94 THE HUMAN BODY 



character of its action, is that it remains in a condition of inac- 

 tivity except when in receipt of definite stimulation. A moment's 

 thought will show the importance of this property. The usefulness 

 of a muscle depends quite as much upon its ability to be inactive 

 when not wanted as upon its property of contracting when con- 

 traction is desired. The difficulties frequently experienced by 

 sufferers from chorea (St. Vitus Dance) illustrate this point suffi- 

 ciently. 



For the study of activity in isolated muscles some form of arti- 

 ficial stimulation must be employed. By far the most satisfac- 

 tory is an electric shock such as may be generated by a small in- 

 duction coil. By carrying fine wires from the terminals of the 

 coil to opposite ends of the muscle the latter becomes a part of the 

 circuit. Thus when the coil discharges a spark the muscle re- 

 ceives it and is stimulated. Besides being peculiarly effective as 

 stimuli induction shocks have the advantage of being easily modi- 

 fied in intensity, and, when not excessive, of having no injurious 

 effect on the tissue. 



A Simple Muscular Contraction. When a single electric shock 

 is sent through a muscle, it rapidly shortens and then rapidly 

 lengthens again. The whole series of phenomena from the mo- 

 ment of stimulation until the muscle regains its resting form is 

 known as a simple muscular contraction or a "twitch": it occupies 

 in frog's muscle about one-tenth of a second. So brief a move- 

 ment as this cannot be followed in its details by direct observa- 

 tion, but it is possible to record it and study its phases at leisure. 

 This may be done by firmly fixing the upper tendon of an isolated 

 muscle, M, Fig. 47, and attaching the other end at d to a lever, Z, 

 which can move about the fulcrum /: the end of the long arm of 

 the lever bears a point, p, which scratches on a smooth smoked 

 surface, S. Suppose the surface to be placed so that the writing 

 point of the lever is at a; if the muscle now contracts it will raise 

 the point of the lever, and a line ac will be drawn on the smoked 

 surface, its vertical height, cm, being dependent, first, on the ex- 

 tent 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/d, the more will the actual shortening of the muscle 

 be magnified. With the lever shown in the figure this magnifica- 

 tion would be about ten times, so that one-tenth of cm would be 



