5 EXPERIMENTAL PHYSIOLOGY 



it accurately a longer nerve and faster rate of movement must be 

 taken. This is obtained by the use of the pendulum myograph, upon 

 which the contraction of the human thumb muscles is recorded. The 

 electrodes used consist of wash-leather pads soaked with strong salt 

 solution. One electrode, large and flat, is fixed against the skin of the 

 upper part of the back, the other smaller one being applied respectively 

 over the median nerve at the elbow and over the brachial plexus above 

 the clavicle; the length of nerve between these points is about 12 

 inches. The muscle-contraction in this case is recorded by tambours 

 or by the pince myographique (p. 37). 



Effect of various substances on nerve- conduction : carbonic acid ; 

 ether vapour ; chloroform vapour. Take a nerve-muscle preparation 

 and lay the nerve across and partly sunk in a ring of putty or 



^^ /TTjfifiW J 



CO^JJ 



FIG. 43. Effect of carbon dioxide on conduction in nerve. 



plasticine placed upon a glass slide, to which a tube is cemented so 

 that a current of air charged with gases or vapours can be conducted 

 over the nerve. A cover glass is placed upon the ring : the end of the 

 nerve projects beyond this and rests upon a pair of electrodes (Fig. 43). 

 The slide is fixed to the myograph cork by plasticine and the muscle 

 is attached to the cork and lever in the usual way. 



Find the minimal stimulus which will produce contraction of the 

 muscle ; then pass a current of CO 2 over the intervening nerve, and 

 notice its effect in blocking the nerve-impulse. Remove the CO 2 by a 

 current of air, and repeat the observation. 



Other experiments may be made with ether vapour and chloroform 

 vapour instead of CO 2 . It will be found that ether acts like C0 2 , but 

 more powerfully. Chloroform vapour is more powerful than ether ; 

 after a short exposure to it the nerve does not recover its power of 

 conduction on readmitting air ; it has, in fact, been killed. 



