LXX.] REACTION-TIME. 327 



(b.) The observer suddenly pulls on (H), thus discharging the spring (F), 

 which vibrates and produces a note. The moment the observed person hears 

 the sound, he presses the knob (K) and raises the writing-style. Of course, 

 a curve is recorded, and it is easy to calculate the time which has elapsed 

 between the emission of the sound and the reaction by the observed person. 

 Numerous observations must be made, and the mean taken. 



(c.) The instrument may also be used for vision, i.e., when the slide (B 6) 

 on being moved uncovers a painted disc. 



(d.) In the more complete form of the apparatus, a key is fixed on one side 

 of the apparatus, so that an electrical current is made or broken at the 

 moment the spring begins to vibrate. The key is placed in the primary 

 circuit of the induction-machine, and the electrodes of the secondary battery 

 are applied to any part of the skin, the observed person depressing the knob 

 (K) when he feels the stimulus. One can thus make numerous experiments 

 on the " Reaction -Time " from different parts of the body. 



W. G. Smith has devised another simple method (see Journal of Physiology, 

 xvii. ; Proceedings of Physiological Society, Nov. 1894). 



6. Inhibition of Equilibration Movements. 



Take an uninjured frog, place it on its back, and observe that it will not 

 lie in this position, but immediately rights itself. Tie pretty firmly a thick 

 string round each upper arm. This in no way interferes with the movements 

 of the frog ; but on placing the animal on its back, it no longer rights itself, 

 but continues to lie in this position for a long time. It may be moved or 

 pulled by the legs, yet it does not regain its normal attitude. Notice the 

 modification of the respiratory movements. 



7. Kirch er's Experimentum Mirabile. 



(a.) Take a hen and gently restrain its movements. Bring its bill in con- 

 tact with a table. With a piece of white chalk draw a line directly outwards 

 from its bill. Hold the animal steadily for a few seconds, and on removing 

 the hands gently, it will be found that the hen lies 

 quiescent and does not move for a considerable time. It . w . 



may be rolled to one side or the other, yet it lies / S 



quiescent. 



(b.) Take a hen, gently restrain its movements, then lay 

 a straw or white thread over the base of its bill. In a short 

 time the animal becomes quiescent. Note the alteration of C^C3 * 



the heart-beat and the depth and number of the respira- (^MM 3 



tions. 



8. Eeactions of Frog without Cerebral Hemispheres. 



In the frog, as shown in fig. 248, the parts of the brain 

 are arranged one behind the other. The guide on the 

 surface of the skull to the posterior end of the cerebral . Olfactory bulb- 

 hemispheres is a line connecting the front margins of the i. Cerebral hemi- 

 two exposed tympanic membranes. The brain may be ^ph eres -,2. Optic 

 exposed in a narcotised frog either by means of a small beihim;*. Med- 

 trocar or by severing the parts with a knife. After removal uUa oblongata. 

 of the cerebral hemispheres, place a little cotton wool in 

 the wound to prevent bleeding. The student is not permitted to do this 

 operation. 



(a.) Immediately after the operation the frog lies flat on any surface with its 

 legs extended, but after the shock of the operation, i.e. , in about an hour, it draws 

 up its legs and assumes the attitude and appearance of an intact frog, but it 



