THE HUMAN BODY. 



smoked surface, its vertical height, cm, being dependent, first, 

 on the extent of the shortening of the muscle, and second, on 

 the proportion between the long and short arms of the lever: 

 the longer fp is as compared with fd, the more will the actual 

 shortening of the muscle be magnified. With the lever shown 

 in the figure this magnification would be about ten times, so 

 that one tenth of cm would be the extent of the shortening 

 of the muscle. Suppose, next, the smoked surface to be moved 

 to such position that the writing point of the lever touches it 

 at i, 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 8 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 8 was moving while that line was being traced: 

 for example, if we know the rate of movement 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 -3^ of a second; and each tenth of io correspond 

 to YJ-Q- 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 8, a nearly vertical, due to the shorten- 

 ing of the muscle; the resulting line is a curve known 

 as the curve of a simple muscular contraction. Let the 

 surface 8 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 con- 

 tract at the very instant of stimulation, but at an appreciably 

 later time, and during the interval the lever draws the hori- 

 zontal line tu\ this period, occupied by preparatory changes 

 within the muscle, is known as the period of latent excitation. 

 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 



