48 



EXPERIMENTAL PHYSIOLOGY 



The interval is relatively small compared with the total latency period of 

 the muscle-nerve preparation. It can be rendered more evident if the nerve (not 

 the muscle) be cooled (p. 24). To measure 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 means of two tambours or by the pince myographique (p. 35). 



Effect of various agencies on nerve-conduction : blocking of 

 nerve by current ; carbonic acid ; ether vapour ; chloroform. Take 



a nerve-muscle preparation and lay the nerve across and partly 

 imbedded in a ring of putty or soft modelling clay placed upon a glass 

 slide, to which a tube is cemented so that a current of C0 2 can be 



FIG. 43. EFFECT OF OAEBON DIOXIDE ON CONDUCTION IN NERVE. 



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). 



Find the minimal stimulus which will produce contraction of the 

 muscle ; then pass a current of C0 2 over the intervening nerve, and 

 notice its effect in blocking the nerve-impulse. Remove the C0 2 by 

 a current of air, and repeat the observation. 



Other experiments may subsequently be made with ether vapour 

 and chloroform vapour instead of C0 2 . It will be found that ether 

 acts like C0 2 , but more powerfully. Chloroform vapour is more 

 powerful than ether ; and after a short exposure to it the nerve 

 does not recover its power of conduction on readmitting air ; it 

 has, in fact, been killed. 



Blocking by a galvanic current. The blocking effect of a constant current 

 has been made use of in Bernstein's experiment on fatigue (p. 43). To exhibit 



