62 



THE CONTRACTILE TISSUES. 



ary coil is developed with unimpeded rapidity. We shall see later on that a 

 rapidly developed current is more effective as a stimulus than is a more slowly. 



FIG. 14. 



Diagram of an Induction Coil : + positive pole, end of negative element ; negative pole, 

 end of positive element of battery ; K, Du Bois-Reymond's key ; pr. c. primary coil, current 

 shown by feathered arrow ; sc. c. secondary coil, current shown by unfeathered arrow. 



developed current. Hence the making shock, where rapidity of production is 

 interfered with by the self-induction of the primary coil, is less effective as a 

 stimulus than the breaking shock whose development is not thus interfered with. 



The strength of the induced current depends, on the one hand, on the strength 

 of the current passing through the primary coil that is, on the strength of the 

 battery. It also depends on the relative position of the two coils. Thus a second- 

 ary coil is brought nearer and nearer to the primary coil and made to overlap it 

 more and more ; the induced current becomes stronger and stronger, though the 

 current from the battery remains the same. With an ordinary battery, the sec- 

 ondary coil may be pushed to some distance away from the primary coil, and yet 

 shocks sufficient to stimulate a muscle will be obtained. For this purpose, how- 

 ever, the two coils should be in the same line ; when the secondary coil is placed 

 crosswise, at right angles to the primary, no induced current is developed, and at 

 intermediate angles the induced current has intermediate strengths. 



When the primary current is repeatedly and rapidly made and broken, the 

 secondary current being developed with each make and with each break, a rapidly 

 recurring series of alternating currents is developed in the secondary coil and 



holder are connected through binding screws in the floor of the moist chamber with the wires 

 x', y', and these are secured in the key, one on either side. To the same key are attached the 

 wires x", y", coming from the secondary coils s. c. of the induction coil D. This secondary coil 

 can be made to slide up and down over the primary coilpr. c., with which are connected the two 

 wires x'" and y'"; x'"is connected directly with one pole for instance, the copper pole c. p. of the 

 battery E; y'" is carried to a binding screw, a, of the Morse key F, and is continued as y'^ from 

 another binding screw, b, of the key to the zinc pole z. p. of the battery. 



Supposing everything to be arranged, and the battery charged ; on depressing the handle ha, 

 of the Morse key F, a current will be made in the primary coil pr. c., passing from c. p. through 

 x"' to pr. c., and thence through y" to a, thence to 6, and so through y"~ to z. p. On removing the 

 finger from the handle of F, a spring thrusts up the handle, and the primary circuit is in conse- 

 quence immediately broken. 



At the instant that the primary current is either made or broken, an induced current is for the 

 instant developed in the secondary coil s. c. If the cross-bar h in the Du Bois-Reymond's key be 

 raised (as shown in the thick line in the figure), the wires x", x', x, the nerve between the elec- 

 trodes and the wires y, y', y", form the complete secondary circuit, and the nerve consequently 

 experiences a making or breaking induction-shock whenever the primary current is made or 

 broken. If the cross-bar of the Du Bois-Reymond key be shut down, as in the dotted line h' in 

 the figure, the resistance of the cross-bar is so slight compared with that of the nerve and of the 

 wires going from the key to the nerve, that the whole secondary (induced) current passes from 

 x" to y" (or from y"io x") along the cross-bar, and practically none passes into the nerve. The 

 nerve, being thus " short-circuited," is not affected by any changes in the current. 



The figure is intended merely to illustrate the general method of studying muscular contraction ; 

 it is not to be supposed that the details here given are universally adopted, or, indeed, the best 

 for all purposes. 



