MUSCLE. 21 



change of form and physical condition, (2) an electrical change, 

 (3) chemical changes, and (4) evolution of heat. 



Changes in Form and Physical Condition. These may be studied 

 in the muscle-nerve preparation of a frog by fixing the upper attach- 

 ment of the gastrocnemius muscle and attaching the tendo Achillis by 

 a thread to the short arm of a lever ; the long arm carries a writing 

 point, and a weight can be attached to it if so desired. The muscle, 

 when it contracts, pulls on the lever, and the movement of the writing 

 point can be recorded on a smoked moving drum. 



When a single induction shock of suitable strength is applied to the 

 muscle, it responds by a single contraction or twitch. A graphic record 

 of such a contraction is shown in fig. 2. 



FIG. 2. Simple muscle twitch. 



The vertical line marks the moment at which the stimulus was applied. Below 

 is a time-record, each double vibration representing T J second. 



The contraction lasts about one-tenth of a second, and the tracing 

 shows three parts, namely, (1) a short latent period, during which the 

 muscle shows no visible change : (2) a period of shortening of the muscle, 

 whereby the lever is raised ; and (3) a period of relaxation, during which 

 the muscle returns to its former length. The length of the latent 

 period is due partly to the inertia of the lever and recording apparatus. 

 This source of error can be avoided by interposing a muscle between a 

 source of light and a rapidly moving photographic plate ; the thickening 

 of the muscle, when it contracts, is photographed, and the interval 

 between the application of the stimulus and the beginning of contrac- 

 tion is measured. This interval, which is only 0'0025 second, repre- 

 sents the true latent period. The relaxation is not an active process, 

 but is due to the weight of the lever pulling the muscle back to its 

 former length : if an isolated muscle lying on mercury, and there- 

 fore not subject to any tension, is made to contract, it relaxes very 

 imperfectly. 



