ANATOMY OF THE GUNNER. 437 



base of the neck, instead of in the thorax as in the higher Ver- 

 tebrates. The heart still preserves its primitive division ; on 

 the other hand, the swimming-bladder is a special adaptation 

 of the piscian type, while the frequent absence of the pan- 

 creas is a peculiarity of fishes the meaning of which is not 

 yet understood. 



The brain (B] does net occupy the whole of the cranial 

 cavity, but is imbedded in a large accumulation of celiular 

 tissue. In order to study the brain satisfactorily, it should 

 be exposed from above, laying bare at the same time the optic 

 nerves and muscles. The two olfactory lobes are followed 

 by two lobes (H], the cerebral hemispheres, and immediately 

 behind them two larger lobes (Q), the corpora bi- or qitadri- 

 gemina (optic lobes, not optic thalami) ; further back follows 

 a single median lobe (Ob), the cerebellum, somewhat conical 

 in shape and resting upon the medulla oblongata (M), from 

 which spring various nerves, and which, tapering backward, 

 is continued as the spinal cord. In front appear the very large 

 and conspicuous optic nerves (Op}, the right nerve passing 

 obliquely to the left eye, the left nerve to the right eye 

 running under the right nerve, but forming no chiasma ; 

 each optic nerve is a plaited membrane, folded somewhat 

 like a fan when shut up, an arrangement occurring only 

 among fishes. In a side-view of the brain (Fig. 400, B}, the 

 mode of origin of the optic nerves and their origin from the 

 optic lobes can be clearly seen ; it further shows the various 

 forms of the lobes of the brain, and the large inferior lobes 

 (L) below the corpora quadrigemina ; these lobes are very 

 remarkab!j " o homologize. 



The ( ^ Dckets, separated by an interorbital 



septum The eyeball has the form of an ob- 



late spl oved, as in all Vertebrates, by four 



recti an nuscles. The recti spring from around 



the exit nerve from the brain-case, and thence 



diverge into different parts of the eyeball ; 



above ; erior (Rs) ; towards the interorbital 



septurr nus (Ri), opposed to the last is the 



rectus id below is the rectus inferior, not 



shown 11 Teleosts both oblique muscles, the 



