THE NERVOUS SYSTEM 



525 



the second pair of sensory nerves of amphioxus may be the homologues 

 of the ophthalmic nerves of craniates. The four mixed nerves posterior 

 to the ophthalmic have been compared respectively with the trigeminal, 

 facial, glossopharyngeal, and vagus. It is probable, however, that at 

 least three segmental nerves are represented in the vagus nerve of verte- 

 brates. Of the somatic motor nerves of vertebrates, the oculomotor is 

 possibly represented in the first ventral root of amphioxus. 



CIRRI 



AMPHIOXUS. 



VELUM 



Fig. 436. — A diagram showing the distribution of the anterior nerves of amphioxus. 

 The two anteriormost nerves of amphioxus lie anterior to the myotomes and are purely 

 sensory. The remaining nerves are either mixed like those shown in the figure or are 

 purely motor. The former alternate, the diagram shows, with the myotomes, while the 

 latter are metameric in position. (Redrawn after Hatschek slightly modified.) 



The number of cranial nerves varies in craniotes. Fishes and amphibia 

 have ten pairs; reptiles, birds, and mammals have twelve. These, 

 beginning with the anteriormost, are: 



I. Olfactory. Special visceral afferent. 



II. Optic. Sensory. A specialized fiber tract of the brain. 



m. Oculomotor. Somatic efferent, with some visceral efferent fibers. 



IV. Trochlearis. Somatic efferent. 



V. Trigeminus. General somatic afferent and visceral efferent. 



VI. Abducens. Somatic efferent. 



VII. Facialis. Mixed visceral afferent and efferent. Special somatic afferent. 

 Vm. Acoustic. Special somatic atTerent. 



IX. Glossopharyngeal. General and special visceral mixed. (?) Somatic sensory. 



X. Vagus. General and special visceral mixed. Special somatic afferent. 



XI. Accessory. (?) Somatic efferent, and general and special visceral efferent. 

 Xn. Hypoglossal. Somatic efferent. 



