496 



PHYSIOLOGY 



CHAP. 



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 lief ore 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- 

 ^ner^SSSn: brain iii this class of fish produces no 



2) 



FIG. 251. Brain of Sqtudius 



noticeable disturbance ; we may, therefore, 



brain ; D, myelencephalon conclude that the parts of the liei'VOUS 



system remaining intact suffice for the com- 

 plete execution of all the higher nervous functions. 



These are certainly represented in the mid- and 'tweeii-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 Sterner, 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 FlG . 25 2.-Bram ot 

 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. 



(Steiner.) en, nasal capsule; bo, 

 olfactory bulb: A, prosencephalon; 

 A', optic thalami or 'tween-brain ; P>, 

 optic lobes or mid-brain ; C, meten- 

 cephalon or hind-brain ; D, myelen- 

 cephalon or bulb, from which the 

 vagus nerve emerges. 



