Part 26

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.

a, M. tragicus lateralis; b, M. jugulohyoideus; c, M. pterygoideus externus; d, partially cut surface of M. pterygoideus internus; e, M. styloglossus; f, M. genioglossus; g, M. geniohyoideus; h, M. hyoglossus; i, M. glossopharyngeus; j, M. constrictor pharyngis medius; k, M. constrictor pharyngis inferior; l, M. stylopharyngeus; m, M. sternohyoideus (cut); n, M. cricothyreoideus; o, M. sternothyreoideus (cut); p, M. thyreohyoideus. 1, mandible; 1′, angular process of mandible; 2, stylomandibular ligament; 3, bulla tympani; 4, trachea; 5, œsophagus; 6, thyroid gland; 7, isthmus of the thyroid gland.]

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.