22 ESSENTIALS OF PHYSIOLOGY, 



A certain strength of stimulus is necessary in order to produce any 

 visible shortening of a muscle ; a further increase in the strength of the 

 stimulus, beyond this point, causes the muscle to contract more strongly, 

 until finally the contraction becomes maximal. The varying degree of 

 the response of the muscle to varying strength of stimulus is due to the 

 fact that a weak stimulus affects only a few fibres, whereas a strong 

 stimulus throws into contraction a large number of fibres ; when all the 

 fibres are stimulated, the shortening is maximal. Each fibre, however, 

 if it contracts at all, gives the maximal contraction of which it is capable 

 for the conditions under which it is placed, whatever the strength of the 

 stimulus. This is known as the "all or none law," and holds good 

 whether the muscle is stimulated directly or through its nerve. 



When a muscle contracts, the contraction travels from the point of 

 stimulation in the form of a wave at the rate of 5 to 6 metres per second 

 in mammals and 3 to 4 metres in cold-blooded animals. This rate can be 

 measured by resting two levers on a muscle, one at the middle, the other 

 at one end, and applying a stimulus to the opposite end of the muscle. 

 The lever nearer the stimulated point will rise earlier than the one at 

 the opposite end of the muscle ; and if this interval is measured and 

 the length of muscle between the two levers is known, the rate at 

 which the wave travels can be calculated. The length of the wave is 

 measured by multiplying the rate at which it travels by its duration at 

 any one point ; it varies in frog's muscle from 150 to 300 millimetres. 

 / A rise of temperature quickens, and a fall of temperature delays, 

 /every phase of the contraction. If the muscle is lifting a small load, ** 

 the lever often rises higher at a high than at a low temperature, since 

 the sudden jerk given to the lever by the rapid contraction imparts to C 

 it a greater momentum than when the pull on the lever takes place I 

 more gradually. On the contrary, when a heavy weight is attached to 

 the lever, a slowly contracting cooled muscle may be more effective in 

 raising the lever than the rapidly contracting warmed muscle. These 

 differences are of purely mechanical origin, and the actual force of the 

 contraction for a given load and for the same strength of stimulus 

 remains unchanged between 5 C. and 20 C. Cooling increases the 

 excitability of muscle, and maximal stimuli should therefore be used in 

 studying the effect of temperature on the height of contraction. 

 Prolonged exposure to a temperature of C. destroys the vitality of 

 muscle. 



If a muscle is repeatedly stimulated, the height of the contractions 

 diminishes, and all phases of the contraction, including the latent 

 period, are prolonged ; finally, the muscle may fail to contract in 

 response to a stimulus. This condition constitutes fatigue. 



