Part 34

=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.

On the left side the longus capitis and one head of the levator scapulæ ventralis have been cut, in order to show the rectus capitis anterior minor. a, M. longus capitis; b, M. rectus capitis anterior minor; c, M. levator scapulæ ventralis (c′, its atlantal head; c″, its occipital head); d, M. rectus capitis lateralis; e, M. obliquus capitis superior; f, M. longissimus capitis; g, M. longus colli (g′, its cervical portion; g″, its thoracic portion); h, part of M. scalenus. 1, bulla tympani; 2, ventral atlanto-occipital membrane; 3, ventral arch of atlas; 4, first rib.]

=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.