ANATOMY OF THE CUNNER. 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 nct 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 optic 
nerves and muscles. The two olfactory lobes are followed 
by two lobes (#7), the cerebral hemispheres, and immediately 
behind them two larger lobes (@Q), the corpora bi- or quadri- 
gemina (optic lobes, not optic thalami) ; further back follows 
a single median love (Cd), 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 
(LZ) below the corpora quadrigemina ; these lobes are very 
remarkable and difficult to homologize. 
The eyes lie in two sockets, separated by an interorbital 
septum (Fig. 397, 8). The eyeball has the form of an ob- 
late spheroid, and is moved, as in all Vertebrates, by four 
rectt and two obliqgui 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 (8) 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 
