514 HANDBOOK OF PHYSIOLOGY. 



reaching its highest point, the lever begins to descend, in consequence 

 of the elongation of the muscle. At first the fall is rapid, but then be- 

 comes more gradual until the lever reaches the abscissa or base line, and 

 the muscle attains its pre-contraction length, indicated in the figure by 

 the line c'. The stage occupies y|-^ second. Very often after the main 

 contraction the lever rises once or twice to a slight degree, producing 

 curves, one of -which is seen in fig. 330. These contractions, due to the 

 elasticity of the muscle, are called most properly (d) stage of elastic 

 after-vibration, or contraction remainder. 



The latent period has been found by exact methods of determination 

 to be only ^-5- second in length. The remainder of the time indicated 

 above is occupied in the propagation of the impulse along the nerve and 

 in overcoming the resistance of the apparatus used for recording the 

 curve. 



Accompaniments of Muscular Contraction. 



(1.) Heat is developed in the contraction of muscles. Becquerel and 

 Breschet found, with the thermo-multiplier, about .5 C. of heat pro- 

 duced by each forcible contraction of a man's biceps; and when the 

 actions were long continued, the temperature of the muscle increased 1. 

 This estimate is probably high, as in the frog's muscle a considerable 

 contraction has been found to produce an elevation of temperature equal 

 on an average to less than -| C. The cause of the rise of temperature 

 is the increased chemical activity at the time of contraction. As we 

 have already seen (Animal Heat), muscles produce heat even when 

 uncontracted. 



(2.) Sound is produced, as mentioned above, when voluntary muscles 

 contract. Wollaston showed that this sound might be easily heard by 

 placing the tip of the little finger in the ear, and then making some 

 muscles contract, as those of the ball of the thumb, whose sound may be 

 conducted to the ear through the substance of the hand and finger. A 

 low shaking or rumbling sound is heard. The sound is due to the vi- 

 bration of the individual muscle fibres. Experimentally it has been 

 found that the number of vibrations corresponds to the number of ex- 

 citations, and that muscle exhibits no normal rate of vibration, except 

 in so far as a rate is expressed in the discharge of nerve impulses from 

 the cells controlling the muscle. Nerve cells do not send out a single, 

 but a series of impulses. Moreover, the muscle sound corresponds to 

 the rate at which the muscle is stimulated. 



Helmholtz found that, in the voluntary contraction of muscle, only 

 reeds having a vibration of 18-20 per second were thrown into motion; 

 and since this rate is too slow to produce a tone, he concluded that the 



