36 



EXPERIMENTAL PHYSIOLOGY 



time, and ordinates at right angles to this indicate for this particular 

 example changes in length of the muscle. The muscle was 26 mm. 

 long ; consequently a f 1 has been drawn at right angles to ad and of 

 three times that length (78 mm.) because the recording lever in tracing 

 the curve magnified the shortening three times. FjFg has been drawn 

 parallel to ad and divided at f,, f 2 , f 3 ... by a series of points 

 which follow one another at intervals of four vibrations of the tuning- 

 fork. Hence f, f 2 , f 2 p 3 , . . . &c. each represent -^th of a second. 

 Then f 2 g 2 , f 3 g 3 . . . f 9 d have been drawn parallel to a f, to cut 

 the curve in g 2 , g 3 . . . . In this way we are able to state that at the 

 instant a, at which the muscle was stimulated, its length was ^ a f. 



gLth of a second later its length was £ f 2 g 2 

 5^ths ditto ditto ^ f 3 g 3 



/oths ditto ditto ^ f 4 g 4 



5 %ths ditto ditto 3 F 9 D 



It should be remembered that in a great number of these graphic 

 records the true interpretation to be placed on the curves is one similar 



Fig. 40, 



-Simple Twitch of a Gastrocnemius. Time Tracing, 

 Magnification, 5. 



200 per sec. 



to the above. As a rule, however, we shall find it sufficient to take 

 into account only changes in length, width, &c, according to the move- 

 ment which is being recorded. 



For purposes of comparison fig. 40 is introduced. This is a curve 

 obtained in the same way, but given by a gastrocnemius. It shows the 

 same features as those of fig. 38. The latent period is 0-01 sec. ; the 

 period of contraction 005 sec. ; and that of relaxation 0-1 sec. The 

 magnification in this case was five-fold. 



In recording a single muscle contraction by the method we have 

 just employed, there is one factor in the method which causes an 

 inaccuracy in the result we are aiming at, and as it leads to several im- 

 portant considerations we must examine into it with some detail. 

 This is that the recording lever must possess a certain amount of mass, 

 and, as a consequence, is unable to follow alterations in length in 

 an absolute manner, more particularly when those alterations are 



