103 



Place a fresh paper on the drum and let it revolve slowly. 

 Adjust a flame under the arm of the muscle warmer and 

 stimulate the muscle with a maximal break current at inter- 

 vals of 5°. Indicate the temperature upon the curves as 

 before. 



"The heig-ht of contraction is least at the freezing point 

 of the muscle ( — 5 degrees). It rises from the freezing- point 

 to zero ; falls from zero to 19 degrees ; increases to 30 

 degrees, which is the maximum ; from 30 to 45 degrees dimin- 

 ishes again, and at 45 degrees the frog's muscle usually enters 

 into a state called rigor caloris ; the muscle becomes opaque, 

 inelastic, resistant to the touch, shortens very considerably 

 and undergoes chemical changes of great importance. The 

 duration of contraction lessens with the rising temperature, 

 being least at 30 degrees. Above 30 degrees the duration 

 remains practically unchanged. The latent period is in- 

 creased at low temperatures, diminished at high. Above 30 

 degrees the excitability to electrical stimuli diminishes 

 steadily ; it disappears almost entirely before rigor is 

 reached . " — Porter. 



348. — The Influence of Load Upon the Contrac- 

 tion OF Muscle. A muscle to which a load is suspended is 

 said to be " loaded." If a muscle contracts against a small 

 and constant resistance, so as to be extended by a constant 

 force during its contraction, the curve described by a light 

 lever attached to it is termed '^isotonic." If a muscle 

 contracts against a large resistance, e. g., a strong spring, 

 so that it can shorten very little, the curve described by a 

 lever attached to it is termed '^isometric." The latter is flat- 

 topped, /. e., it shows a period of maintenance at maximum 

 contraction. The muscle reaches its maximum tension sooner 

 than its maximum shortening and maintains the maximum 

 tension longer than the maximum shortening. As ordinarily 

 employed, the myograph gives an isotonic curve. 



Arrange the apparatus as for a simple twitch. Arrange 

 a nerve-muscle preparation and attach to a lever weighted 

 only with the scale pan. Obtain a 'tracing as thus arranged. 

 Increase the load by adding a 10 gram weight and get tracing. 

 Increase the load still further by adding 30, SO and 100 grams 

 respectively and compare the tracings. 



