CH. XIV.] VELOCITY OF NERVE IMPULSES 159 



cession to obtain only one electrical response near the seat of excitation which has 

 been cooled, while two such responses occur in a more peripheral warmer region. 



Excitability and conductivity. It is necessary to distinguish between these two 

 properties of nerve. Changes in excitability, and in the power of conducting nerve 

 impulses, do not necessarily go together, as shown in the following experiment : 

 The nerve of a frog's leg is led through a glass tube, the ends of which are sealed 

 with clay, care being taken that the nerve is not compressed. The tube is provided 

 with an inlet and outlet, so that gases may be passed through it. Two pairs of 

 electrodes are arranged, so that the nerve can be stimulated either within or outside 

 the little gas chamber. If carbon dioxide or ether vapour is passed through the 

 tube, both excitability and conductivity are in time abolished, but excitability 

 disappears first ; at this stage, if the nerve is stimulated by an induction shock 

 inside the tube, the muscle does not respond, but on stimulating the nerve at the 

 end distant from the muscle and outside the tube, the muscle contracts. The nerve, 

 therefore, is not excitable, though it will conduct impulses. At a later stage shocks 

 administered by either pair of electrodes provoke no contraction. When the 

 poisonous vapour is replaced by air, the nerve recovers, and conductivity returns 

 before excitability. If alcohol vapour is used conductivity is stated to vanish before 

 excitability. 



Gotch has shown that cold applied to a nerve acts very much like carbonic 

 acid. Intense cold will cause disappearance of both excitability and conductivity ; 

 but cold of such a degree which abolishes the excitability of the nerve to induction 

 shocks, increases its excitability to the constant current, and also to mechanical 

 stimuli. 



Velocity of a Nerve Impulse. 



This may be measured, as was first done by Helmholtz, in motor 

 nerves as follows : a muscle-nerve preparation is made with as long 

 a nerve as possible ; the nerve is stimulated first as near to the 

 muscle, and then as far from the muscle, as possible. The moment 

 of stimulation and the moment of commencing contraction is 

 measured by taking muscle-tracings on a rapidly moving surface in 

 the usual way, with a time- tracing beneath. The contraction ensues 

 later, when the nerve is stimulated at a distance from the muscle, 

 than in the other case, and the difference in the two cases gives 

 the time occupied in the passage of the impulse along the piece of 

 nerve, the length of which can be easily measured. 



A similar experiment can be performed on man by means of the 

 transmission myograph (see p. 105). If a tracing of the contraction 

 of the thumb muscles is taken, the two stimuli may be successively 

 applied through the moistened skin, first at the brachial plexus below 

 the clavicle; and secondly, at the median nerve at the bend of 

 the elbow. 



The same method may be employed in man for determining the 

 rate of transmission in sensory nerves. A man is told to make a 

 given signal, such as to open a key in an electrical circuit, when he 

 receives a stimulus such as an induction shock applied to one of his 

 toes; the time between the excitation and the reply is easily 

 measured. A second experiment is then performed in the same way, 

 except that the stimulus is applied to another part of his body ; for 

 instance, his knee. The time interval is again measured, and found 



