26 PEACTICAL PHYSIOLOGY 



to a deformation of the curve (Fig. 3$), in which the hump near 

 the top of the up stroke of the lever is caused by the muscle re- 

 sponding to the second stimulus (see Effect of two successive stimuli, 

 Chap. VII, p. 40). If with a slowly revolving drum it is desired to 

 send into the nerve a single stimulus, it is only necessary to place the 

 secondary coil at such a distance from the primary that the break 

 but not the make shock is effective. 



The curve (Fig. 32) occupies about yV^ths sec. and can be divided 

 into three parts. 



(1) The first part extends from the point at which the stimulus 

 reached the nerve to that at which the contracting muscle began to 

 raise the lever. This is the latent period, and is seen to last about 



yj^th of a sec. During this 

 period several distinct processes 

 take place ; (a) a nervous impulse 

 has to pass down the strip of 

 nerve between the point stimu- 

 lated and the muscle, this will 

 occupy about y^^ths of a second 

 (see Velocity of nervous impulse). 

 Of the remaining T 8,-ths (b) the 



FIG. 33. Contraction of the same preparation . v ' 



as in Fig. 32, recorded on a drum revolving at passage of the nerVOUS impulse 

 a slower rate. The hump near the top of the 



upstroke of the lever represents a second con- . along the fine motor 



traction in response to the break shock. Time n . . 



marker, 100 per sec. (A.P.B.) endings OCCUplCS about 



sec., and (c) the latent period of the muscle itself about y^^-ths of a 

 sec. This in turn is due to several factors, of which two must be 

 mentioned. When muscle fibres begin to contract a certain time must 

 elapse before the muscle is able to exert a sufficient pull to move the 

 recording lever ; in other words, there is instrumental inertia to be 

 overcome. Again, when muscle, which is highly extensible, begins 

 to contract, every part of every fibre does not simultaneously begin to 

 shorten ; but the contracted part of a fibre stretches at first the 

 uncontracted part, and is therefore not united to the lever by a rigid 

 connection. It is only when the tension in the stretched part has 

 sufficiently increased, or the fibre as a whole has passed into a state of 

 contraction, that the lever begins to be pulled upon. 



(2) The second period extends from the point at which the lever 

 begins to rise to the point highest above the base-line. This is the 

 period of active contraction or shortening of the muscle, and occupies 

 about y^ths of a sec. 



(3) The third portion extends from the highest point of the curve to 

 the point at which the curve rejoins the base-line. This is the period 



