84 VETEKINAKY PHYSIOLOGY 



offers a resistance, and this resistance varies with the condition 

 of the neurons involved, possibly with the condition of the 

 dendrites which form the synapses. 



If the toe of a frog deprived of its brain is pinched, the leg 

 is drawn up ; but if a dose of strychnine is first administered, 

 even touching the toes causes a violent spasm of every muscle in 

 the body. The resistance is decreased. If, on the other hand, 

 a dose of bromide of potassium has been administered, or if 

 ice be put on the back of the animal, much more powerful 

 stimulation is required to produce any reaction. The resistance 

 is increased. The activity of the central synapses may be 

 increased or diminished in various ways, and hence it is never 

 easy to predicate the ultimate result of any stimulation of the 

 nervous system. But, other things being equal, the strength 

 of stimulus applied to the first neuron that is, the extent of 

 excitation directly affects the extent of the resulting action. 

 (Practical Physiology.} 



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. Further, it is dependent on the vitality of 

 the nerve. Death of tbe nerve, as when it is heated to 47 C., 

 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 at about 

 300 million metres per second, a velocity much higher than that 

 of 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, 



