138 



Hitchcock's anatomy 



270. Rapidity of Muscular Contraction.— The rapidity 

 of muscular movement is equally wonderful. The pulsations 

 of the heart in children can often be counted as high as 200 

 per minute. Some persons can pronounce 1500 letters in a 

 minute (of course combined in words), each one requiring the 

 contraction and relaxation of one or more muscles, and both 

 occupying 3 oth of a minute, or jVth of a second. The 

 muscles, too, which move the wings of some insects must 

 contract many thousands of times every second in order to 

 produce the musical tone or humming that is frequently heard 

 when their wings are in motion. 



271. Duration of Muscular Contraction— Intrinsic and 

 Available Force — Shortening of Muscular Fiber— Muscu- 

 lar Sense— Sound and Heat.— The length of time which 

 muscles may be on the stretch is astonishing, but especially 

 so among the lower orders of animals soon to be mentioned. 



The intrinsic force of a muscle is no measure of its avail- 

 able power. Thus the deltoid, if able to act in a perpendicu- 

 lar direction, would raise 1000 pounds ; while, acting at the 

 great mechanical disadvantage (that it does) of passing over the 

 head of the humerus, it can not lift 50 pounds held in the 

 hand. 



The amount of shortening of the fiber in muscular contrac- 

 tion is differently stated by physiologists. The statements 

 are from one third to one sixth the length of the muscle. 

 That is, a muscle three inches long can by contraction be- 

 come only two inches, or one six inches long become five 

 inches. It is probable, however, that the former statement is 

 the nearest to the truth, and that a muscle is shortened from 

 one half to one third its original length. 



The muscles give but little evidence of sensibility ; that is, 

 perception of objects by pressure or touch ; but when fatigued of 

 overworked, they give a painful sensation, and also give to theil 



270. State instances of great rapidity of muscular movement, as in pulsations of 

 children's hearts and the power of articulating words. 271. State the distinction be- 

 tween the intrinsic and available force of muscles. How much does muscular flbei 

 shorten during its contraction ? What is muscular sense ? 



