294 PKACTICAL PHYSIOLOGY 



CHAPTER IV. 



COMPARISON OF THE SHORTENING IN A SINGLE CONTRACTION 

 AND IN TETANUS. 



AT first sight there seems to be a great difference between the form and 

 height of a tetanus curve and of a single twitch in response to a 

 maximal stimulus ; since the record of a voluntary muscular contraction 

 closely resembles a tetanus curve in form and height, it would seem to 

 follow that a voluntary contraction must be of the nature of a tetanus 

 and cannot be a single contraction. This difference between the height 

 of a single contraction and of a tetanus curve is, however, more apparent 

 than real, and is at any rate not a fundamental distinction between 

 the two. 



In order to investigate this point, arrange the apparatus for stimu- 

 lating a gastrocnemius-sciatic preparation with single maximal induction- 



Fio. 175. Comparison of the shortening of a gastrocnemius during a single 

 isotonic contraction S, during complete tetanus T, and during a series of pro- 

 gressively after-loaded contractions S to S 6 . (A.P.B.) 



shocks, using a simple key in the primary circuit, and a myograph-lever 

 provided with an after-loading screw. Place a weight of 20 grms. near 

 the axis of the lever, bring the writing point on to a stationary drum and 

 record a base-line by rotating the drum by hand. ^ Stimulate the 

 muscle with a single maximal shock. Rotate the drum on a short 

 distance and stimulate the muscle by a tetanising current for a second 

 or two ; a contraction of about double the height of the single contrac- 

 tion will be recorded. Rotate the drum again and repeat the single 

 stimulus ; the contraction will be of its former height. Now raise the 

 after-loading screw until the writing point is just at the top of the 



