PISCES FISHES. 



529 



mals under consideration would seem to represent the spinal recur- 

 rent of the human subject ; it supplies the swimming-bladder and 

 the muscles of the shoulder. 



(571.) All the above nerves posterior to the optic arise from a 

 chain of ganglia constituting the medulla oblongata ; but above these 

 are situated other important masses entering into the composition 

 of the encephalon, from which no nerves take their origin, viz. the 

 cerebral hemispheres and the cerebellum. 



(572.) The cerebral hemispheres in all the Vertebrata are un- 

 doubtedly the seat of the mental powers; and, as this portion of 

 the brain becomes developed and perfected, brutality and stupidity 

 give place to sagacity and intelligence. 



In the higher quadrupeds, and more especially in man, the pro- 

 portionate size of the hemispheres of the brain is so enormous that 

 they overlap and conceal all the parts we have been describing ; 

 but, as we descend to lower forms, their relative dimensions become 

 gradually smaller and their structure less complicated, until in 

 fishes, the least intelligent of all the creatures belonging to this 

 great division of the animal kingdom, they are found in such a 

 rudimentary condition that they are frequently far inferior in size 

 even to the olfactory or optic ganglia (Jig. 232, c). 



The lobes representing the hemispheres in fishes (Jig- 284, b) 

 are quite smooth externally, and within are hollowed into a large 

 ventricle, in the floor of which is seen the upper surface of the optic 

 ganglia (fig. 234, B, d). They present none of that complica- 

 tion of parts met with in the brains of higher orders : their 

 inner surface is lined with transverse fibres (A), and a simple com- 

 missure passes pi^ 234. 



across the an- 

 terior part of 

 the ventricle, 

 bringing the 

 two sides into 



kA 



communication 



with each other; 



behind the com- 



missure a pas- 



sage leads to the 



third ventricle, ABC 



the infundibulum, and the pituitary gland. 



(573,) The cerebellum (Jig. 234, a) is at (/nee recognisable from 



