m_), teres major (Fig. 77, _c_), subscapularis (Fig. 77, _a_), and near the origin with the rectus abdominis (Fig. 73, _k_), the scalenus (Fig. 73, _f_-_f‴_), and the obliquus externus (Fig. 68, _p_). Inner (medial) surface with the serratus posterior superior (Fig. 73, _l_) and its tendon, and with the intercostales externi (Fig. 73, _m_). At its insertion the muscle touches the rhomboideus. The cranial edge is united to the caudal edge of the levator scapulæ (Fig. 73, _h_).

_Action._--Depressor of the scapula.

=M. levator scapulæ= (Fig. 73, _h_).

_Origin._--From the dorsal tubercles of the transverse processes of the last five cervical vertebræ and from the ligaments between the tubercles. A slip sometimes arises from the atlas.

_Insertion_ (Fig. 78, _d_) into a triangular area on the medial surface of the scapula near its vertebral border. It is continuous with the insertion of the serratus anterior.

_Relations._--Outer (lateral) surface with a mass of fat separating it from the clavotrapezius (Fig. 68, _d_) and cleidomastoid (Fig. 65, _h_), and with the subscapularis (Fig. 77, _a_). Inner (medial) surface with the splenius (Fig. 73, _b_), the tendon of the serratus posterior superior (Fig. 73, _l_), and the cervical portion of the longissimus dorsi (Fig. 69, _f″_). Near the insertion the muscle touches the occipitoscapularis (Fig. 73, _a_) and rhomboideus. Caudal border united with the serratus anterior (Fig. 73, _i_).

_Action._--Draws the scapula cranioventrad.

B. MUSCLES OF THE BACK (interconnecting parts of the vertebral column) (Fig. 69).--The muscles connected with the vertebral column form a mass which is less markedly differentiated into distinct muscles than is the case in the limbs. A great longitudinal mass of fibres begins in the sacral region and extends along the vertebral column to the head. This mass contains fibres running in various directions and attached at one or both ends to the sacrum, the innominate bones, the spinous, transverse, and articular processes, and to the arches, of the vertebræ; to the ribs and to the head, and having in general the function of moving the vertebræ in various ways, or of moving parts connected with the vertebræ. In some regions, especially the cervical, it is possible to distinguish clearly distinct muscles which have been differentiated from this mass. In the greater part of its extent, however, it is possible to distinguish only more or less closely interconnected bundles,--the more clearly marked of which have received special names.