CH. IX.T CONTRACTION OF MUSCLE. 



CHAPTER IX. 



CONTRACTION OF MUSCLE. 



MUSCLE undergoes many changes when it contracts; they 

 may be enumerated under the following five heads : 



1. Changes in form. 



2. Changes in extensibility and elasticity. 



3. Changes in temperature. 



4. Changes in electrical condition. 



5. Chemical changes. 



In brief, each of these changes is as follows : 



1. Changes inform. The muscle becomes shorter, and at the 

 same time thicker. The amount of shortening varies so that the 

 length of the muscle when contracted is from 65 to 85 per cent, 

 of what it was originally. Up to a certain point, increase of the 

 strength of the stimulus increases the amount of contraction. 

 Fatigue diminishes and up to about 33 C. the application of heat 

 increases the amount of contraction. Beyond this temperature 

 the muscular substance begins to be permanently contracted, 

 and a condition called heat rigor, due to coagulation of the 

 muscle proteids, sets in a little over 40 C. 



What the muscle loses in length it gains in width ; there is 

 no appreciable change of volume. 



Among the changes in form must also be mentioned those 

 changes in the individual muscular fibres which require a 

 microscope for their investigation ; these have been already 

 considered (see p. 90). 



2. Changes in elasticity and extensibility. The contracted 

 muscle is more stretched by a weight in proportion to its length 

 than an uncontracted muscle with the same weight applied to it; 

 the extensibility of contracted muscle is increased ; its elasticity 

 is diminished. 



3. Changes in temperature. When muscle is at work or 

 contracting, more energetic chemical changes are occurring than 

 when it is at rest ; more heat is produced and its temperature 

 rises. 



4. Changes in electrical condition. A muscle when it contracts 

 undergoes a diphasic variation in its electrical condition. 



5. Chemical changes. These consist in an increased consump- 

 tion of oxygen, and an increased output of waste materials such as 



