20 PRACTICAL PHYSIOLOGY 



and fixes the lower end of the femur ; the other wire from the Da 

 Bois key is joined to a piece of capillary copper wire which has been 

 threaded by means of a needle through the tendo-Achillis. In this 

 way the current can be passed through the length of the muscle, and 

 the very fine wire will not cause any obstruction to the free movement 

 of the muscle when it contracts. Bring the writing point on to the 

 surface of the stationary drum. 



With the secondary coil at 20 cm. and the Du Bois key open, make 

 and break the primary circuit, no contraction will take place. Gradu- 

 ally move up the secondary coil towards the primary, opening and 

 closing the key in the primary circuit at each new position. With the 

 secondary coil at about 16 cm. the muscle will contract at break but 

 not at make, showing that the break induction shock is stronger than 

 the make-shock. The contraction is recorded on the drum by a nearly 

 vertical line, and shows a minimal contraction in response to a minimal 

 stimulus ; the make-induction shock is still a sub-minimal stimulus 

 and no contraction results. Rotate the drum on a short distance by 

 hand, move the secondary coil up 1 cm. and stimulate again. Repeat 

 this process, moving the drum on after each contraction and increasing 

 the strength of the stimulus after each make and break of the primary 

 circuit (Fig. 27). As the strength of the stimulus is increased the 

 contraction at break increases in height rapidly at first and then more 



FIG. 27. Heights of contraction of a muscle with different strengths of stimuli. 

 M marks the make and B the break of the primary circuit. The numbers refer to 

 the distances in cms. of the secondary from the primary coil. (A.P.B.) 



slowly until, with the secondary coil at about 7 cm., a point is reached 

 beyond which the height does not increase. At 7 cm., therefore, the 

 break-shock and the contraction which it causes are maximal. All stimuli 

 intermediate in strength between minimal and maximal are called 

 sub-maximal. At a certain point the make-shock will be found to 

 become an effective stimulus and cause a minimal contraction. As 

 the make-shock is increased in strength, the contraction rapidly 

 increases in height until, with the secondary coil at about 7 cm., 



