THE CONTRACTION OF MUSCLE 301 



(ii) The muscle may be enclosed in a small cliamber 

 free from oxygen and made to contract several times. 

 If now we examine the gas in the chamber in which 

 this excised muscle has been contracting we cannot obtain 

 satisfactory evidence of any escape of carbonic acid from 

 the muscle during its contraction. That, however, the 

 muscle within the body richlij supplied with oxygen does 

 give off carbonic acid during its contraction is clearly 

 shown by the fact that the venous blood coming from a 

 contracting muscle contains relatively more carbonic acid 

 than it does wlien tlie muscle is at rest. 



(iii) The muscle becomes slightly warmer ; this can 

 only be due to the fact that heat is formed during the 

 coMtraction. The rise of temperature is slight, but may 

 always be observed. The evolution of heat, like the 

 oxidation processes with which it is associated, is pro- 

 bably associated with the formation, after the contraction, 

 of fresh contractile material. 



(iv) The muscle undergoes certain electrical changes. 

 At the moment of commencing contraction the muscle 

 becomes like a small battery cell, and can send an electric 

 current through a wire whose ends are suitably applied to 

 the surface of the muscle.^ 



7. The Tetanic Contraction of Muscles. — When 

 experimenting with a muscle-nerve preparation, as in the 

 preceding section, it is easy to stimulate the nerve twice 

 in such rapid succession that tlie second stimulus is given 

 while the muscle is in a state of contraction resulting 

 from the first. In this case the muscle responds to the 

 second stimulus as well as to the first ; in other words it 

 conti'acts still more while already contracting. The 

 second contraction takes place on top of the first, is 

 rather less in amount than the first, and is added on to 

 the first. If now a rapidly successive series of stitnuli be 



1 See also Lesson XI., where the electrical changes of an active nerve, 

 which are essentially the same as those of a contracting muscle, are 

 described iu greater detail. 



