590 MOTION. 



about I of heat produced by each forcible contraction of a 

 man's biceps ; and when the actions were long continued, 

 the temperature of the muscle increased 2. It is not 

 known whether this development of heat is due to chemi- 

 cal 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. 233). 

 And Nasse suspects that to it is due the higher tempera- 

 ture of the blood in the left ventricle ; for he says that 

 this fluid is always warmer in the left ventricle than in the 

 left auricle, and that the blood in the latter is but little 

 warmer than that on the right side of the heart. But these 

 experiments need confirmation. 



Sound is said to be produced when muscles contract for- 

 cibly. Dr. 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 height 

 and loudness of the note being in direct proportion to 

 the force and quickness of the muscular action, and to 

 the number of fibres that act together, or, as it were, in 

 time. 



The two kinds of fibres, the striped and unstriped, have 

 characteristic differences in the mode in which they act on 

 the application of the same stimulus; differences which 

 may be ascribed in great part to their respective differences 

 of structure, but to some degree possibly, to their respec- 

 tive modes of connection with the nervous system. When 

 irritation is applied directly to a muscle with striated 

 fibres, or to the motor nerve supplying it, contraction of 

 the part irritated, and of that only, ensues ; and this 

 contraction is instantaneous, and ceases on the instant 

 of withdrawing the irritation. But when any part with 

 unstriped muscular fibres, e.g., the intestines or bladder, is 



