NERVOUS IRRITABILITY. 367 



for nervous action ; the quickness of the senses and the promptitude 

 of the will frequently varying to a perceptible degree. In the case of 

 a voluntary movement, the period consumed is occupied by three dif- 

 ferent processes, namely : 1. The act of volition, in the brain ; 2. The 

 transmission of the motor impulse, through the spinal cord and nerves, 

 to its destination ; and 3. The excitement of the muscle to contrac- 

 tion. In the case of a sensation, there are three analogous successive 

 acts, namely : 1. Reception of the impression by the sensitive mem- 

 brane ; 2. Transmission of the stimulus through the nerve toward the 

 brain ; and 3. Its perception in the brain as a conscious sensation. It 

 is important to determine the rapidity of nervous communication in each 

 direction. 



Methods of Determining the Rate of Transmissionof the Nerve Force. 

 The rate of transmission of the nerve force, first measured by 

 Helmholtz,* has since been investigated by different observers with 

 essentially similar results. The principle adopted is in all cases the 

 same. Muscular contraction is excited by a stimulus which p; 

 through two nerves of different length, or through two different lengths 

 of the same nerve ; the delay in contraction, when the stimulus passes 

 through the longer of these routes, gives the time required to traverse 

 the additional distance. 



These experiments were first performed on separated nerves and 

 muscles of the cold-blooded animals. The gastrocnemius muscle of a 

 frog is prepared, with a portion of the sciatic nerve attached. A gal- 

 vanic battery with an induction apparatus is also provided, so that the 

 closure of the battery circuit will produce an instantaneous current in 

 the induction coil. This induced current is first applied to the muscle, 

 and the time noted which intervenes between the closure of the circuit 

 and the muscular contraction. This represents the period required for 

 the excitement of the muscular fibres, and was found by Helmholtz to 

 be about y^ of a second. If the stimulus be now applied to the nerve 

 near its termination in the muscle, the interval is not perceptibly 

 changed. But if it be applied at a point one, two, or three centimetres 

 distant, a retardation is manifested in the muscular contraction ; and 

 this retardation becomes greater as the distance between the muscle 

 and the point of stimulation is increased. 



The intervals of time in these experiments have been measured by 

 various contrivances, the most successful of which is an automatic 

 registering apparatus like that of Marey (page 284). Upon the surface 

 of the revolving cylinder the extremity of a tuning-fork, vibrating 500 

 times per second, traces an undulating- line (Fig. 96, a) which records 

 the time occupied in moving from one point to another. A straight 

 horizontal line i b) is also traced upon the same surface by the extremity 

 of a slender lever, the other end of which forms part of the galvanic 

 circuit. The closure of the circuit is accomplished by a movement 



* Couaptes Rendus de 1' Academic des Sciences. Paris, 1851, tome xxxiii., p. 262. 



