214 



PHYSIOLOGY 



that the make or break current will be uniform so long as the distance of the 

 coils remains constant. We are not able however to say by how much the 

 current will increase as the secondary coil is moved, say, from 11 to 10 cms. 

 distant from the primary coil. If it is required to know the exact increment 

 in the exciting current which is used, it is necessary to graduate the induction- 

 coil by sending the induction shocks, obtained at different distances of secondary 

 from primary coil, through a ballistic galvanometer. 



Another method which may be adopted for the excitation of muscle or 

 nerve is the discharge of a condenser. The advantage of this method is that 

 we can determine not only the amount of electricity discharged through the 

 preparation, but the actual energy employed. If two plates of metal separated 

 from one another by a thin insulating layer of dielectric such as air, glass, mica, 

 or paraffined paper, be connected with the two poles of a battery, each plate 



acquires the potential of the pole of the battery 

 with which it is connected, and receives therefrom 

 a charge of electricity (positive or negative). If 

 the connections be broken the two plates retain 

 their charge. If now they be connected by a wire 

 they discharge through the wire, and if a nerve be 

 inserted in the course of the wire, it may be excited 

 by the discharge. 



The amount of electricity, that may be stored 

 up in this way, will depend on the extent of the 

 plates and their proximity to one another, as well 

 as on the E.M.F. of the charging battery. In order 

 to get great extent of surface, a condenser is built 

 up, as in the diagram (Fig. 49), of a very large 

 FIG. 49. Diagram to show num ber of plates of tinfoil, separated by discs of 

 the mode of construction of mica or paraffined paper. Alternate discs are 

 a condenser. connected together : thus 1, 3, 5 are connected to 



one pole, while 2, 4, 6 are connected to the other. 



The rheocord is used to modify the amount or strength of current flowing 

 through a preparation. One form of it is represented in Fig. 50. A constant 

 source of current at B causes a flow of electricity from a to b through a straight 

 wire. As the resistance of this wire is the same throughout its length, the 

 fall of potential from a to b must be constant. The nerve, or whatever pre- 

 paration that is used, is connected with the straight wire at two points, at a 

 and at c, by means of a sliding contact or rider. Supposing that there is an 

 electromotive difference of one volt between a and b, it is evident that if c is 

 pushed close to b, the E.M.F. acting on the nerve will be also one volt. The 

 E.M.F., however, may be made as small as we like by sliding c nearer to a. 

 Thus if ab is one metre, and there is a difference of one volt between the two 

 ends, then if c be one centimetre from a, the E.M.F. acting on the nerve will be 

 Ttnr v0 ^- Thus we alter the current passing through the nerve by altering the 

 E.M.F. which drives the current. 



If a weak current from a Daniell's cell (or any other form of 

 battery) be passed through a muscle or any part of its nerve, at the 

 make of the current the muscle gives a single sharp contraction 

 a muscle-twitch. In this contraction the whole of the muscle fibres 

 may be involved. During the passage of the current no effect is 

 apparently produced and the muscle seems to be quiescent, though 

 on careful observation we may see that there is a state of continued 



