745 



plate to the position in which the key is just opened, and allowing 

 the lever to trace here a vertical line (or, rather, an arc of a circle). 

 The portion of the time-tracing between this line and a parallel line 

 drawn through the point at which the contraction begins gives the 

 latent period. 



Helmholtz measured the length of the latent period by means of the 

 principle of Pouillet, that the deflection of a magnet by a current of 

 given strength and of very short duration is proportional to the time 

 during which the current acts on the magnet. He arranged that at 

 the moment of stimulation of the muscle a current should be sent 

 through a galvanometer, and should be broken by the contraction of 

 the muscle the moment it began. In this way he obtained the value 

 of Y^JJ second for the latent period of frog's muscle. The tendency of 



ILL 



Fig. 251. Spring Myograph. A, B, iron uprights, between which are stretched the 

 guide-wires on which the travelling plate a runs; k, pieces of cork on the guides 

 to gradually check the plate at the end of its excursion, and prevent jarring; 

 b, spring, the release of which shoots the plate along; A, trigger-key, which is 

 opened by the pin d on the frame of the plate. 



later observations has been to make the latent period shorter. Burdon 

 Sanderson found that the change of form begins in unweighted or very 

 slightly weighted muscle with direct stimulation in y^y second after, 

 and the electrical change (p. 824) simultaneously with, the excitation. 

 It is known that the apparent latent period depends upon the resistance 

 which the muscle has to overcome in beginning its contraction. 



The maximum shortening, or ' height of the lift,' depends upon the 

 length of the muscle, the direction of the fibres, the strength of the 

 stimulus, the excitability of the tissue, and the load it has to raise. 



In a long muscle, other things being equal, the absolute shortening, 

 and therefore the maximum height of the curve, will be greater than 

 in a short muscle; in a muscle with fibres parallel to its length the 

 sartorius, for instance it will be greater than in a muscle like the 

 gastrocnemius, with the fibres directed at various angles to the long 

 axis. For stimuli less than maximal, the absolute contraction increases 



