708 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



this the frog tends first to move the head in the direction opposite to the motion 

 of the support, and then to follow with movements of the body. If the optic 

 thalami are removed (Fig. 201, C\ the power of balancing is lost, because, 

 although the movements of the head still occur, those of the body are abolished. 

 A frog thus operated on and deprived of the hemispheres and thalami exhibits 

 the lack of spontaneity which is usually described as following the loss of the 

 hemispheres alone, but which is not a necessary consequence of this operation, 

 as the preceding experiments show. 



A frog possessed of the mid-brain and the parts behind it (Fig. 201, C) 

 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 necessarily 

 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 swim- 

 ming movements according to light stimuli acting through the eye, jumping 

 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 18 centimeters high. They 

 are at a loss when the edge of the box has been finally attained, and vainly 

 reach into space from this position. In the water they swim " dog- fashion," 

 and 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 (Fig. 201, E) puts the frog in a con- 

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

 tion at 6 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 

 are the only defects noted (Fig. 201, D). These results hold for symmetrical 

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

 rical in the inter-brain, mid-brain, or bulb (Fig. 201, F, a and b\ 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- 



