28 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Conditions which Determine the Effect of Excitation. 



The result of the irritation of nerve and muscle is dependent on two sets 

 of conditions — namely, conditions which determine the irritability; condi- 

 tions which determine the efficiency of the irritant. 



It will be necessary for us to study the second set of conditions first — for, 

 before we can judge of the irritability and the effect of various influences upon 

 it, we must consider how far the activity of the nerve and muscle is depend- 

 ent on the character, strength, and method of application of the irritant. 



Conditions ■which Determine the Efficiency of Irritants. — Some of 

 these conditions can be best studied on nerves, while others are more ap- 

 parent in their effects on muscles. The most useful irritant for purposes of 

 study is the electric current. Mechanical, thermal, and chemical irritants are 

 likely to injure the tissue, and are not manageable, whereas electricity, if not 

 too strong, can be applied again and again without producing any permanent 

 alteration, and can be accurately graded as to strength, place, time, duration 

 of application, etc. Of course, the results obtained by the use of a given 

 irritant cannot be accepted for others until verified. The conditions which 

 determine the effectiveness of the electric current as an irritant may be 

 classed as follows : (a) The rate at which the intensity changes, (b) The 

 strength of current, (c) The density of current. (<1) The duration of 

 application, (e) The angle of application. (/) The direction of flow. 



Irritating Effect of the Electric < 'urrent. — Luigi Galvani, Professor of 

 Physics at Bologna, 1791 (or, according to some, his wife Lucia), observed the 

 legs of frogs which had been prepared for the kitchen, and had been suspended 

 by brass hooks from an iron balcony, make convulsive movements every time 

 the wind blew them against the iron. He repeated the experiment in his 

 laboratory, and decided that the frogs had been excited to action by electric 

 currents developed within themselves ; he looked upon the metals which he 

 had used merely as conductors for this current. Volta, Professor of Natural 

 Philosophy at Pavia, repeated Galvani's experiment, and concluded that 

 there had been an electric current developed from the contact of the dissimilar 

 metals with the moist tissues of the frog. In accordance with this idea he con- 

 structed the voltaic pile, and this was the starting-point of the science of 

 electricity of to-day. 



Although it is true that, under certain conditions, differences in electric 

 potential sufficient to excite muscles to contraction can be developed in the 

 animal body, the contractions of the frog's leg which Galvani observed were 

 due to the metals which he employed. The experiment can be easily per- 

 formed by connecting a bit of zinc to a piece of curved copper wire, and bring- 

 ing the two ends of the arc against the moist nerve and muscle of a frog. A 

 stronger and more efficient shock can be obtained from a Daniell or some other 

 voltaic cell. 



A I><ii, i>ll cell (Fig. 5) is composed of a zinc and copper plate, the former dipping 

 into dilute sulphuric acid, the latter into a strong copper-sulphate solution. Although 

 gravity will keep these liquids separated, if the cell is to be moved about it is better 



