NEKVE-IMPULSE IN SENSORY NERVES AND REACTION TIME. 573 



mentioned length of nerve. [The time is easily ascertained by causing a tuning-fork of a 

 known rate of vibration to write its movements under the curves.] 



3. In the sensory nerves of man, the velocity of the impulse is probably about 

 the same as in motor nerves. The rates given vary between 94 to 30 metres [280 

 to 90 feet] per second (v. Helmholtz). 



Method. Two points are chosen as far apart as possible, and at unequal distances from the 

 brain, and they are successively excited by a momentary stimulus, e.g., an opening induction 

 shock applied successively to the tip of the ear and the great toe. The moment of the applica- 

 tion of the stimulus is indicated on the registering surface. The person experimented on is 

 provided with a key attached to an electric arrangement, by which he can mark on the register- 

 ing surface the moment he feels the sensation in each case. 



Reaction Time. The time which elapses between the application of the stimulus and the 

 reaction is called the " reaction time." It is made up of the time necessary for conduction in 

 the sensory nerve, that for the process of perception in the brain, for the conduction in the 

 motor nerves to the muscles, by which the signs on the registering surface were made, and 

 lastly by the latent period (p. 480). The reaction time is usually about 0'125 to 0*2 second 

 ( 374). 



Pathological. The conduction in the cutaneous nerves is sometimes greatly delayed, in 

 alterations of the cutaneous sensibility, in certain diseases of the spinal cord ( 364). The 

 sensation itself may be unchanged. Sometimes only the conduction for painful impressions is 

 retarded, so that a painful impression on the skin is first perceived as a tactile sensation, and 

 afterwards as pain, or conversely. When the interval of time between these two sensations is 

 long, then there is a distinctly double sensation (JYaunyn). It is rarely that voluntary move- 

 ments are executed much more slowly from causes depending on the motor nerves, but 

 occasionally the time between the voluntary impulse and the contraction is lengthened, but 

 there may be in addition slower or longer continued contraction of the muscle. In tabes 

 dorsalis or locomotor ataxia, the discharge of reflex movements is delayed ; it is slower with 

 thermal stimuli (60*) than with cold ones (0*52 C., Eicald). 



338. DOUBLE CONDUCTION IN NERVES. Conductivity is that property 

 of a living nerve in virtue of which, on the application of a stimulus, it transmits 

 an impulse. [The nature of a nerve-impulse is entirely unknown ; we may con- 

 veniently term the process nerve-motion, but there is some reason to believe that 

 nerve energy is transmitted by some sort of molecular vibration.] The conductivity 

 is destroyed by all influences or conditions which injure the nerve in its continuity 

 (section, ligature, compression, destruction by chemical agents) ; or which abolish 

 the excitability at any part of its course (absolute deprival of blood ; certain 

 poisons, e.g., curara for motor nerves ; also strong anelectrotonus, 335). 



Law of Isolated Conduction. Conduction always takes place only in the con- 

 tinuity of fibres, the impulse never being transferred to adjoining nerve-fibres. 



Double Conduction. Although apparently conduction in motor nerves takes 

 place only in a centrifugal direction towards the muscles, and in sensory nerves in a 

 centripetal direction, i.e., towards the centre ; nevertheless, experiment has proved 

 that a nerve conducts an impulse in both directions, just as in a non-living con- 

 ductor. If a pure motor or sensory nerve be stimulated in its course, an impulse 

 is propagated at the same time in a centrifugal and in a centripetal direction. 

 This is the phenomenon of " double conduction." 



Proofs. 1. If a nerve be stimulated, its electro-motive properties are affected 

 both above and below the point of stimulation (see Negative Variation in Nerves, 

 332). 



2. Electrical Nerves. If the posterior free-end of the electrical centrifugal 

 nerves of the malapterurus be stimulated, the branches given off above the point 

 of stimulation are also excited, so that the whole electrical organ discharges its 

 electricity (Babuchin, Mantey). 



3. Kiihhe's Experiments. The sartorius of the frog has no nerve-fibres at its 

 upper and lower ends. If the lower end be cut off, and if the lower third of the 

 muscle be suspended and divided vertically, on stimulating mechanically one apex 

 of the muscle, then the impulse passes in the motor nerves centripetally to the place 



