122 INSTRUMENTS. [CH. X. 



To keep the preparation fresh during an experiment, it should 

 be covered with a glass shade, the air of which is kept moist by 

 means of wet blotting paper. A somewhat elaborate form of 

 moist chamber is shown in fig. 144. 



The last piece of apparatus necessary is a time-marker, so that 

 the events recorded in the myogram can be timed. The simplest 

 time-marker is a tuning-fork vibrating 100 times a second. This 

 is struck, and by means of a writing-point fixed on to one of 

 the prongs of the fork, these vibrations may be written beneath 

 the myogram. More elaborate forms of electrical time-markers 

 or^chronographs are frequently employed. 



The Simple Muscle Curve. 



We can now pass on to results, and study first the result 

 of a single induction shock upon a muscle-nerve preparation. 



Fig. 145. Simple muscle curve. (M. Foster.) 



A single momentary stimulation causes a single or simple mus- 

 cular contraction, or as it is often called a twitch. The graphic 

 record of such a contraction is called the simple muscle curve. 

 One of these is shown in the preceding figure (fig. 145). 



The upper line (m) represents the curve traced by the end of 

 the lever in connection with a muscle after stimulation of the 

 muscle by a single induction-shock : the middle-line (I) is that 

 described by a lever, which indicates by a sudden drop the exact 

 instant at which the induction-shock is given. The lower wavy 

 line (t) is traced by a tuning-fork vibrating 200 times a second, 

 and serves to measure precisely the time occupied in each part of 

 the contraction. 



