172 PHYSIOLOGY OF NERVE [CH. XV. 



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 and 

 thermal stimuli. 



Velocity of a Nerve Impulse. 



A nervous impulse is not electricity ; compared to that of electri- 

 city its rate of propagation is extremely slow. It has been measured 

 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. 122). 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. 



Another method, largely employed by Bernstein, is to take the 

 electrical change as the indication of the impulse. The rheotome is 

 the instrument used. If fig. 169 (p. 139) is referred to, and a long 

 nerve substituted for the muscle-nerve preparation, the stimulus is 

 applied at one end, and the change in the electrical condition of the 

 nerve is recorded by the galvanometer, which is connected to the 

 other end of the nerve. The time measurement is effected by the 

 adjustment of the rheotome, which must be such as to tap off the 

 electrical change at the moment it occurs, 



