158 PHYSIOLOGY OF NERVE [CH. XIV 



control the nutrition of the fibres are situated within the cord for the 

 anterior roots, and within the spinal ganglia for the posterior roots. 



Changes in a Nerve during Activity. 



When a nerve is stimulated, the change produced in it is called a 

 nervous impulse ; this change travels along the nerve, and the pro- 

 pagation of some change is evident from the effects which follow : 

 sensation, movement, secretion, etc. ; but in the nerve itself very little 

 change can be detected. There is no change in form ; the most deli- 

 cate thermopiles have failed to detect any production of heat, and 

 we are almost completely ignorant of any chemical changes. The 

 only alteration which can be readily detected as evidence of this 

 molecular change in a nerve is the electrical one. Healthy nerve is 

 iso-electric, but during the passage of a nervous impulse along it 

 there is a very rapid diphasic variation, which travels at the same 

 rate as the nervous impulse. This is similar to the diphasic change 

 in muscle, and can be detected and measured in the same way. 



Waller regards the current of action of any excitable tissue as an index of the 

 magnitude of action, and records the movement of the galvanometer by photograph- 

 ing the excursion of the spot of light on a moving photographic plate. He has in 

 this way obtained records from muscle, nerve, retina, skin, plant tissues, etc. He 

 points out that the only available index of action within the nerve itself is this 

 electrical sign of activity, whereas in muscle the mechanical action can be compared 

 with its accompanying electrical changes. The amount of contraction in a muscle 

 caused by excitation of its nerve is only a very rough, or even a fallacious, indica- 

 tion of the excitability of the nerve, because the nerve is connected to the muscle by 

 motor end-plates, and these, as we have already seen, are fatigued long before the 

 nerve shows any sign of fatigue. 



Using this method, Waller has obtained a number of interesting results on the 

 variation in nerve action produced by drugs and other agents. He finds that the 

 effect of carbonic acid is to cause a diminution, and finally disappearance of the 

 galvanometric response ; when this gas is replaced by air the nerve recovers, and the 

 action-currents increase. Ether acts similarly ; but with chloroform recovery is 

 difficult to obtain. Small doses of carbonic acid increase the action-currents, and 

 Waller considers that the staircase effect in muscle (p. 101), and the similar progres- 

 sive increase noted in the action-currents of nerve as the result of repeated stimula- 

 tion are due to the evolution of this gas during activity. 



This hypothesis has been recently confirmed by Bseyer and Frohlich. They 

 have shown that peripheral nerves participate in respiratory exchanges, using up 

 oxygen and producing carbonic acid in minute but measurable amounts. In the 

 absence of oxygen, stimulation ceases after some hours to evoke the activity of a 

 nerve, but on readmission of the gas recovery is almost instantaneous. The store 

 of oxygen so obtained will again keep up nervous activity for a considerable 

 time even although no fresh oxygen is supplied. This illustrates the great power 

 nerve has in repairing itself and in storing oxygen. 



There can be no doubt that the existence of the electrical variation is as a rule 

 the index of the excitatory alteration in a nerve. But in the present state of our 

 knowledge we are not justified in assuming that it gives an absolutely faithful 

 record. The electrical variation can be detected in a nerve for many days after its 

 removal from the body ; although the electrical change is a concomitant of the 

 real excitatory process, the former may be therefore perceptible when other evidence 

 of the existence of the latter fails. Moreover, Gotch and Burch have obtained 

 further evidence of the dissociation of the electrical response from the excitatory 

 process. In the frog's sciatic nerve, it is possible with two stimuli in rapid sue- 



