THE SOMATIC MOTOR DIVISION. 1 93 



The most cephalic of the ventral spinal nerves form the 

 so-called cervical plexus. This is closely related to the brachial 

 plexus, some nerves usually contributing to both. The rami 

 arising from the cervical plexus go to innervate muscles extending 

 betvi^een the pectoral girdle and the region of the tongue, commonly 

 called the tongue musculature. This musculature is formed by 

 buds from a number of myotomes in the cervical region, and, in 

 vertebrates which possess gills, these muscles are separated from 

 the dorsal muscles and from the place of origin' of the nerves by 

 the expansion and shifting backward of the gill apparatus. In 

 consequence, the nerves destined to the tongue muscles must 

 run around behind and forward beneath the gills. As the more 



Rib 



R.I) 



Fig. 104. — The constitution of the cervical plexus in a selachian, Hexanchus. 

 After Fiirbringer. vg., vagus; ■w,x,y,z, spino-occipital nerve roots; i and 2, spinal 

 nerves; the letters d and v indicate respectively the dorsal and ventral roots; Rdl, 

 rami for the dorso-Iateral trunk muscles; Rsbsp, rami for the subspinal muscles; 

 Rib, rami for the interbasal muscles. 



anterior nerves run back to get around the gills they unite with 

 the more caudal ones and so give rise to the cervical plexus (Fig. 

 104). In higher vertebrates this plexus becomes somewhat 

 modified. The anterior part of it, consisting of nerve roots arising 

 in the occipital region and innervating the muscles of the tongue, 

 becomes relatively independent of the plexus and is known as 

 the hypoglossal nerve. The remainder of the plexus gives rise 

 to rami supplying various muscles of the neck and is even con- 

 nected with the spinal accessory nerve which belongs to the visceral 

 motor system. 

 13 



