NERVOUS CENTRES OF FISHES. 
365 
proceeding from it, are largest in those animals in which the 
brain is smallest. 
453. It is in Fishes that we find the brain least developed, 
and the cerebral hemispheres bearing the smallest proportion 
to the other parts. On opening the skull, we usually observe 
four nervous masses (three of them in pairs) lying, one in 
front of the other, nearly in the same line with the spinal 
cord. Those of the first pair are olfactory ganglia , or the 
ganglia of the nerves of smell (fig. 192 a, ol). In the Shark, 
and some other Fishes, these are separated from the rest by 
peduncles or foot-stalks (b, ol) ; a fact of much interest, as 
explaining the arrangement 
which we find in Man (§ 458). 
Behind these is a pair of gan¬ 
glionic masses ( c h), of which 
the relative size varies con¬ 
siderably in different fishes 
(thus in the Cod they are 
much smaller than those 
which succeed them, whilst 
in the Shark they are much 
larger); these are the cerebral 
hemispheres. Behind these, 
again, are two large masses 
(op), the optic ganglia, in 
which the optic nerves termi¬ 
nate. And at the back of these, overlying the top of the 
spinal cord, is a single mass, the cerebellum (ce) ; this is seen 
to be much larger in the active rapacious Shark, the variety 
of whose movements is very great, than in the less energetic 
Cod. The spinal cord ( sp ) is seen to be divided at the top by 
a fissure, which is most wide and deep beneath the cerebellum, 
where there is a complete opening between its two halves. 
This opening corresponds to that through which the oesophagus 
passes in the Invertebrata ; but, as the whole nervous mass of 
Yertebrated animals is above the alimentary canal (§ 74), it 
does not serve the same purpose in them; and in the higher 
classes the fissure is almost entirely closed by the union of the 
two halves of the cord on the central line. 
454. In Reptiles we do not observe any considerable 
advance in the character of the brain, beyond that of Fishes; 
