86 IRRITABILITY AND CONTRACTILITY [CH. VIII. 



Contraction of Muscle. 



Muscle undergoes the following changes when it contracts : 



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 proteins, 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. 68). 



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 contract- 

 ing, 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 variation in its electrical condition. 



5. Chemical changes. These consist in an increased consumption 

 of oxygen, and an increased output of waste materials such as car- 

 bonic acid, and sarcolactic acid. After prolonged contraction the 

 muscle consequently acquires an acid reaction. 



These five sets of changes will form the subjects of the following 

 five chapters. 



