PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 229 



more quickly by tactile irritatiou. If the impressiou is on the eye, the 

 reaction of the hand takes place in one-flfth of a second ; if it is on the 

 ear, in one-sixth of a second; and on the skin, in one-seventh of a second. 

 Thus the physiological periods are among- themselves as the numbers 

 one-tifth, one-sixth, one-seventh. But does this period of, for examjile, 

 one-seventh of a second, which elapses between the moment when the 

 skin is irritated and that when the observer moves his hand, corre- 

 spond entirely to a psychical act? We answer no; it is necessary that 

 the sensory impression should have time to reach the brain ; cerebral 

 perception and volition being then accomplished, time is required for the 

 motor impression to reach the muscle and determine its motion. Nor 

 is this all. The impression once produced passes through the uerve 

 with a known velocity, but this impression is not instantaneously pro- 

 duced ; it needs time to take form, to be completed before becoming 

 ready to traverse the nerve. It is not sufficient, therefore, to deduct 

 from the total duration of one-seventh of a second, the duration of the 

 passage of the sensory nerve-force and of the motor nerve-force, to con- 

 clude that the remainder appertains to the psychical act. These experi- 

 ments do not exhaust the subject. They do not make known the dura- 

 tion of the cerebral act, nor even whether it has a duration. 



M. Bonders instituted in the meanwhile a series of experiments des- 

 tined to remove all doubts. His method consists in augmenting the 

 l)h3'siological period until the measurement of the intercalated intel- 

 lectual operation can be clearly observed. 



The following is the plan of one of his experiments: 



First case. — The observer knows that an electric shock will be given 

 to his right foot, while the signal of reaction is to be given by the right 

 hand. 



Second case. — The observer does not know which foot will receive the 

 irritation, and is instructed to give the signal by the hand of the irri- 

 tated side. 



The physiological period measured in the two cases was longer in the 

 latter by about one-fifteenth of a second. It is clear, all the other con- 

 ditions being the same, that the difference in question represents the 

 time necessary to determine upon which side the irritatiou had taken 

 place, and to direct to the right or left side, conformably with the idea 

 acquired, the action of the will. Consequently the solution of a di- 

 lemma reduced to the most simple form is a cerebral action existing 

 during one-fifteenth of a second. It is thus experimentally established 

 that a cerebral action has a duration. We finally become assured that 

 this duration is augmented in proportion as the psychical action be- 

 comes more complicated, and that it is diminished when the action be- 

 comes simplified. 



Instead of exciting the organs of touch, Bonders next experimented 

 as follows on visual irritation : 



First case. — The observer executes a movement with the right hand 

 upon the appearance of a white light. 



Second case. — A white light and a red light are employed, and the sig- 

 nal of reaction is to be given with the right hand for the white light, 

 and with the left hand for the red light. Under these conditions the 

 solution of the dilemma consumes in the mind a more considerable 

 time. Oij the contrary, in the case of auditory stimulation, the dilemma 

 was solved by the mind in less time than in the case of visual stimula- 

 tion. The author of these researches attributes this superiority of one 

 sense over another to a facility derived from habit or exercise ; and, in 

 fact, repetition tends to equalize the action in the case of the two hands. 



