156 PAUL S. McKIBBEN 



of the paraphysis, causing more marked change in the form of 

 the fore-brain than occurs when a part of the parasphenoid is 

 removed. Fixation of the brain of any amphibian may preserve 

 more closely the normal relations as seen in the fresh condition 

 if the precautions above mentioned are heeded. 



Variability 



The optic apparatus in Necturus, when compared with that 

 in some other tailed Amphibia, seems poorly developed. The 

 habits and reactions of Necturus, so far as known, also indicate 

 the comparatively slight functional importance of the eye. Con- 

 siderable variability is noted in the eye-muscle nerves in their 

 passage from the brain to the orbit and in their ultimate branches. 

 The relations of the trunks of the oculomotor and abducent nerves 

 in Necturus seem fairly constant, but the trochlear nerve shows 

 some remarkable differences in different individuals and often 

 varies greatly on the two sides of the same head. Such varia- 

 bility in the course and relations of the eye-muscle nerves in 

 Amphibia has previously been noted by Coghill ('02 and '06) 

 and by Norris ('08). As pointed out by these authors, conclu- 

 sions concerning cranial nerves in Amphibia can not be drawn 

 accurately from a study of single or even several specimens. 



In no two of the twenty-four eyes of the twelve individuals 

 studied carefully do the eye-muscle nerves appear exactly alike. 

 The descriptions and figures which follow illustrate the average 

 condition, special note being made of some of the most interesting 

 variations. 



N. oculomotorius 



The oculomotor nerve is formed by two or three bundles of 

 fibers which spring from the ventral surface of the mid-brain 

 about 0.6 mm. cephalad to the lateral pouches of the medulla 

 oblongata and about 0.4 mm. from the midline. The nerve runs 

 directly laterad above the saccus vasculosus (fig. 2) and inclining 

 slightly cephalad reaches its foramen which lies about 3.7 mm. 

 caudad to the optic foramen in the parietal bone (fig. 4). The 

 foramen for the oculomotor nerve Hes usually about 1.1 mm. 



