PRACTICAL EXERCISES 703 



towards the primary, testing at every new position whether the shock 

 is perceptible. 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. 261. 

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

 the drum, as there shown, but connect it as in Fig. 258. 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 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, holding the leg in the left hand, slit up the fascia which 



FIG. 258. ARRANGEMENT OF COIL FOR SINGLE SHOCKS. 



connects the external and internal groups of muscles on the back 

 of the thigh. Complete the separation 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 arc 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 off. 

 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. 



