PRACTICAL EXERCISES 781 



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 

 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. 278. 

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

 the drum, as there shown, but connect it as in Fig. 274. 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 back- 

 bone 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 incision 

 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 dissected out, as 

 follows : Remove the 

 skin from the thigh, and, . pjg. 274. Arrangement of Coil for Single Shocks. 

 holding the leg in the 



left hand slit up the fascia which connects the external and internal 

 groups of muscles on the back of the thigh. Complete the separa- 

 tion with the two thumbs. Cut through the iliac bone, taking 

 care that the blade of the scissors is well pressed against the bone, 

 otherwise there is danger of severing the sciatic plexus. Now divide 

 in the middle line the part of the spinal column which remains above 

 the urostyle A piece of bone is thus obtained by means of which the 

 nerve can be manipulated without injury. Seize this piece of bone with 

 the forceps, and carefully free the sciatic plexus and nerve from their 

 attachments right down to the gastrocnemius muscle, taking care not 

 to drag upon the nerve. The muscles of the thigh will contract, as the 

 branches going to them are cut. This is an instance of mechanical 

 stimulation. Now pass a thread under the tendo Achillis, tie it, and 

 divide the tendon below it. Strip up the tube of skin that covers the 

 gastrocnemius, as if the finger of a glove were being taken oft. Tear 

 through the loose connective tissue between the muscle and the bones 

 of the leg, and divide the latter with scissors just below the knee. Cut 

 across the thigh at its middle. 



Fix the preparation on the cork plate of the myograph by a pin passed 



