374 



CHORDATE ANATOMY 



EVOLUTION OF THE CRANIAL NERVES 



The two anterior pairs of nerves of Amphioxus are purely sensory, 

 while all posterior dorsal nerves are mixed. The second pair of sensory 

 nerves of Amphioxus may be the homologues of the ophthalmic nerves 

 of craniotes. 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 vertebrates. Of the somatic motor 



CIRRI 



AMPHIOXUS. 



VELUM 



Fig. 331. — A diagram showing the distribution of the anterior nerves of Amphioxus. 

 The two most anterior 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.) 



nerves of vertebrates, the oculomotor is possibly represented in the first 

 ventral root of Amphioxus. 



The number of cranial nerves varies in craniotes. Fishes and amphib- 

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

 beginning with the most anterior, are: 



I. Olfactory. Special afferent. 



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



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

 rV. Trochlearis. Somatic efferent. 



V. Trigeminus, (ieneral somatic afferent and visceral efferent. 



VI. Abducens. Somatic efferent. 



