THE CRANIAL NERVES. 167 



IL THE PERIPHERAL NERVOUS SYSTEM. 



E. The Cranial Nerves. 



(To facilitate reference the original arrangement of this part 

 has been altered.) 



[There are ten pairs of cranial nerves in the frog-, which are 

 numbered in order from before backwards. The mode of origin of 

 these nerves, and their deeper relations with the respective parts of 

 the brain to which they belong, have already been described.] 



1. The olfactory nerve (iV. olfadorius), [(Figs. 97, 98, J 02, 103, 

 1 1 1 7) runs a very short coru'se only a few lines in length, and 

 escapes from the cranium by an opening in the cartilage of the 

 sphenethmoid into the nasal cavity, where it divides into two branches, 

 each of which breaks up into a brush of filaments, to be distributed 

 in the olfactory mucous membrane. The nerve contains no white 

 fibres]. (See organ of smell, p. 385.) 



2. The optic nerve (N. oj^tictis), (Figs. 97, 102, 103, in //) 

 [arises, as already described, by the optic tract, and joins with its 

 fellow at the optic chiasma or commissure, where part of the fibres 

 pass over to the opposite side (according- to Michel all the fibres 

 cross). Each optic nerve then courses outwards, piercing' the car- 

 tilage of the cranium and so reaching the eyeball]. (See organ 

 of sight, p. 408.) 



3. The motor oculi (N. ocuJomotorius, Ocido-motor, Motor com- 

 munu), (Figs. 102, iii, 112, 113 7/7). From its origin it courses 

 ovitwards and forwards, perforating with a slight obliquity the carti- 

 laginous wall of the cranium, just in front of the Gasserian gan- 

 glion (Fig. 116 777); on reaching the orbit it divides into two 

 branches, between the Levator hulhi and the Bectus intertins and in- 

 ferior. One branch. Ramus superior, runs over and parallel to the 



Ramus ophtlialmicus Trig, and enters the under surface of the 

 Rectus superior ; the second, lower branch. Ramus inferior, supplies 

 filaments to the Rectus internns and inferior and to the Obliqims in- 

 ferior. Apparently it exchanges fibres with the ophthalmic division 

 of the trigeminal nerve. 



[That portion of the motor-oculi between its branch to the 

 Rectus superior and to that to the Rectus internus contains a 

 number of nerve cells ; according to Schwalbe {I. c, p. 235 and 

 Pt. XII, Fig. 4) these cells are arranged in four groups or ganglia ; 



