496 



PHYSIOLOGY 



CHAP. 



FIG. 26i.-Brain of stilus 

 cephaius. (steiner.) A, 



prosencephalon, witholfac- 

 tory nerve above ;,mesen- 



Cyclostomes and Teleosteans the cerebral mantle consists of a single 



layer of ectodermal cells. 



According to Steiner no disturbances in 

 the movements are seen after excising the 

 fore -brain of Squalius cephalus, a teleostean 

 (Fig. 251); the animal moves as though it 

 were intact. If offered a living worm it 

 chases, catches, and swallows it. If a bit 

 of string of much the same size is thrown 

 into the water, the animal comes up in the 

 same way, but turns off before catching it, 

 or rejects it from its mouth. It is able to 

 select its food, and recognises its companions 

 that have not been operated on. Steiner's 

 experiments show that ablation of the fore- 

 brain in this class of fish produces no 

 noticeable disturbance; we may, therefore, 

 myeiencephaion conclude that the parts of the nervous 

 system remaining intact suffice for the com- 

 plete execution of all the higher nervous functions. 



These are certainly represented in the mid- and 'tween-brain. 



When the optic lobes are excised, according to Steiner, the animal 



loses its power of maintaining equilibrium, and lies on its side, or 



back, motionless, with relaxed fins. 



But the return of spontaneous move- 

 ment, some time after the operation, is 



not excluded : Steiner did not continue 



his observations long enough. 



According to Steiner, removal of 



the anterior brain in Selachians, as 



in the dog-fish (Scyllium canicula, 



Fig. 252), causes immobility for many 



hours and even days, unless the animal 



is artificially stimulated. Bethe was 



unable to confirm this observation, 



as he found that removal of the fore- 

 brain did not abolish spontaneous 



movements. The animals certainly 



no longer feed spontaneously, but 



this is due not to ablation of the 



fore -brain, but to destruction of the 



olfactory lobes, as is proved by the 



fact that excision of the latter alone 



produces the same effect. On the 



other hand, attentive observation of the normal dog-fish shows 



that it is largely guided by the sense of smell in seeking its food ; 



the Squalius, on the contrary, by the visual sense. 



FIG. 252. Brain of Scyllium canicula. 

 (Steiner.) en, nasal capsule ; 60, 

 olfactory bulb: A, pro.sencephalon ; 

 A', optic thalami or 'tween-brain ; B, 

 optic lobes or mid-brain ; C, meten- 

 cephalon or hind-brain ; D, myelen- 

 cephalon or bulb, from which the 

 vagus nerve emerges. 



