97 



of Contraction. The curve is at first convex to the abscissa, 

 or base line, which means that the rate of contraction is at 

 first very slow as seen by the acute angle which the first part 

 of the curve makes with the abscissa ; it then rapidly increases 

 as shown by the increasing- inclination to the abscissa, and very 

 soon reaches a maximum rapidity. From this, again, there is 

 another change in rate, this time in the reverse direction, for 

 the curve becomes concave to the abscissa line and gradually 

 shortening becomes slower until at last it ceases, when the 

 tangent to the curve becomes parallel to the abscissa. 



c , ordmatc 



Fig. 24 



Fig. 24, ah, latent period ; he, period of contraction ; cd, period of relax- 

 ation. 



(3). The third portion of the tracing is from the highest 

 point (c) to the point (d) at which the lever again reaches 

 the abscissa. This part is called the Period of Relaxation. 

 The terminal point (d) is often a difficult one to determine 

 with any accuracy because the lever does not come instantly 

 to rest ; but as it always possesses some inertia, it oscillates 

 for a time about a mean position which it ultimately reaches. 



340. Arrange a nerve-muscle preparation in the moist 

 chamber so as to record its contraction upon the drum. Con- 

 nect the battery with the induction coil introducing into the 

 primary circuit a make and break key and an electro-magnet. 

 Use also a short-circuiting key in the secondary circuit. Ar- 

 range the writing tip of the lever from the electro-magnet, so 

 that it will write just below and on the same vertical plane as 

 the muscle lever. Spin the drum at a fairly rapid rate by 

 hand and use single induction shocks by breaking the primary 

 circuit. The lever of the electro-magnet will indicate the 

 instant the current is sent into the nerve-muscle preparation. 

 (The lever rises and falls alternately as the current is made 



