CHAP. IT.] THE CONTRACTILE TISSUES. 81 



We might take this disappearance of irritability as marking 

 the death of the preparation, but it is followed sooner or later by 

 a curious change in the muscle, which is called rigor mortis, and 

 which we shall study presently ; and it is convenient to regard 

 this rigor mortis as marking the death of the muscle. 



The irritable muscle, then, when stimulated either directly, the 

 stimulus being applied to itself, or indirectly, the stimulus being 

 applied to its nerve, responds to the stimulus by a change of 

 form which is essentially a shortening and thickening. By the 

 shortening (and thickening), the muscle in contracting is able to 

 do work, to move the parts to which it is attached ; it thus sets 

 free energy. We have now to study more in detail how this 

 energy is set free and the laws which regulate its expenditure. 



