THE SKELETON AND MUSCLES 341 



To each muscle also a nerve passes, distributing its nerve 

 fibers through the connective tissues to the muscle cells. 

 Without its nerve-supply, any muscle would be useless, 

 for the nerve centers must initiate and control every motion. 



Hygiene of muscles ; necessity for exercise. Muscular 

 activity is absolutely essential to healthy living, and should 

 therefore be as much a daily habit as eating or sleeping. 

 Take a vigorous walk for a mile or two and note the result. 

 The day may be cold, but the body is soon aglow with 

 warmth, the heart beats more vigorously, the breathing is 

 deep and invigorating. What is taking place in the body? 

 The muscles are undergoing chemical change. Carbon 

 dioxide and other wastes are being thrown into the blood, 

 and large quantities of heat are being liberated so that the 

 temperature of the body rises considerably. This calls 

 for a rapid supply of blood to the muscle cells, that there may 

 be a sufficient supply of food and oxygen. The heart 

 responds by a quickened beat, driving the blood to the needy 

 tissues. The heat regulating apparatus is brought into 

 play. The small arteries of the skin dilate, and perspiration, 

 laden with water and waste, is secreted by the glands of the 

 skin. The blood-vessels of the internal organs constrict, 

 reducing the blood flow to these organs. The respiratory 

 movements become deeper and more frequent, and, probably 

 most beneficial of all, the lymph circulation in the tissues 

 is improved. The lymph having no propelling organ like 

 the blood, is dependent upon the bodily movements, par- 

 ticularly the muscular movements, for its flow. 



Muscular activity not only aids in the proper circulation 

 of food and oxygen, but in keeping the muscles at a proper 

 tension. An unused muscle becomes soft and flabby, and 

 tires easily when demands are made upon it. Those forms 

 of exercise are most valuable that call into play the greatest 

 number of muscles. Walking is perhaps the best exercise 

 because it exercises most of the body muscles and the 



