FUNCTIONS OF THE BRAIN-STEM AS A WHOLE 619 



In the true fishes the cerebrum is but slightly developed, and in the lampreys 

 and bony fishes the cortex consists of only a simple layer of epithelial cells. 



After extirpation of the cerebrum from a bony fish (Squalius cephalus, Fig. 

 274) the animal moves exactly like a normal animal, and, according to Steiner, 

 it is quite impossible to discover anything anomalous in its movements. When 

 an earthworm is thrown to the fish, it makes a rush for the booty, seizes it while 

 it is still falling and devours it. A cord of about the same dimensions thrown 

 into the water may or may not be seized, but is never eaten. A decerebrated 

 fish may be even fastidious about its food; spurning fish worms but taking 

 crumbs of bread from the surface of the water. When one red wafer and four 

 white ones are thrown to it, the 

 fish regularly chooses first the red 



and then the white ones. It does IISS? 12$!Sf Nasal capsules 



not move to take the food from the ^^F^^^^M^ 



observer's hand, but will take it \ ^J " Olfactory bulb 



from a long string. Finally, the ^ Forebrain 



decerebrated fish will exchange ca- 



resses with its uninjured compan- Midbrain 



ions. From these observations we (optic lobes) 



may conclude that suppressipn of " H j nd J )r b ai 5J 



the cerebrum in this genus is of fec^J 



no particular consequence that to JPH" ^meduSaobiongata) 



judge from the behavior of the ^IIP" Vagus nerve 



animal after the operation, the 

 parts remaining are sufficient for 

 the discharge of all the central 

 functions. FIG. 275. The brain of Scyllium canicula, a dog- 



We have no experiments which shark, after Steiner. 



give us any clew as to the impor- 

 tance of the 'tweenbrain in the bony fish. But Steiner has reported some in which 

 he removed both the midbrain and the 'tweenbrain along with the cerebrum. 

 Following this operation the animal would lie entirely motionless on its side or 

 on its back, with the fins hanging perfectly limp. Hence we can say that the 

 higher functions of the central nervous system are dependent upon the 'tween- 

 brain and the midbrain, but just what share each one takes we do not yet know. 



The selanchians also (dog shark, Scyllium canicula, Fig. 275) withstand 

 removal of the cerebrum without suppression of their movements. After a few 

 rounds about the tank the animal lies quietly on the bottom of the tank for 

 many hours or even days at a time, Steiner having scarcely ever seen one in 

 motion when it was not excited by some external stimulus. Besides, the animal 

 does not spontaneously take food, but its inability to do so is not the result of 

 loss of the cerebrum itself, but is rather due to the functional loss of its olfactory 

 lobes, 1 which of course is a necessary consequence of the operation. Careful 

 investigation of the normal dogfish confirms this indication that it seeks food 

 entirely by the sense of smell. 



Simultaneous removal of both the 'tweenbrain and the cerebrum likewise 

 produces only insignificant effects. Since this operation involves destruction of 

 the optic nerves, such animals are of course blind ; and yet they can swim in a 

 perfectly normal manner. One observes, however, that after some time, which 

 appears to be shorter after removal of the forebrain alone, the animal clings to 



The olfactory lobe consists of the olfactory bulb and the olfactory tract. ED. 



