426 NERVOUS IRRITABILITY 



excitement of the muscular fibres to a state of contraction. In the case 

 of a sensation, there are also three analogous successive acts, namely : 

 1. The reception of the impression by the sensitive membrane; 2. Trans- 

 mission of the stimulus through the nerve fibres inward; and 3. Its per- 

 ception in the brain as a conscious sensation. It is an important phy- 

 siological problem to determine the degree of rapidity with which the 

 transmission of stimulus takes place through the nerve fibres in either 

 direction ; and it has recently become a matter of practical interest in 

 relation to pathology. 



Methods of determining the Rate of Transmission of the Nerve Force. 

 The measurement of the rate of transmission of the nerve force was 

 first accomplished by Helmholtz, 1 and has since been carried out by a 

 number of different observers with essentially similar results. The 

 principle adopted is in all cases the same. Muscular contraction is 

 excited by a stimulus which passes 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 greater of these 

 two distances, gives the time required for its transmission by the inter- 

 vening nerve fibres. 



These experiments were first performed upon nerves rnd muscles 

 freshly separated from the body in the cold-blooded animals. The gas- 

 trocneinius muscle of a frog is prepared, with a portion of the sciatic 

 nerve attached. A galvanic battery with an induction apparatus is also 

 provided, so that the closure of the circuit of the battery will produce 

 an instantaneous electric current in the induction coil. By this means 

 the stimulus of the induced current is first applied to the muscle itself, 

 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 themselves, and was found in the 

 experiments of Helmholtz to be about T ^ of a second. If the stimulus 

 be now applied to the nerve in immediate proximity to the muscle, the 

 above interval is not perceptibly altered. But if it be applied to the 

 nerve at a point one, two, or three centimetres distant, a decided retarda- 

 tion is manifested in the muscular contraction ; and this retardation 

 becomes greater as the length of the nerve, 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 depend upon the use 

 of an automatic registering apparatus, on the principle of that employed 

 by Marey. 2 In this apparatus, a card, with its surface blackened by 

 smoke, moves by clockwork, with uniform velocity, in a horizontal 

 direction. Upon this card the extremity of a diapason or tuning fork, 

 vibrating 500 times per second, traces an undulating line (Fig. 146, a) 

 which records the time occupied by the card in moving from one point 



1 Comptes Eendns de 1'Academie des Sciences. Paris, 1851. tome xxxiii. p. 262. 



2 Du Mouvemcnt duns les Fonctioris de la Vie. Paris, 1868, p. 422. 



