Part 31

Insertion into an area ventrad of the medial part of the lambdoidal crest beneath the insertion of the complexus (Fig. 69, b) and the biventer cervicis (Fig. 69, a).

Relations.--Dorsal surface with the biventer cervicis (Fig. 69, a). Ventral surface with the rectus capitis posterior medius (Fig. 71, d) and the obliquus capitis inferior.

Action.--Raises the snout.

=M. rectus capitis posterior medius= (Fig. 71, d). (This is in man a part of the rectus capitis posterior major.)--It lies beneath the rectus capitis posterior major (Fig. 71, a).

a, M. rectus capitis posterior major; b, M. obliquus capitis inferior; c, M. semispinalis cervicis; d, M. rectus capitis posterior medius; e, M. obliquus capitis superior; f, M. longus atlantis; g, M. scalenus (part of cervical portion); h, cut end of M. levator scapulæ.]

Origin from the cranial end of the axial spine. It forms a triangular prismatic mass which passes craniad and has its

Insertion into an elongated area on the occipital bone, ventrad of the median half of the lambdoidal crest.

Relations.--Dorsal surface with the rectus capitis posterior major (Fig. 71, a). Ventral surface with the atlas and the rectus capitis posterior minor.

Action.--Assists the rectus capitis posterior major.

=M. rectus capitis posterior minor.=--A small triangular muscle beneath the rectus capitis posterior medius (Fig. 71, d).

Origin by the apex of the triangle from the cranial border of the dorsal arch of the atlas for about two millimeters laterad of the median line. The muscle passes over the ligament which connects the atlas to the occiput and adheres to it.

Insertion into an elongated area on the occipital bone, ventrad of the insertion of the rectus capitis posterior medius for about five millimeters next to the median line.

Relations.--Dorsal surface with the rectus capitis posterior medius. Ventral surface with the posterior atlanto-occipital membrane.

Action.--Assists the rectus capitis posterior major.

=M. obliquus inferior= (Fig. 71, b).

Origin from the whole lateral surface of the spine of the axis. The parallel fibres form a thick mass which passes laterad and craniad.

Insertion into the dorsal surface of the transverse process of the atlas along a narrow area near its lateral margin.

Relations.--Outer surface with the rectus capitis posterior major (Fig. 71, a), the complexus (Fig. 69, b), and the obliquus superior (Fig. 71, e). Inner surface with the axis.

Action.--Rotates the head.

=M. obliquus capitis superior= (Fig. 71, e; Fig. 72, e).--This is a triangular muscle passing from the outer border of the atlas laterad of the recti capitis posteriores to the occiput.

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.