. The chordates. Chordata. Mammalia: Skin, Muscles, Skeleton 593 within itself. Movements of the head in relation to the neck are effected by muscles extending between the skull and more posterior parts. Especially significant for movements of the head on the neck is a complex set of small muscles extending from the atlas and axis to the adjacent occipital region of the skull. The surfaces are so shaped that the joint between skull and atlas provides for up-and-down move- ments of the head, while right-and-left movements result from rotation of the atlas on the pivot provided by the odontoid p
. The chordates. Chordata. Mammalia: Skin, Muscles, Skeleton 593 within itself. Movements of the head in relation to the neck are effected by muscles extending between the skull and more posterior parts. Especially significant for movements of the head on the neck is a complex set of small muscles extending from the atlas and axis to the adjacent occipital region of the skull. The surfaces are so shaped that the joint between skull and atlas provides for up-and-down move- ments of the head, while right-and-left movements result from rotation of the atlas on the pivot provided by the odontoid process of the axis (Fig. 108). On the ventral side of the neck is an important set of paired muscles (Figs. 454, bottom; 459,460) having no relations to the vertebral column, but extending from anterior parts of the thoracic skeleton (anterior end of sternum and first thoracic ribs) forward to the posterior region of the skull (the sternomastoid muscle) or to parts of the visceral skeleton (sternohyoid and sternothyroid muscles). The latter two, in their ventral position and in their occasional possession of inscriptiones tendineae, resemble the rectus muscles of the abdomen. Such of these ventral longitudinal muscles as attach to parts of the visceral skeleton, or extend between parts of it—all innervated by the hypoglossal nerve (XII)—correspond to the hypobranchial muscles of Anamnia. The group includes the chief muscles of the tongue (genioglossus and hyo- glossus; Fig. 459), the geniohyoid (connecting lower jaw and hyoid), and some small muscles of the laryngeal apparatus. The sterno- mastoid, innervated by XI, is probably of branchiomeric origin (p. 96). Lower jaw-I Tongue Genioglossus Hyoglossus Stylohyoid ligament Hyoid n+n. Sternum—\-% Clavicle Scapula gl^ Pectoral girdle Fig. 459. Hypobranchial muscles: diagrammatic. (A) Shark: ventral view. (B) Mammal: lateral view. Exact homology of particular muscles can hardly be recognized, but the relations to skeletal par
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