116 



THE BRAIN OF FISHES 



In many of the lower Fishes the eyes are very rudimen- 

 tary. In the young Lamprey two pigment spots replace 

 the single ' eye spot ' of the Lancelot. In the genus 

 Myxine the eyes are represented hy small bodies, which, 

 though in connection with slender optic nerves, are 

 covered over by muscle as well as by skin. The ocular 

 muscles for moving the eyeball are absent in many Fishes ; 

 this is the case even in the Gar-Pike, in which, though 



Fig. 51. 



Fig. 50.— Brain of Perch, upper surface. ''Owen after Cuvier.) a, Cerebellum ; 

 b, optic lobes ; c, cerebral lobes ; i, olfactory ganglia ; g, medulla ; }), n, r, s, t, cra- 

 nial nerves. 



Fig. 51.— Brain of Perch, under surface. (Owen after Cuvier.) a, Medulla ; e. 

 hypoaria ; /, pituitary body ; n, optic nerves, decussating ; c, cerebral lobes ; i, olfac- 

 tory ganglia ; p, q, r, s, t, cranial nerves. 



small, the eyes are at the surface. In the great majority 

 of Fishes, however, these organs are large and attain a 

 remarkable development. 



The optic lobes are usually the largest divisions of the 

 brain in osseous fishes, as in the Perch (fig. 50), and 

 they are commonly united by one or more transverse com- 

 missures. Each of them generally contains a distinct 

 cavity or ' ventricle,' and they often bear on their under 

 surface two smaller ganglionic projections, known as 



