192 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



XI. Accessory Nerve. — This and the next are regarded as cranial 

 nerves occurring only in the amnio tes; but in the elasmobranchs 

 nerves are given off from the vagus which innervate the trapezius 

 muscle, and much the same conditions are found in the amphibia. 

 In these classes the centre of the nerve is in the medulla oblongata. 

 In the amniotes the centre of the accessory nerve is in both medulla 

 and spinal cord, parts of it so far back that the posterior rootlets may 

 emerge from the cord near the seventh cervical nerve. These rootlets 

 unite in a trunk which runs forward, inside the cranium, between the 

 dorsal and ventral roots of the spinal nerves, and leaves the skull with 

 or near the vagus. The accessory is a motor nerve and in the amni- 

 otes supphes the trapezius and sternocleidomastoid muscles which 

 move the shoulder girdle. 



XII. The hypoglossal nerve acquires its full development in the 

 amniotes, though apparently it has its equivalents in the ichthyop- 

 sida. In the adult it is purely motor, but in its development in 

 several mammals ganglionated dorsal roots occur which later disap- 

 pear. The rootlets of the adult are usually two or three in number, 

 sometimes more. They unite to form the hypoglossal nerve which 

 usually unites with the anterior cervical nerves to form a cervical 

 plexus, from which the main trunk goes to the hypoglossal muscles 

 and to the retractors of the tongue (in birds to the syrinx as well). 



In many fishes there are small occipital nerves which leave the skull behind 

 the vagus, these sometimes having (notidanid sharks) a ganglion on a dorsal 

 root, roots and ganglia reappearing in the embryonic stages of some elasmo- 

 branchs, to be lost in later development. These occipital nerves pass backward, 

 dorsal to the gill clefts and then forward to innervate the hypobranchial 

 (hypoglossal) muscles developed from the myotomes at the posterior end of the 

 head (fig. 145, hyp. n). They also supply some of the muscles of the pectoral 

 fin. These occipital nerves are the probable homologues of the hypoglossal 

 nerves of the amniotes, which have been taken into the cranium in the higher 

 t group. 



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THE SENSORY ORGANS 



The sensory organs are to receive information both from without 

 and from various parts of the body — alimentary tract, muscles, 

 joints, etc. — and to transform it into stimuli to be carried by the 

 nerves to the ganglia, usually those of the central nervous system. 

 This information varies in character and the organs consequently 

 differ in structure according to the impressions they are to receive. 



With very few exceptions the characteristic portions of the organs, 



