120 



THE BRAIN OF FISHES 



Though the Cerebellum itself does not appear to be 

 immediately connected with any nerves, the Medulla 

 Oblongata, from which this part is an outgrowth, is 

 remarkable in Fishes, as well as in other vertebrates, for 

 the number and importance of the nerves with which it is 

 connected. Indeed, if the limits of the Medulla aro 

 taken to be those originally denned by Willis and most 

 anatomists anterior to Haller (1763), they will include the 

 ' crura cerebri ' ; and in that case all the Cranial Nerves 

 (that is, the nerves which pass inwards or outwards 

 through holes in the cranium), except the olfactory and 

 the optic, would have to be described as in direct con- 

 nection with the medulla oblongata. 



The Cranial Nerves of Fishes and of Amphibia are, 

 with few exceptions, similar in number and nature to those 

 existing throughout the vertebrate series, so that they may 

 with advantage be here enumerated. According to the 

 classification of Willis (1664), which is generally followed, 

 they are said to consist of nine pairs, counting from before 

 backwards. (See figs. 46, 57, 58.) 



1st Pair. Olfactory. 

 2nd Optic. 



3rd Motor oculi communis ; supplying all but two of 

 the muscles of the eyeball and the circular 

 fibres of the iris. 



4th Trochlearis ; supplying the superior oblique muscle 

 of the eye. 



/'Large root : the nerve of general sen- 

 sibility for the side of the head, 

 face, &c. 



5th Trigeminus 



Small root : supplying muscles con- 



nected with the jaw (muscles of 



mastication). 



6th Motor oculi externus ; supplying the external 

 , rectus muscle of the eyeball. 



