

PRACTICAL EXERCISES 615 



sected out, a.s follows : Remove the skin from the thigh, and, 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 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 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 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. 



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

 passed through the cartilaginous lower end of the femur, and attach 

 the thread to the upright arm of the lever by one of the holes in it. 

 Hang not far from the axis by means of a hook a small leaden weight 

 (5 to 10 grammes) on the arm of the lever which carries the writing- 

 point, and move the myograph plate or the muscle-nerve preparation 

 until this arm is just horizontal. Fasten the electrodes from the 

 secondary coil on the cork plate with an indiarubber band ; lay the 

 nerve on them ; and cover both muscle and nerve with an arch of 

 blotting-paper moistened with normal saline, taking care that the 

 blotting-paper does not touch the thread. Adjust the writing-point 

 to the drum. Begin with such a distance between the coils that a 

 break contraction is just obtained on opening the key in the primary 

 circuit, but no make contraction. The lever will trace a vertical line 

 on the stationary drum. Read off on the scale of the induction 

 machine the distance between the coils, and mark this on the drum. 

 Now allow the drum to move a little, still keeping the writing-point 

 in contact with it; then push up the secondary coil i centimetre 

 nearer the primary, and close the key. If there is a contraction, let 

 the drum move a little before opening the key again, so that the lines 

 corresponding to make and break may be separated from each other. 

 If there is still no contraction at make, go on moving the secondary 

 up, a centimetre (or less) at a time, till a make contraction appears. 

 When the coils are still further approximated, the make may become 

 equal in height to the break contraction, both being maximal, i.e., as 

 great as the muscle can give with any single shock (Fig. 190). 



(c) Attach a thin insulated copper wire to each terminal of the 

 secondary. Loop the bared end of one of the wires through the 

 tendo Achillis, and coil the other round the pin in the femur, so 

 that the shocks will pass through the whole length of the muscle. 

 Repeat the experiment of (), with direct stimulation of the muscle. 



