Part 24

Origin from the outer surface of the premaxilla near the suture along the ventral border of the narial opening. The fibres diverge and pass caudad into the upper lip within the superficial fibres of the orbicularis oris.

Insertion.--The skin of the upper lip just craniad of the whiskers. Some of the fibres pass into the pad on which the whiskers rest.

Action.--It carries the lip craniad.

=M. quadratus labii inferioris.=--A thin flat band extending almost the length of the lower lip.

Origin from the alveolar border of the mandible, between the molar tooth and the canine. The fibres pass dorsad into the lower lip, where they intermingle with those of the orbicularis oris. The muscle is perhaps not constant.

Action.--Depressor of the lower lip.

Sometimes a few fibres arising from the cranial portion of the alveolar border of the mandibula pass mediad, seeming to join corresponding fibres from the opposite side. Such fibres would constitute a M. transversus menti.

B. DEEP MUSCLES OF THE HEAD.

(a) Muscles of Mastication.

=M. digastricus= (Fig. 65, b).--The digastric is a thick prismatic muscle lying mediad of the angle of the lower jaw, connecting it with the base of the skull.

Origin by fleshy fibres from the outer surface of the jugular process of the occipital bone, and by a thin tendon from the tip of the mastoid process and from the ridge between the mastoid and the jugular processes. The muscle passes craniad, becoming at the same time broader and more voluminous.

Insertion.--The ventral border of the mandible craniad of the caudal border of the molar tooth. The insertion area extends slightly onto both surfaces of the bone and is triangular, with its apex craniad.

Relations.--Outer (ventral) surface with the following, beginning at the caudal end: the rectus capitis lateralis, the cleidomastoid (h), the submaxillary gland (2), the stylohyoid (d), a large lymph-gland (3), and the integument. Inner (dorsal) surface with the jugulohyoid, the tympanic bulla, the masseter (a), the hyoglossus, and the mylohyoid (c).

Action.--Depressor of the lower jaw.

=M. masseter= (Figs. 65 and 66, a).--The masseter forms the projecting mass so prominent in the cat behind and below the eye. Its outer surface is covered by a strong aponeurosis. The muscle is divided into three layers which are distinct as to origin, insertion, and direction of fibres, but which are otherwise not distinct.

The =superficial layer= takes

Origin by the superficial aponeurosis from the ridge which runs lengthwise of the outer surface of the malar bone, and by fleshy fibres from the surface of the malar bone ventrad of this ridge. The fibres pass obliquely caudad to their insertion.

Insertion.--The caudal half of the lateral margin of the ventral border of the mandible, by a sheet of fascia which lies on the inner surface of the muscle. The superficial fibres curve ventrad of the mandible and are inserted into a tendinous intersection between this muscle and the internal pterygoid (Fig. 66, c).

The =middle layer= takes

Origin from the ventral border of the malar bone, from opposite the molar tooth to within three millimeters of the mandibular fossa. The fibres pass ventrad and have their

Insertion into the mandible at the ventral border of the external coronoid fossa by means of the aponeurosis which covers the inner surface of the muscle.

The left side shows the first layer of muscles beneath the skin and skin-muscles; the right side shows the second layer, except in the head region. a, M. masseter; b, M. digastricus; c, M. mylohyoideus; d, M. stylohyoideus; e, M. sternohyoideus; f, M. thyreohyoideus; g, M. sternomastoideus; g′, M. sternothyreoideus; h, M. cleidomastoideus; i, M. levator scapulæ ventralis; j, M. clavotrapezius; k, M. clavobrachialis; l, M. pectoralis major; m, M. pectoantibrachialis; n, part of M. scalenus; o, M. pectoralis minor; p, M. xiphihumeralis; q, M. latissimus dorsi; r, M. epitrochlearis; s, caput longum of M. triceps brachii; t, M. biceps brachii; t′, bicipital arch; u, M. acromiodeltoideus; v, M. brachialis. 1, parotid gland; 2, submaxillary gland; 3, lymphatic glands; 4, external ear; 5, external jugular vein; 6, manubrium; 7, first rib; 8, common carotid artery; 9, molar gland.]

The =deep layer= takes

Origin by a strong tendon from the ventral border of the zygoma just craniad of the mandibular fossa. The fibres pass cranioventrad, diverge and have their

Insertion into the external coronoid fossa.

Relations.--Outer surface with the following, beginning at the caudal border: the external ear, the parotid gland (Fig. 65, 1), the platysma, the submentalis, the zygomaticus (major), the zygomaticus minor. Ventral border with a lymph gland caudad of the angle of the mouth (Fig. 65, 3), and the digastric muscle (b). Inner surface with the bones.

Action.--A very powerful elevator of the lower jaw.

=M. temporalis= (Fig. 63, n).--The temporal muscle is the great mass taking origin from the temporal fossa and having its insertion on both surfaces and both borders of the coronoid process of the mandible. It may be divided into two layers, deep and superficial.