132 THE HUMAN BODY. 



from the moment of stimulation until the muscle regains 

 ) its resting form is known as a "simple muscular contraction'' 

 I or a " tioitch." It occupies in the frog about one tenth oi 

 a second and is separable into three portions. First, there 

 elaspes a time, after the stimulation and before the com- 

 mencement of the shortening, which is known as the " lost 

 | time" or the period of latent excitement. This lasts about 

 one hundredth of a second, and represents the time during 

 which molecular changes preparatory to the contraction are 

 taking place in the muscle fibres. Then follows the short- 

 ening, at first slow, then rapid, then slower again up to a 

 maximum, and occupying rather more than half of the re- 

 maining time; the elongation occupies the remainder of the 

 time taken up in the contraction. In warm-blooded ani- 

 mals, the duration of a simple muscular contraction is even 

 less than one tenth of a second and all its stages are quick- 

 ened. In any given animal, cold increases the time taken 

 in a muscular contraction and also impairs the contractile 

 power, as we find in our own limbs when "numbed" with 

 cold. Moderate warmth on the other hand (up to near 

 the point at which heat-rigor is produced) diminishes the 

 duration of the contraction; so that t4ie molecular changes 

 in a muscular fibre are clearly eminently susceptible to slight 

 changes in its environment. The contractility of a muscle 

 does not depend upon a vital force, entirely distinct from 

 ordinary inanimate forces, but upon an arrangement of its 

 material elements, which is only maintained under cer- 

 tain conditions and is eminently modifiable by the sur- 

 roundings. 



Physiological Tetanus. It is obvious that the ordinary 

 movements of the Body are not brought about by such tran- 

 sient muscular contractions as those described in the last 

 paragraph. Even a wink lasts longer than one tenth of a 

 second. Our movements are, in fact, due to more prolonged 

 contractions which may be described as consisting of several 

 \ simple twitches fused together, and known as " tetanic- 

 1 contractions"; it might be better to call them "compound 

 ' contractions," since the word tetanus has long been used 

 by pathologists to signify a diseased state, such as occurs- 



