GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 43 



before we can judge of the irritability and the effect of various innu,nces up 

 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, and dura- 

 tion 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. 



(6) The strength of current. 



(c) The density of current. 



(d) The duration of application. 



(e) The angle of application. 

 (/) The direction of flow. 



Irritating Effect of the Electric Current. Galvani, in seeking to find the 

 effect of atmospheric electricity upon the animal body, suspended frogs by 

 copper wires from an iron balcony, and observed the remarkable fact, that 

 when the wind blew the legs against the balcony the muscles of the frogs 

 twitched. He repeated the experiment in his laboratory, and concluded that 

 the frogs had been excited to action by electric currents developed within them- 

 selves ; he looked upon the metals which he had used merely as conductors for 

 this current. Volta, Professor of Natural Philosophy at Pavia, repeated Gal- 

 vani'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 constructed the voltaic pile, and 

 this was the starting-point of the electric science 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 Daniell 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. Alt! 

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

 to enclose one of them in a porous cup. A common form of cell consists of 

 in the middle of which is a porous cup ; outside the cup is the sulphuric acid 



