44 EXPERIMENTAL PHYSIOLOGY 



and '065 sec. respectively. Two things are happening in these 

 later twitches which account for this difference. In the first place, 

 the muscle is taking in any ' slack ' there may be. Secondly, and 



Fig. 47. — Twitches Taken under the Principle of After-loading. 



more important, it is gradually increasing its tension until it is able 

 to lift the load. The first part of such a twitch is therefore isometric ; 

 but beyond a certain point it suddenly becomes isotonic, and its 

 shortening is then registered. 



ALTERATIONS IN THE SIMPLE TWITCH BROUGHT 

 ABOUT BY VARIOUS CONDITIONS 



I. The Influence of Temperature. — The differences in a simple 

 twitch, brought about when the same muscle is at different tem- 

 peratures, may be studied in many ways. If we are recording the 

 twitches by a crank lever the muscle may be laid upon a metal base 

 arranged so that it can be heated. Thus, in one form, the base is 

 hollow so that water at different temperatures can be circulated 

 through it. In another a stout metal wire is soldered to it, which is 

 immersed in water at different temperatures, and so its temperature 

 raised or lowered as required. In another form the muscle is sus- 

 pended in a small moist air chamber made of hollow metal walls 

 through which water is circulated. The chamber is completely closed 

 except by a small orifice at the bottom through which the thread 

 passes, attaching the lower end of the muscle to the recording 

 lever. One of the most convenient forms is shown in fig. 48. This 

 is to be employed for the purpose in the following experiment : — 



Experiment 4. — Arrange the apparatus as for taking a simple twitch. 

 Fit a cork c (fig. 48) tightly on to the lower end of the metal L- piece a b c. 

 A weight w is hung round the little pulley d of the recording lever, so that 

 it rotates the lever upwards. A muscle preparation is then made and its 

 lower end fixed firmly by a pin to the upper edge of the cork c. The best 

 preparation for the purpose is a hyoglossus or a sartorius, but a gastrocne- 

 mius may also be used. The upper end of the muscle is then fixed by a fine 



