266 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



will croak when stroked on the back. When the optic lobes have been 

 removed this reaction becomes more difficult to obtain, but it is not necessa- 

 rily abolished, neither is the characteristic fling of the legs in swimming. At 



the same time, a frog with its optic lobes can direct both its jumping and 

 swimming movements according to light stimuli acting through theeye,jump- 

 ing around and over obstacles which form a shadow in its path, and climbing 



out of the swimming tank on the lighter side. This power is lost when the 

 optic lobes have been removed. 



When the anterior end of the bulb (pars commissuralis — Stieda) has been 

 also removed then the frog becomes incessantly active, creeping about and 

 not coming to rest until he has run himself into some corner. Schrader found 

 such frogs capable of clambering over the edge of a box eighteen centimeters 

 high. They are at a loss when the edge of the box lias been finally attained, 

 and vainly reach into space from this position. In the water they swim "dog- 

 fashion, 1 ' but only upon special stimulation do they make a spring. 



If more of the bulb is removed, the bearing of the frog departs more and 

 more from the normal, and is only temporarily regained in response to strong 

 stimulation : nevertheless, co-ordinated movements can be obtained when the 

 bulb down to the calamus scriptorius has been removed, and only when the 

 movements of the arms are directly affected by the damage of the upper end 

 of the cord does the inco-ordination become constant. 



A section through the optic lobes at a t Fig. 116, E) puts the frog in a con- 

 dition similar to that following the isolated removal of the lobes, while a sec- 

 tion at I, has the curious effect of causing the animal to move backward upon 

 stimulation of the toes. 



When the small ridge which forms the cerebellum in the frog has been 

 removed a slight tremor of the leg-muscles and a loss of precision in jumping 

 arc the only defects noted (Fig. 116, />). These results hold for symmetrical 

 removal of the divisions of the encephalon. When the removal is unsym- 

 metrical in the inter-brain, mid-brain, or bulb (Fig. 116 F, a and //), there is 

 more or less tendency to forced positions or forced movements. 



As a rule, action is most vigorous on the side of the body associated with 

 the greater quantity of nerve-tissue. This relation appears as a natural result 

 of the greater effectiveness of the incoming impulses when entering a larger 

 group of central cells. Indeed, the removal of the different portions of the 

 central system in the frog is accompanied by a progressive loss in responsive- 

 ness, stronger and stronger stimuli being required to induce a reaction. This 

 hold- true down to the anterior end of the bulb, the removal of which, on the 

 contrary, sets free the lower centres, so that the frog becomes incessantly 

 active, dust how this release is effected is not easy to explain, but further 

 removal is again followed by the loss of responsiveness. 



Passing next to the bird, as represented by the pigeon, the observations of 

 Schrader arc the most instructive. 1 The removal of the hemispheres from 

 the bird involves taking away the mantle and the basal ganglia, the chiasm a 

 1 Archwfur die gesammte Physiologie, 1888, Bd. xliv. 



