700 



THE CEREBRAL CORTEX. 



itself in the water, it swims in a straight line, and, unlike other fishes 

 which loiter by the way smelling at this and nibbling at that, it keeps 

 on its course as if compelled by some irresistible impulse, and only stops 

 when it reaches the side of the vessel, or when worn out by pure 

 nervous and muscular fatigue. Apparently there is some essential 

 difference between the frog, which sits for ever quiet, and the fish, 

 which goes on indefinitely, but the same explanation is applicable 

 to both. In the one case there is no external source of irritation ; in 

 the other, the contact of the mobile water with the body surface 

 acts as a continual external stimulus to the natatory mechanism. As 

 this is constant, so the fish continues to swim until arrested by 

 some insurmountable obstacle or by pure nervous and muscular 

 fatigue." 1 



This description of the brainless fish is, however, only applicable to 

 osseous fishes, and only to those osseous fishes from which the optic 

 thalami have been removed as well as the corpora striata and hemi- 

 spheres. For if only the hemispheres and corpora striata are cut away, 

 the thalami and optic lobes being left intact, the behaviour of the 

 animal is little, if at all, different from that of normal fishes. It moves 

 to and fro in the water along with its fellows, nibbling at passing 

 objects in the manner characteristic of these animals, distinguishing- 

 edible from non-edible material (worms from bits of thread resembling 

 them), and even showing a preference for bait (wafers) of certain 

 colours. 2 And still less is the description applicable to elasmobranch 

 fishes, although the alteration here is in the opposite direction. For 

 the shark, deprived of its hemispheres and corpora striata, remains 

 perfectly immobile, even although the optic thalamus and optic lobes 

 may be completely intact. The difference is due to the fact that the 

 elasmobranch depends mainly upon its olfactory apparatus as a guide 

 to its movements, and in removing the cerebral hemispheres the 

 olfactory lobes are necessarily also removed. In fact, it has been 

 shown by Steiner that removal of the olfactory lobes alone, in the 

 shark, has an effect similar to the removal of the hemispheres. The 

 osseous fish, on the other hand, mainly depends upon influences reaching 

 it through its visual organs, and the greater part of the central 

 apparatus of vision remains intact. In reptiles also, the development of 

 the cerebral hemispheres goes largely hand in hand with that of the 

 olfactory apparatus. 3 



" The results of ablation of the cerebral hemispheres in pigeons have 

 been described in great detail by Eolando, Flourens, Longet, Vulpian, 

 and others. A pigeon so mutilated continues able to maintain its 

 equilibrium and to regain it when disturbed. When placed on its back, 

 it succeeds in regaining its feet. When pushed or pinched, it marches 

 forward. Should it happen to step over the edge of the table, it will flap 

 its wings until it regains a firm basis of support. When thrown in the 

 air, it flies with all due precision and co-ordination. Left to itself, it 

 seems as if plunged in profound sleep. From this state of repose it is 

 easily awakened by a gentle push or pinch, and looks up and opens its 

 eyes. Occasionally, apparently without any external stimulation, it 



1 Ferrier, loc. eit. 



2 J. Steiner, "Die Functionen d. Centralnervensystem, u.s.w. 

 Fische," Braunschweig, 1888. 



3 See Edinger, Anat. Aw,, Jena, 1893, Bd. viii. S. 305. 



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