ME:\rOIRS OF THE NxVTIO^^AL AOxVDEMY OF SCIENCES. 



411 



This sliows that when the i-eaction is made with the foot tlie dehiy may be partly due to a moro 

 difficult coordination in the higher centers. In earlier experiments made by one of us it was also 

 found that a time about 30 a longer was required to make a movement with the organs of spoecli 

 than with the hand. The reactiontiine seems, however, to be the same for tlic right and left 

 hands. Thus, for 6' the reaction-time on light was 14G (j with the right hand, and 147 a with the 

 left hand. 



Part III. — EEACTio>is on Touch. 



In the case of reaction experiments with dermal stimuli tVie electric shock has mostly been 

 used, as it is easy to apply the shock to ditteient parts of the body. We have, however, seen that 

 the iihysiological efl'ects of the saock vary greatly on different parts of the body, and even at the 

 same ])oint they can not be kept constant. The reaction-time following a touch or blow can be 

 measured without difficulty beyond the inconvenience of applying the blow to dilferent parts of 

 the body. We have found that the same objective force of blow is followed by the same subjective 

 sensation more nearly than in the case of electric stimulation. On different parts of the body the 

 same blow, iiuleeil, calls forth dift'erent sensations, the sensations being more intense when the 

 part is harder, as over a bone. But the difference is not so great as in the case of the electric 

 shock, and at the same point the same sensation can be given time after time and day after day. 

 The probable error is conseiiuently smaller than in the case of electric shock; indeed, the variable 

 error in our experimeuts on touch is much smaller than in any reaction-time experiments hitherto 

 published. We have, for example, measured ten successive reaction times, as follows: 102, 102, 

 100, 100, 100, 100, 100, 100, 101, 100 a. We have in this series a mean variation or variable error 

 of 0.7 (T, which of course includes the error of the apparatus, with which readings are only made 

 to thousandths of a second. Such experiments certainly demonstrate great constancy in complex 

 physiological processes, the variable error comparing favorably with that of many phy.sical 

 measurements. 



We used three methods for applying the blow, the last of which was much the best. We 

 shall, however, describe brietly our earlier experiments, as the methods might prove useful in 

 clinical work when the greatest accuracy is not necessary. In our first experiments we tied 

 telegraphic keys to the limb. These were atljustetl so that a surface about 10 mm. across touched 

 the spot. When the arm of the key was hit with a hammer the circuit was broken, and at the 

 same time the knob was forced against the skin. In this case a contact was broken, not closed, 

 ami the chrou<iscoi)e was started by means of a secondary circuit. The current could pass either 

 through the contact of the key or the coils of the electro-magnet of the chronoscope. As the 

 resistance in the chronoscope is great, the current would pass almost exclusivel}' through the key, 

 but when the contact at the key was broken the current would go to the chronoscope and start 

 the hands. In this case a secondary current must also be used in regulating the chronoscope by 

 means of the gravity chronometer. 



The keys were applied on the lower ami upper arm at the points A and D, and three intensi- 

 ties were used — a gentle pressure, a medium blow, and a hard blow. The gentle pressure was 

 barely jierceptible, as the surface was always in contact with the skin. The results of 2,400 

 reactions, ten being made in succession at intervals of two seconds, are given in Table IX. 



Taule IX. — I'ressiirc strong, meilium, iiiid iceah-. roiiits A (Did D. J and (' observers. 2,400 rtacti'iiis. 



