24 EXPERIMENTAL PHYSIOLOGY 



round the uppermost end of the length of nerve displayed, and the 

 nerve may be cut across above the thread. Then, holding it up 

 gently by the thread and passing a pair of straight scissors below and 

 parallel to the nerve, its branches to the thigh muscles are successively 

 severed, and the nerve is separated as far as the back of the knee- 

 joint. Notice that as each branch is snipped the muscles which it 

 supplies contract (mechanical stimulation). Lay the nerve thus 

 isolated upon the gastrocnemius muscle. Then cut through the middle 

 of the femur, and clear the attachments of the thigh muscles away 

 from the lower end of that bone. You now have a preparation 

 consisting of the knee-joint with portions of the femur and tibia, the 

 gastrocnemius muscle, and the sciatic nerve; this is termed a 

 muscle-nerve preparation. 



For many experiments a longer nerve is needed. To obtain this 

 the sciatic nerve is to be exposed as before from behind, but not tied 

 or cut ; it should indeed not be touched in its course by any instru- 

 ment. Then seize the urostyle with forceps, and cut it and the 

 muscles attached to it entirely away with scissors. The nerves 

 previously seen behind the kidneys are now exposed from the back ; 

 they are continuous on each side with the corresponding sciatic nerve. 

 To isolate the nerve along its whole length, sever the attachment of 

 the ilium to the sacrum. Split the end of the spinal column longi- 

 tudinally with scissors, and, holding one half with forceps, lift it up 

 obliquely, but not at too sharp an angle, along with the nerves issuing 

 from it to form the sciatic. Gradually dissect out the nerve from 

 above down as before described, snipping the lateral branches with 

 scissors (without touching the main nerve) until the back of the knee 

 is reached. Lay the long piece of nerve thus isolated upon the 

 gastrocnemius muscle, and, as before, cut through the middle of the 

 femur, clearing the attachments of the thigh-muscles away from 

 its lower end ; you now have a nerve-muscle preparation with long 

 nerve. Place a piece of paper impregnated with paraffin on the frog- 

 cork, and lay the nerve out clear of the muscle, fixing a pair of 

 electrodes so that the nerve lies across them. Keep both muscle and 

 nerve but especially the latter wet with normal salt solution or 

 Ringer's solution. 



With either of these nerve-muscle preparations the following experi- 

 ments, which are, for the most part, similar to those already performed 

 upon the tongue, are to be made. Note down all your results. 



Excitation by galvanic current. Determine that making or 

 breaking the circuit of a cell or battery is a stimulus to the nerve, 

 whereas the passage of the current usually x produces no obvious 

 1 For exceptions see Chapter XI, 



