238 NERVE COMPONENTS OF BONY FISHES. 



originate from a different position from the others and are 

 innervated by smaller nerve fibres. Their significance is 

 unknown. All of the eye-muscle nerves derive most of 

 their fibres from their nucleus of the same side, but some 

 from the nucleus of the opposite side and doubtless some 

 from the fasciculus longitudinalis dorsalis. The eye- 

 muscle nerves of Menidia do not conform to the scheme 

 given by Allis for the ganoids and teleosts, and an exam- 

 ination of his v^hole system leads to the conviction that its 

 phylogenetic value is greatly in need of confirmation. 



Section 10. — The Optic Nerve. 



This nerve is enormous and is composed of very small 

 fibres. The chiasm is the simple crossing with the left 

 nerve uppermost which is typical for teleostomes. The 

 cranial wall at the point of emergence is membranous. 

 The nerve is of the broad, plicated, ribbon-shape so com- 

 mon among the bony fishes, consisting of three laminae. 

 One septum enters from the dorsal side, the other from 

 the ventral. The adult structure certainly favors the view 

 of Studnicka ('96) that this form is derived by the folding 

 of a ribbon-shaped nerve, rather than that of Deyl ('95) 

 that it is derived from a cylindrical nerve by the intrusion 

 of connective tissue septa. This form must be regarded 

 as an adaptation to secure the proper nourishment of the 

 nerve in these large-eyed forms, as Studnicka points out, 

 and Deyl's attempt to deduce phylogenetic conclusions 

 from the forms of the optic nerve can hardly have much 

 value, as the character is too variable and too liable to 

 convergence. For further morphological considerations 

 see the papers cited above. 



