440 , ZOOLOOT. 



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 not 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, la3dng bare at the same time the optic 

 nerves and inuscles. The two olfactory lobes are followed 

 by two lobes {H), the cerebral hemispheres, and immediately 

 behind them two larger lobes {Q), the corpora hi- or quadri- 

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

 a single median lobe [Gb), the cerebellum, mm.ewh.z,t 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 (I^'ig. 400, IJ), the 

 mode of origin of tiie 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 inferwr lobes 

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

 remarkable and difficult to homologize. 



The eyes lie in two sockets, separated by an interorbital 

 septum (Pig. 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 superior (Rs) ; towards the interorbital 

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

 rectus externus (Re), and below is the rectus inferior, not 

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



