CONDUCTION WITHIN NERVE CELLS. 



611 



Even with the same sense differences are observed, for it has been 

 noted that the response to an electrical stimulus of the skin of the 

 forehead may demand a longer time than an electrical stimulus of the 

 hand ; x hence, probably, the difference of acuteness of perception of 

 the different terminal organs influences the result. 



In the case of the eye and ear, the length of path being approximately 

 the same, the reaction times might have been expected to be equal, 

 whereas this is far from being the case. It is a significant fact that 

 there are a larger number of synapses in the nerve chain along which 

 nervous impulses, started by the falling of light upon the retina, pass 

 to reach the sensorium, than is the case with the auditory path, for in 

 the retina alone there are at least two synapses, namely, between the 

 cone-fibres and the inner granules, and between the inner granules and 

 the ganglion cells, whereas in the cochlea there are no synapses, these 

 being first met with in the part of the auditory path which lies in the 

 medulla oblongata (Fig. 308). 



Another explanation which has been put forward regarding the length of 

 the optical reaction time, depends upon the hypothesis that the peripheral 

 excitation of the nerve fibres is of the nature of a chemical change, as con- 

 trasted with the mechanical excitation of auditory and tactile nerve fibres. 

 This, if true, may account for a part of the delay, but can hardly be responsible 

 for the whole, since it is not likely to be longer (nor even so long) in the case 

 of the retinal elements as in muscle, where it is certainly not more than 

 - 0025 second (see article on " Muscle" in this volume, p. 169). 



The reaction time is dependent upon many conditions, and is therefore 

 extremely variable. The following are the most important sources of varia- 

 tion : — 



1. The temperament of the individual. — Contrary to what might perhaps 

 have been expected, individuals of a phlegmatic temperament have, as a rule, 

 a shorter reaction time than those of a mercurial temperament — probably 

 because they are less liable to have their attention distracted from the act 

 which they are performing. 



2. Age. — In old age the reaction time becomes slow. 



3. Practice and attention. — These tend up to a certain point to diminish 

 the time. 



4. Fatigue, on the other hand, tends to increase it. 



5. Intensity of stimulus, — The reaction time is shortened by increasing 

 the intensity up to a certain point ; this shortening may be very marked. 



6. Season. — The reaction time is shorter in winter than in summer. 



7. Drugs. — Alcohol and morphia increase, coffee diminishes it. It is 

 further affected by various other drugs. It also varies in disease. 



The total reaction time is of course compounded of several factors. 

 There is — (1) The time of propagation of the stimulus to the terminal 

 nerve apparatus (which may be practically disregarded as far as most 

 sensations are concerned) ; (2) the latency (inertia) of the terminal or 

 recipient apparatus ; (3) the time occupied by the passage of the nerve 

 impulses to and within the lower centres ; (4) the transmission from 

 these to the higher centres (sensory and association centres) and that 

 occupied by changes taking place in the nerve cells here ; (5) the trans- 

 mission to and within the motor cerebral centres ; (6) the passage of the 

 volitional impulses herein generated to the nerve cells of the spinal cord ; 

 (7) the generation of fresh nerve impulses in these cells ; (8) their passage 



1 Kries and Auerbach, Arch. f. Physiol., Leipzig, 1877 ; Hall and Kries, ibid., 1879, 

 Suppl., S. 1 ; Vintschgau, Arch. f. d. ges. Physiol., Bonn, 1880, Bd. xxii. S. 87. 



