92 THE HUMAN BODY 



to such position that the writing point of the lever touches it at t, 

 and, the muscle being left at rest, the surface to be moved evenly 

 from left to right; the horizontal line io would then be traced, its 

 length depending on the distance through which S moved during 

 the time the lever was marking on it: and it is clear that if S move 

 uniformly, and we know its rate of movement, we can very readily 

 calculate from the length of io how long S was moving while that 

 line was being traced: for example, if we know the rate of move- 

 ment to be ten inches per second, and on measurement find io to 

 be an inch long, the time during which the surface was moving 

 must have been ^ of a second ; and each tenth of io correspond to 

 1^5 of a second. 



If we set the recording surface in motion and while the lever 

 point is tracing a horizontal line cause the muscle to contract, the 

 point will be raised as long as the muscle is contracted, and the 

 line drawn by it will be due to a combination of two simultaneous 

 movements a horizontal, due to the motion of S, a nearly verti- 

 cal, due to the shortening of the muscle; the resulting line is a 

 curve known as the curve of a simple muscular contraction. Let the 

 surface S be placed so that the writing point is at q and then be set 

 in uniform motion from left to right at the same rate as before 

 (ten inches per second). When the point is opposite t, stimulate 

 the muscle by an electric shock; the result, until the muscle has 

 fully lengthened again, will be the curve tuvwxy, from which many 

 things may be learned. In the first place we see that the muscle 

 does not commence to contract at the very instant of stimulation, 

 but at an appreciably later time, and during the interval the lever 

 draws the horizontal line tu; this period, occupied by preparatory 

 changes within the muscle, is known as the latent period. Then the 

 muscle begins to shorten and the lever to rise, at first slowly from 

 u to v, then more rapidly, and again more slowly until the summit 

 of the contraction is reached at w. The muscle does not now in- 

 stantly relax, but only gradually passes back to the resting state: 

 beginning at w, we see the descent of the curve is for a time slow, 

 then more rapid, and finally slow again from x to y, when the con- 

 traction is completed and the lever once more traces only the 

 horizontal line yp, due to the continued movement of the record- 

 ing surface. The curve then shows three distinct phases in the 

 contraction : the latent period ; the period of shortening ; the period 



