vin CONDUCTIVITY AND EXCITABILITY OF NERVE 59 



sections of the fibres is in striking disproportion with the cross- 

 section of the trunk. This is evident on comparing the incon- 

 siderable axis-cylinder of the single fibre in the electrical nerves of 

 Malapterurus with the area covered by the sections of its innumer- 

 able ramifications (Fritsch reckons the increase at 346,760 

 times) : examination of any muscle that is rich in bifurcating 

 nerve-fibres proves that the section of the trunk is far exceeded 

 by the sum of the sections in the branches nearest to it. This 

 is obviously not derived from increase of medullary sheath, and 

 must be due to the axis-cylinders, so that two possibilities only 

 remain in support of Schultze's hypothesis ; the fibrils must either 

 become thicker towards the periphery, or diminish in number at 

 the cost of the stroma neither of which can be demonstrated 

 (Kiihne). 



The study of nervous excitation is much complicated by the 

 fact that the excitatory process is not associated with any directly 

 perceptible alterations within the nerve. We are everywhere 

 thrown back upon the effects at its peripheral end, foremost 

 among which, as a delicate indicator of the changes taking place 

 in the nerve, is muscular contraction. Muscle striated muscle in 

 particular is the surest index of nerve-excitation, and we owe 

 nearly all our knowledge of the physiological properties of peri- 

 pheral nerve-fibres to experiments on motor nerves. On stimu- 

 lating any motor nerve there is a strikingly rapid reaction from 

 the muscle (whatever the distance of the excited point), without 

 any perceptible interval between moment of stimulation and 

 commencement of contraction, no matter what point of the nerve 

 is excited. This formerly led to very exaggerated statements of 

 the rate at which these alterations in the nerve were conducted, 

 and it was held to be incalculable. 



Helmholtz (13) was the first who succeeded in measuring the 

 rate of conductivity in nerve, by means (in the first instance) of 

 Pouillet's method of time-measurement (Fig. 161), in which a 

 battery current is closed at P by a switch when C is opened, at 

 the moment of excitation, and broken again at B when the muscle 

 begins to contract. During the short interval between P and B 



O O 



the current passes through the galvanometer G, and causes a 

 perceptible deflection of the magnet proportional with the dura- 

 tion of closure. If two points of the nerve are stimulated, one 

 remote from the muscle (a), the other as near it as possible (&), 



