440 ZOOLOGY. 



base of the neck, instead of in the thorax as in tlie 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 not occupy the whole of the cranial 

 cavity, but is imbedded in a large accumulation of cellular 

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

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

 nerves and muscles as in Fig. 400. Most anteriorly are two 

 lobes (//), the cerebral hemispheres, and immediately behind 

 them two larger lobes (Q), the corjjora hi- or quadri-gemina 

 (optic Inhes, not optic thalami) ; further back follows a single 

 median lobe {Cb), the cerebellum, somewhat conical in shape 

 and resting upon the medtdla oblongata (3/), from which 

 spring various nerves, and which, tapering backward, is con- 

 tinued as the sjiinal cord. In front arise the very large and 

 conspicuous optic nerves (O^j), 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 oi:)tic 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), the 

 curious disjiosition of the optic nerves 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 quadri- 

 gemina ; these lobes are very remarkable and difficult to 

 homologize. 



The eyes lie in two sockets, separated by an interorbital 

 septum (Fig. 400, S). The eyeball has the form of an ob- 

 late spheroid, and is moved, as in all Vertebrates, by four 

 recti and two obliqui muscles. The recti spring from around 

 the exit of the optic nerve from the brain-case, and thence 

 diverge to be inserted into different parts of the eyeball ; 

 above is the rectus siqierior (Rs) ; towards the interorbital 

 septum (S) rectus interjius (Ri), opposed to the last is the 

 r.ectus externus {Re), and below is the rectus inferior, not 

 shown in the figure. In Teleosts both oblique muscles, the 



