STRUCTURE OF THE VERTEBRATES 299 



nomastoid and cleidomastoid muscles, which unite to form one 

 in the human. The nerve also innervates the trapezius. 



Nerve XII, Hypoglossal. Like the accessory, the hypoglossal 

 is motor. It has several roots on the cord which pass into the 

 cranium and out to the retractors and muscles on the ventral 

 side of the tongue and jaw. 



The relationships of the cranial nerves and their evident 

 homology with spinal nerves, have an interesting bearing on the 

 metamerism of the face and head. The long discarded theory of 

 Owen (1846) that the skull is metameric has no foundations in 

 either evolution or embryology; but the metameric origin of the 

 jaws and muscles of the face is now well established. First 

 developed the hypocranial structures of the cyclostomes, and 

 the metameric muscles of the eye. Following this the anterior 

 gill arches became modified into jaws and hyoid apparatus, 

 and the posterior gill clefts were pushed closer under the chon- 

 drocranium, with the inclusion of their nerves as the ninth and 

 tenth cranial nerves. When the gill clefts disappeared in the 

 amniotes, and the cartilages and muscles became incorporated 

 into the laryngeal and neck region, two other spinal nerves came 

 into the skull, making the twelve of the higher vertebrates. 

 These nerves are all highly specialized, but their relationships 

 correspond perfectly w^ith the other evidence regarding the 

 evolution of the skull. 



