TRANSMISSION OF SENSORY AND MOTOR IMPRESSIONS. 581 



needle of a sensitive thermo-electric apparatus be made to traverse a large 

 nerve like the sciatic, a marked increase of temperature occurs at the mo- 

 ment that its functional activity is excited by mechanical or electrical ex- 

 citation. 



471. The rapidity with which sensory and motor impressions are trans- 

 mitted through the Nerves of the living body is apparently so great, and the 

 means of estimating it at our disposal so few, that it is doubtful whether we 

 shall ever be able to determine it with accuracy ; but the ingenuity of Helm- 

 holtz has enabled him to ascertain with some precision the rate at which the 

 change induced by the passage of an electrical current is propagated along 

 a small portion of a nerve. The following figure and description deserve 



FIG. 194. 



attention, since they exhibit the mode in which physical research can be ap- 

 plied to the most recondite questions of animal life. It is required to measure 

 the time which any excitation takes to travel from the cut end a of the nerve 

 N to its point of entrance b into the muscle M. As the means of excitation, 

 Helmholtz employs an instantaneous induction current in the following 

 mode: At the moment that a constant current passing through the coil p, 

 from the battery K, is broken, an instantaneous induced current passing in the 

 same direction is excited in Q; this is the stimulus acting on the nerve at a. 

 The time-measuring current is formed by a second battery H, into some part 

 of whose circuit a galvanometer E is introduced. It is requisite that the 

 closure of this current should be precisely coincident with the opening of the 

 inducing current from K, by which the induced current at Q, or the stimu- 

 lus, is produced ; this is effected by the lever ABC, whose arm A rests on the 

 metal plate o and completes the circuit K L A P. If now the metal point D 

 be forcibly pressed on B, the time-measuring current passing through H I B 

 D E F G will be closed; but at the same instant, from the depression of the 

 arm B, the stimulus will be applied to the nerve through the opening of the 

 inducing current K L A p, which is the result of the elevation of the arm A. 

 But as soon as the stimulus has reached the muscle M, after traversing the 

 nerve N the muscle contracts, raising with it the hook G, which just dips into 

 the cup of mercury F, and consequently breaking the time-measuring current 

 proceeding from the battery H. The extent to which the galvanometer 

 needle has been deflected indicates the time during which the current has 

 been flowing through the circuit H I B D E F G, that is, the time which has 



