1890.] Changes in Muscle daring "Latent Stimulation" 15 



I have now to submit to you incontrovertible evidence which the 

 photographic method affords, not only that the estimate of the dura- 

 tion of the period of latent stimulation accepted as true ever since 

 Helmholtz's early investigations is very much too long ; but that the 

 final conclusion arrived at by Dr. Yeo a year ago, that it has a real 

 duration of five thousandths of a second, is erroneous. I am further 

 in a position to demonstrate what are the time-relations of the 

 electrical change to the muscular contraction with which it is 

 associated. 



The method of observation consists in projecting the movement to 

 be recorded, whether of the muscle or that of any instrument which 

 serves as an index of change, on a vertical slit on which the vibrations 

 of a tuning-fork and the motion of a signal are also shadowed. Im- 

 mediately behind the slit is a photographic plate, which is carried by 

 an equilibrated pendulum. The approximately uniform rate of motion 

 of the sensitive surface which receives the light-written record is 

 about one meter per second, but is determined in each experiment by 

 reference to the rate of vibration of a tuning-fork. 



The plan adopted for obtaining a photographic record of the earliest 

 trace of change of form, was based on the by no means new considera- 

 tion that the effect of an instantaneous stimulus is in the first instance 

 limited to the part of the structure to which it is applied, and, con- 

 sequently, may fail to produce any measurable change of form of the 

 whole muscle ; the parts which first contract doing so at the expense 

 of the as yet relaxed parts which are connected with them. This 

 consideration is applicable not only to the case in which the muscle 

 is excited directly, but also to that in which it is excited through its 

 nerve ; for in the latter case, each fibre is first stimulated at the spot 

 at which it receives its nerve. 



In the experiments on direct excitation, the muscles used were the 

 gastrocnemius and sartorius of the frog. In the former the movement 

 of contraction was communicated to a light index, which was sup- 

 ported by a fine spring. One end of the index rested on the muscle, 

 while the other occupied the front focus of a projection apparatus, the 

 slit being in the other focus. When the sartorius was used the 

 surface of the muscle was itself brought for a moment into the focus, 

 at the seat of excitation. The unavoidable exposure of the structure 

 to the electric light, which this method involved, lasted scarcely more 

 than a second. In successful experiments, the interval between 

 excitation and the beginning of the contraction was 2^ thousandths 

 (= ^Q) of a second. 



In ft photographic record of a succession of events no time-error is 

 possible, provided that the rate of movement of the recording surface 

 remains unaltered, for, if I may so express myself, an event cannot 

 be seen photographically before it happens. It is therefore certain 



