123 



PRACTICAL EXERCISES] 



RELATING TO THE PHYSIOLOGICAL PROPERTIES OF THE 



CONTRACTILE AND EXCITABLE 

 TISSUES. 



I. Modes of Excitation. 



Electrical Excitation. The requirements for the purpose are 

 Batteries (Grove's or Daniell's), an Induction coil, wires, two keys, and suit- 

 able Electrodes. The Induction apparatus used is that of Prof, du Bois-Rey- 

 mond. (See " Handbook," p. 351, fig. 298.) The key ordinarily used is also 

 that of du Bois-Reymond. The Electrodes are made as follows : Cement 

 with sealing-wax two copper wires, each about three inches long and pointed 

 at one end, into two pieces of glass tube two inches long, just large enough to 

 contain the wires. The ends of the wires must project about half an inch 

 from the glass tubes, and must be coated on all sides, excepting one, with 

 yealing-wax. Bind the two glass tubes together with strong thread and 

 solder fine copper wires to the blunt ends of the electrodes. 



I. Use Of th.6 Induction Coil. a. For single induction shocks. 

 Connect a Daniell cell by copper wires with the two upper screws which are 

 directly connected with the ends of the primary coil, interposing a key in the 

 circuit. Insert, the wires from the electrodes in the binding screws of the 

 secondary coil, and place the points of the electrodes against the tongue. 

 Withdraw the secondary coil from the primary and then gradually bring it 

 nearer, opening and closing the key after each approximation. The " break 

 shock" is first felt, and is throughout perceptibly stronger than the "make 

 shock." b. For faradization. Connect the battery wires with the screws at 

 the bases of the brass pillars (C and A in fig. '293). If the platinum-pointed 

 screw (/) is properly adjusted, the hammer begins to vibrate on closing the 

 key, and a series, consisting alternately of " make" and "break" shocks, is felt, 

 which, as the secondary coil begins to cover the primary, becomes unbearable. 

 For many purposes it is desirable to avoid the great disparity which in the 

 ordinary arrangement exists between the opening and closing shocks. This is 

 accomplished by a contrivance known as Helmholtz' modification (see " Hand- 

 book," fig. 294) : A side wire connects the outer pillar with the top screw of 



