86 HUMAN PHYSIOLOGY 



6. The nature of the " impulse " which passes along a nerve 

 is due to changes in the axis cylinder, since this without 

 its sheath can conduct impulses. Secondly, it is dependent 

 on the vitality of the nerve. Death of the nerve at once stops 

 the transmission of an impulse. 



We may at once dismiss the idea that the impulse is due 

 to a flow of electricity. Electricity travels along a nerve 

 with a much higher velocity than the nerve impulse. 



Two possibilities remain. The impulse may be of the 

 nature of a molecular vibration, such as occurs in a stetho- 

 scope which conducts sound vibration, or it may consist of 

 a series of chemical changes such as cause the activity of 

 protoplasm generally. 



In considering this matter it must be remembered that 

 the amount of energy evolved in a nerve impulse need not 

 be great. All it has to do is to start the activity of the part 

 to which it goes. Hence if chemical changes are the basis 

 of the impulse, these may be extremely small in amount and 

 difficult to detect, while at the same time recuperation may 

 be extremely active. 



As a matter of fact, the evidence of chemical changes in 

 nerve fibres is entirely wanting. No change in reaction, 

 no heat production, and no phenomena of fatigue can be 

 demonstrated. 



7. The great function of the cell is to preside over the 

 nutrition of the neuron. If any part of the neuron is cut off 

 from its connection with the cell, it dies and degenerates. 

 In the white nerve fibres this degeneration begins in the 

 white sheath, which breaks down into globules, and under- 

 goes chemical changes, so that it is more readily stained with 

 osmic acid. This is taken advantage of in Marchi's method 

 for tracing degenerated fibres. The change extends through- 

 out the whole extent of the nerve at the same time. The axis 

 cylinder next breaks up, and the nerve corpuscles proliferate 

 and increase, and absorb the remains of the white sheath and 

 of the axis cylinder, so that after about twenty days nothing 

 is left of the nerve but the primitive sheath, filled with the 

 nucleated protoplasts. According to some observers, after a 

 time these nucleated structures begin to throw out processes 

 along the course of the degenerated fibre, and these processes, 



