130 THE HUMAN BODY 



act also to extend the wrist. To secure extension of the fingers 

 while the wrist is flexed the latter position must be maintained 

 by activity of the wrist flexor to overcome the tendency of the 

 finger extensors to extend the wrist. 



Hygiene of the Muscles. The healthy working of the muscles 

 needs of course a healthy state of the Body generally, so that 

 they shall be supplied with proper materials for growth and re- 

 pair, and have their wastes rapidly and efficiently removed. In 

 other words, good food and pure air are necessary for a vigorous 

 muscular system, a fact which trainers recognize in insisting upon 

 a strict dietary, and in supervising generally the mode of life of 

 those who are to engage in athletic contests. The muscles should 

 also not be exposed to any considerable continued pressure, 

 since this interferes with the flow of blood and lymph through 

 them. 



As far as the muscles themselves are directly concerned, exer- 

 cise is the necessary condition of their best development. The 

 muscles are so compactly built that the movement of blood and 

 lymph through them is less free than in other tissues. During 

 the act of contraction, however, the circulation both of blood and 

 of lymph is augmented by the pressure of the muscle upon its own 

 contents. For their proper nourishment most of the muscles are 

 largely dependent upon this self-massage. A muscle which is 

 permanently unused suffers serious impairment of nutrition, and 

 usually degenerates and is absorbed, little finally being left but 

 the connective tissue of the organ and a few muscle-fibers filled 

 with oil-drops. This is well seen in cases of paralysis dependent 

 on injury to the nerves. In such cases the muscles may them- 

 selves be perfectly healthy at first, but lying unused for weeks 

 they become altered, and finally, when the nervous injury has been 

 healed, the muscles may be found incapable of functional activity. 

 The physician therefore is often careful to avoid this by exercising 

 the paralyzed muscles daily by means of electrical shocks sent 

 through the part; passive exercise, as by proper massage, is fre- 

 quently of great use in such cases. The same fact is illustrated 

 by the feeble and wasted condition of the muscles of a limb which 

 has been kept for some time in splints. After the latter have been 

 removed it is only slowly, by judicious and persistent exercise, 

 that the long-idle muscles regain their former size and power- 



