6i 4 A MANUAL OF PHYSIOLOGY 



PRACTICAL EXERCISES ON CHAPTERS IX. AND X. 



i. Difference of Make and Break Shocks from an Induction 

 Machine. Connect a Daniell or other cell B (p. 173) with the two 

 upper binding-screws of the primary coil P, and interpose a spring 

 key K in the circuit. Connect a pair of electrodes with the binding- 

 screws of the secondary coil (Fig. 189). 



Electrodes can be very simply made by pushing copper wires 

 through two glass tubes, filling the ends of the tubes with sealing- 

 wax, and binding them together with waxed thread. The projecting 

 points may be filed, and the nerve laid directly on them, or they 

 may be tipped with small pieces of platinum wire soldered on. 



(a) Push the secondary away from the primary, until no shock can 

 be felt on the tongue when the current from the battery is made or 

 broken with the key. Then bring the secondary gradually up towards 



FIG. 189. ARRANGEMENT OF COIL FOR SINGLE SHOCKS. 



the primary, testing at every new position whether the shock is per- 

 ceptible. It will be felt first at break. If the secondary is pushed 

 still further up, a shock will be felt both at make and at break. 

 From this we learn that for sensory nerves the break shock is 

 stronger than the make. The same can easily be demonstrated for 

 motor nerves and for muscle. 



(b) Smoke a drum and arrange a myograph, as shown in Fig. 192. 

 But omit the brass piece F, and do not connect the primary through 

 the drum, as there shown, but connect it as in Fig. 189. Pith a frog 

 (brain and cord), and make a muscle-nerve preparation. 



To make a Muscle-Nerve Preparation. Hold the frog by the hind- 

 legs back upwards; the front part of the body will hang down, making 

 an angle with the posterior portion. With strong scissors divide the 

 backbone anterior to this angle, and cut away all the front portion of 

 the body, which will fall down of its own weight. Make a circular in- 

 cision at the level of the tendo Achillis, and another at the lower end 

 of the femur, through the skin. The sciatic nerve must now be dis- 



