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EXPERIMENTAL PHYSIOLOGY 



plate (fig. 116). Make a nerve muscle preparation — the sciatic nerve and 

 the whole of the leg below the knee — and place the cut end of the nerve on 

 one clay pad and the middle of the nerve on the other. Great care must be 

 taken of the nerve during its preparation. Place a little normal saline in 

 a watch glass and lift this up until both clay pads touch the fluid. At the 

 contact the leg muscles give a twitch, due to the closure of the current of 

 injury of the nerve. 



Fig. 116. — Kuhne's Expeeiment of Con- 

 traction without Metals. 



Fig. 117. — Arrangement of Apparatus 

 for showing Secondary Contraction. 



Experiment 4. — Secondary contraction. Dissect out two nerve-muscle 

 preparations and place them on a dry glass plate. Lift up the nerve of one, 

 b, fig. 117, and place it in a loop over the muscle of the other preparation, A. 

 Lay the nerve of a over a pair of electrodes, e, in connection with a Du Bois 

 key, k, in a secondary circuit. Close k and connect a battery to the primary 

 coil for tetanising currents. Open the key k and the preparation a passes 

 into tetanus. It is also found that b is thrown into tetanus. On closing K 

 the tetanus of b ceases. 



This experiment is of considerable importance on several accounts. 

 In the first place we can show by physiological means the existence 

 of a current of injury in the muscle of a, for if the nerve of b be 

 sensitive and it is laid across the muscle of a and then its cut end 

 dropped on to some other point of a, at the contact the muscle of b 

 contracts stimulated by the closure of the current of injury of A. In 

 the second place it directly demonstrates the ' negative variation ' or 

 current of action of A, for it is due to the production of this current 

 that the muscle b is stimulated when a is indirectly tetanised. In the 

 third place it teaches us one very important fact with regard to the 

 nature of the physical events occurring in A while it is being tetanised. 

 This results from observing that b passes into tetanus when a is 

 tetanised ; and as a muscle does not contract during the time that a 

 constant current is passing through it, but only on make and break, 

 it follows that the current of action of a must be intermittent. Hence 

 although the change of length of A may remain constant, one at least 

 of the physical factors accompanying that contraction is intermittent. 

 By other methods it has been shown that the rate of oscillation of the 

 current of action of A is the same as that of the excitation. 



