438 SCIENCE PROGRESS. 



a personal bias which garbles the account it gives of the 

 event ? As in historical so in these physiological inquiries, 

 suspicion is cast upon the veracity of a writer by the fact 

 that another account of the same event written by a different 

 hand differs from the first in some important particular. Such 

 a second record is obtained when the muscle is attached to 

 a lever of essentially different construction. In that just 

 described the lever always pulled with its small weight upon 

 the muscle : the condition may be thus described as "iso- 

 tonic " and the curve obtained by it as an " isotonic muscle 

 curve ". 



But it is a familiar experience that muscles during con- 

 traction not only shorten in length but become tense. This 

 stiffening is as much a mood of the mechanical response of 

 muscle as is the shortening and may be communicated to a 

 lever by a very simple device. The muscle is made to pull 

 against a spring, preferably a steel wire fixed at one end 

 and capable of torsion at the other. The torsion of the 

 spring is always counteracting the mechanical tension of the 

 muscle, if the latter increases the former must do so too. 

 The slight movement of the steel wire, if recorded, will 

 indicate any alteration in the amount of the torsion and as 

 it may be easily arranged to move a fine lever, a magnified 

 record of the change can be obtained upon a travelling 

 blackened surface. The shortening- of the muscle when it 

 twitches under these conditions is now so little as to be 

 insignificant ; its length is kept approximately unaltered by 

 the steel wire and it is thus said to be under isometric 

 conditions, whilst the curve which is inscribed by the 

 lever is termed the isometric muscle curve. It is, or rather 

 it professes to be, the history of the development and 

 subsidence of the muscular tension, and we have thus a 

 second independent account of the mechanical change in 

 muscle evoked by a single stimulus. 



When the isometric and isotonic curves are compared 

 it is seen that the two accounts differ in one essential 

 particular, for the time relations of the curves are not 

 identical ; the isometric curve reaches its maximum from 

 tot to t§«t of a second before the isotonic. The existence 



