564 



CHORD ATA. 



the eye is very short-siglited because light is so absorbed by water that 

 objects forty feet away are invisible. With this is connected the cam- 

 panula Halleri. The processus falciformis, a sickle-shaped outgrowth of 

 the choroid, extends from the entrance of the optic nerve into the vitreous 

 body as far as the lens, swelling out into the campanula ; this contains a 

 muscle which draws bacl< the lens and so is an apparatus of accommoda- 

 tion. Near the entrance of the optic nerve is a problematic organ, the 



Fig. 591. 



Fig. 592. 



Fig, 591.— Brain of trout. (After Wiedersheim.) BG, corpus striatum ; GP. pine- 

 alis ; HH, cerebellum ; L'}1, olfactory lobes ; ilH, optic lobes ; NH, Medulla 

 oblongata ; Fail, pallium, in part cut away ; VH, cerebrum ; 1-XII, nerves. 

 (See p. SiH.) 



Fig. .592.— Brain and nasal capsules of Scyllinm oiiulus. (From Gegenbaur.) n, me- 

 dulla ; 6, cerebellum ; c, optic lobes ; d, 'twixt brain ; ;/, cerebrum ; /(, bulbus and 

 tractus olfactorius ; o, nasal capsules. 



choroid gland, consisting largely of blood-vessels (rete mirabile). Chon- 

 drifications and ossifications of the sclera are common. Lids are weakly 

 developed or absent, and only some elasmobranchs have a nictitating mem- 

 brane. 



The ear has a relative size found in no other vertebrates, the labyrinth 

 corresponding well with fig. 575. The labyrinth contains in many teleosts 

 two otoliths, the asteriscus and sagitta, the first being especially large. 

 Experiments show that the ears are primarily for balance, and hearing is 

 doubtful. Strychninized fish do not respond to sound, if in its production 

 mechanical vibrations are avoided. 



Of all sense organs the most noticeable are those of the skin, especially 

 those of the lateral line, which are nowhere else so well developed and 

 which occur elsewhere only in cyclostomes and aquatic amphibia. In 

 fishes a line on either side usually begins at the tail and extends to the 

 head, where it divides into several curved lines (fig. 602, S!). Its position 

 is marked by a groove or a canal in the scales whicli opens to the exterior 

 by numerous canals through the scales. Branches of trigeminus, facialis, 

 glossiipliaryngcus, and especially the lateral branch of the vagus (fig. 570) go 

 to these organs, the latter extending back t(i the tail. Tliese supply special 



