ADDITIONAL TOPICS IN HUMAN BIOLOGY 161 



directed by the conscious portions of our brain. Thus we can close 

 our fingers and open them as we please ; we can move the eyes, the 

 head, and the legs at will. We call all the muscles that are con- 

 trolled by our will power, vol'un-ta-ry muscles (Latin, voluntas = 

 will) . Most of the muscles of the throat, those of the gullet, stom- 

 ach, and intestines, on the other hand, act without any voluntary 

 direction on our part, and they are therefore called in-vol'un-ta-ry. 



222. Conditions necessary for healthy muscles. If one is to 

 have a well-developed and healthy muscular system, four conditions 

 must be fulfilled : the body must be supplied with nutritious food; 

 there must be a generous amount of fresh air; the muscles must be 

 exercised vigorously; and this exercise must be followed by periods of 

 rest. We will now consider in turn how each of these requirements 

 may be met. 



223. Food. We have learned that 75 per cent of muscle is com- 

 posed of water, and that protein is the most important solid ingredi- 

 ent. Mineral matter and fats are also present hi small quantities, 

 even in the leanest of muscle. During the period of growth all these 

 nutrients should be supplied for muscle building, but protein is 

 absolutely essential. Grape sugar is also found to be an important 

 food during muscular contraction. The diet of athletes while they 

 are training for contests is carefully regulated : rare meats, coarse 

 breads, eggs, vegetables, and fruits are supplied in generous quanti- 

 ties ; pastry and fats are reduced to a minimum. Tobacco and al- 

 cohol in any form, however, are absolutely prohibited. Such a diet is 

 undoubtedly far more wholesome to develop a healthy boy or girl, 

 man or woman, than are the rich gravies, pastries, and condiments 

 which are found on too many tables. 



224. Fresh air. Healthy muscle is absolutely powerless, how- 

 ever, unless, in addition to food, it receives a supply of oxygen ; 

 for all muscular energy is produced by oxidation. Impure air, be- 

 sides being deficient in oxygen, contains carbon dioxid and other 

 gases that are exceedingly harmful to the tissues (196). Well- 

 ventilated sleeping rooms are most essential for healthy living, for 



