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 e 

 (thus in the Cod they are 

 much smaller than those of. 

 which succeed them, whilst ce 

 in the Shark they are much 

 larger) ; these are the cerebral sp 

 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 

 Vertebrated 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 ; 



Fig. 192. BRAINS OF FISHES. 

 A, Cod ; B, Shark. 



