156 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



and at the end of from five to ten minutes may be even stronger than it was 

 at the start. Chloroform, likewise, inhibits the activity of the nerve; but it 



is even more vigorous in its action and the nerve is Less likely to recover. 

 The vapor of ethyl alcohol, after a preliminary exhilarating efl'eet, paralyzes 

 the nerve ; under favorable conditions the nerve recovers in a few hours. 

 The action of C0 2 is particularly interesting, since this is one of the normal 

 waste products of the body. In small doses it is found to increase the 

 strength of the current of action, while in large doses it has an anaesthetic 

 effect. The nerve is so sensitive to 0O 2 that even a fiftieth of a milligram 

 suffices to change its irritability : the amount that is contained in the expired 

 air, 4 per cent., suffices to do away with the current of action in three 

 minutes. 



In general, whatever increases the irritability of the nerve increases the 

 negative variation currents which result from its excitation. For instance, 

 if a nerve be excited at regular short intervals with induction shocks of equal 

 strength, there is a staircase-like increase in the negative variation current; 

 moreover, if it be subjected for a short period to tetanic excitation, the cur- 

 rent of action called out by a single shock is found to be increased. 1 Indeed 

 if one photographically records the movements of the galvanometer magnet 

 in such experiments, he obtains a curve closely resembling that obtained by 

 records of muscular contractions when the muscle is so excited (see Fig. 57). 

 Waller suggests that the effect of excitation to increase the irritability of the 

 nerve may be due to the production of C0 2 by the nerve, and a consequent 

 internal stimulation. The fact that the nerve does not fatigue he would 

 explain as the result of rapid restoration of the protoplasm and a rapid 

 neutralization of waste products. 



Apparently the strength of the current of action can be taken as a fair 

 index of the activity of the nerve, and consequently of the strength of the 

 nerve impulse. This view corresponds with the fact that there is a close 

 relation between the strength of the current of action of the muscle and the 

 height of the contraction. Waller 2 and Green 3 found experimentally that a 

 current of action can be detected with difficulty with subminimal irritants; 

 but as the strength of the current is raised above the threshold intensity the 

 Strength of the electrical change increases proportionally to the strength of 

 the excitation, until a point is reached which is far beyond what is needed 

 to excite maximal muscular contractions. After this the increase is less and 

 less, until finally a maximal current of action is reached. This occurs with 

 a strength of excitation much stronger than is required to call out a maximal 

 muscular contraction, and probably beyond the limit of functional action. 

 It is doubtful whether the nerve-cell could excite such a condition in a 

 nerve. 



On account of the great resistance of the nerve, and the short-circuiting 



1 Waller: Lectures <>n .\>iim<il Electricity, 1897, p. 59. 



2 Waller: Brain, xvii. p. 200. 



3 Green : American Journal of Physiology, 1898, p. 104. 



