VII. 12 



FOREBRAIN 



209 



in buoyancy, perhaps due to an influence on the carbonic anhydrase 

 of the gas bladder. 



12. Brain of bony fishes 



The brain of bony fishes is built on the same general functional and 

 structural plan as that of elasmobranchs, namely, the development of a 

 number of separate centres, one concerned with each of the main 

 receptor systems. The forebrain is often large, but it is characterized 



chiasma 

 opticum 



rec. pr. 



Fig. 127 {a). Cross-section of the forebrain of the cod. 

 lat.tr. and tn.tr. lateral and medial tracts between olfactory region and hypothalamus; hyp. 

 hypothalamus; m. membranous roof of forebrain; n.mag. nucleus magnocellularis preopticus; 

 n.opt. optic tracts (that on the right has atrophied in this specimen) ; rec. pr. preoptic recess ; 

 str. hind end of striatum; thai, thalamus; 3rd v. third ventricle. (From Kappers, Huber, 



and Crosby.) 



by great development of its ventral regions (the 'corpus striatum'), the 

 roof being wholly membranous (Figs. 127 (a), 128). This condition is 

 known as 'eversion' and is the very opposite to the inverted or thick- 

 roofed forebrains that are found in the lung-fishes, close to the line 

 of tetrapod descent (p. 278). The whole of this forebrain is reached by 

 olfactory fibres, and there is little evidence that fibres from other 

 receptor centres reach forward to it; it is mostly a smell brain. 

 Extirpation of the telencephalon from various teleosts has not 

 produced changes in locomotion, balance, or vision; there may be 

 slight changes in general activity and social behaviour. No movements 

 have been seen following electrical stimulation of it. 



The diencephalon is not large, since most of the optic fibres end 

 not here, but in the midbrain. The roof is everted to form a pineal 

 body, and this and other parts of the diencephalon may contain 



