34 HAND-BOOK OF PHYSIOLOGY. 



blood. Here we have a sudden restoration of energy: the muscle in this 

 case makes an entirely fresh start from A, and the new fatigue curve is 

 parallel to, and never coincides with the old one. 



A fatigued muscle has a much longer "latent period" than a fresh one. 

 The slowness with which muscles respond to the will when fatigued must 

 be familiar to every one. 



In a muscle which is exhausted, stimulation only causes a contraction 

 producing a local bulging near the point irritated. A similar effect may 



FIG. 286. Fatigue muscle-curves. (Ray Lankester.) 



be produced in a fresh muscle by a sharp blow, as in striking the biceps 

 smartly with the edge of the hand, when a hard muscular swelling is in- 

 stantly formed. 



Accompaniments of Muscular Contraction. (1.) Heat is de- 

 veloped in the contraction of muscles. Becquerel and Breschet found, 

 with the ther mo-multiplier, about 1 Fahr. of heat produced by each forci- 

 ble contraction of a man's biceps; and when the actions were long con- 

 tinued, the temperature of the muscle increased 2. 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 \ 0. It is not known whether this development of heat is due 

 to chemical changes ensuing in the muscle, or to the friction of its fibres 

 vigorously acting: in either case, we may refer to it a part of the heat 

 developed in active exercise (p. 310, Vol. I.). 



(2.) Sound is said to be produced when muscles contract forcibly, as 

 mentioned above. 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. 



