CHAPTER X 

 CRANIAL NERVES 



DETAILS of the peripheral distribution of the several systems of 

 nerve components have been recorded for a considerable num- 

 ber of amphibian species, notably in many important papers by 

 H. W. Norris. The first of this series was Strong's paper ('95) on 

 the larval frog, which was followed by Coghill's description of the 

 cranial nerves of Amblystoma, published in 1902. Their arrangement 

 here may be regarded as typical for the vertebrate phylum as a 

 whole, with no extreme specialization of any system. The constancy 

 of the arrangement of these components at the superficial origins of 

 the nerve roots in all vertebrates is remarkable, in view of the ex- 

 treme diversity of both peripheral and central connections of their 

 fibers and of the enormous differences in the number of fibers repre- 

 sented in the several systems among the various species. Except for 

 the specific differences just mentioned, the chief departures from 

 uniformity of composition of the nerve roots are the suppression in 

 all Amniota of the large lateral-line components of the Ichthyopsida 

 and the correlated differentiation of the cochlear apparatus in the 

 higher classes. 



The central connections of the olfactory and optic nerves and the 

 nervus terminalis are described in the chapters relating to the fore- 

 brain and the midbrain. The other functional systems are discussed 

 in chapters iv and v, and to those general statements some additional 

 details of their arrangement in Amblystoma are given here. 



DEVELOPMENT 



Some peculiar features of the development of the somatic motor 

 roots were mentioned in the preceding chapter. The development of 

 the visceral motor roots was described by Coghill, though many 

 details remain to be filled in. The early development of the sensory 

 systems of root fibers was studied by Coghill ('16, Paper II) and 

 Landacre ('21 and later papers). In Landacre's paper of 1921 the 

 embryos studied were identified as Plethodon glutinosus, but they 

 subsequently proved to be Amblystoma jeffersonianum (Landacre, 

 '26, p. 472). Older stages were described by Kostir ('24). 



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