54 HUMAN PHYSIOLOGY. 



the pitch of the sound corresponds with the number of stimuli a second. 

 A voluntary contraction is attended by a tone having a vibration frequency 

 of about thirty-six a second, which is, however, the first overtone of the true 

 muscle tone, which is caused by a contraction frequency of about eighteen 

 a second. This low tone is inaudible, from the small number of vibrations 

 a second. 



Muscle Fatigue. Prolonged or excessive muscular activity is followed by 

 a diminution in the power of producing work and by an increase in the dura- 

 tion of the muscular contractions. Fatigue is accompanied by a feeling of 

 stiffness, soreness, and lassitude, referable to the muscles themselves. In the 

 early stages of muscular fatigue the contractions increase in height and dura- 

 tion, to be followed by a progressive decrease in height, but an increase in 

 duration, until the muscle becomes exhausted. The cause of the fatigue is 

 the production and accumulation of decomposition products, such as phos- 

 phoric acid and phosphate of potassium, CO 2 , etc. A fatigued muscle is 

 rapidly restored by the injection of arterial blood. 



Work Done. Muscles are machines capable of doing a certain amount of 

 work, by which is meant the raising of a weight against gravity or the over- 

 coming of some resistance. The work done is calculated by multiplying the 

 weight by the distance through which it is raised. Thus, if a muscle shortens 

 four millimeters and raises 250 grams, it does work equal to 1,000 milligram- 

 meters, or one gram-meter. If a muscle contracts without being weighted, no 

 work is done. Equally, when the muscle is over-weighted so that it is unable 

 to contract, no work is done. The amount of work a muscle can do will 

 depend upon the area of its transverse section, the length of its fibers, and the 

 amount of the weight. The amount of work a laborer of 70 kilograms 

 weight performs in eight hours averages 105,605 kilogram-meters, or 340.2 

 foot-tons. 



SPECIAL PHYSIOLOGY OF MUSCLES. 



The individual muscles of the axial and appendicular portions of the body 

 are named with reference to their shape, action, structure, etc. e. g., deltoid, 

 flexor, penniform, etc. In different localities a group of muscles having a 

 common function is named in accordance with the kind of motion it produces 

 or gives rise to e. g., groups of muscles which alternately bend or straighten 

 a joint, or alternately diminish or increase the angular distance between two 

 bones, are known respectively as flexors and extensors; such muscle groups are 

 in association with ginglymus joints. Muscles which turn the bone to which 

 they are attached around its own axis without producing any great change of 



