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 parts and the innervation indicate the 

 homology of the hypobranchial group throughout the vertebrates. 



