152 STUDIES IN PHYSIOLOGY 



Colds and their Prevention. Prolonged exposure to cold, 

 the wearing of wet clothing, or sudden exposure to a draught 

 of air often results in a contraction of the arteries in the 

 skin. The blood is thus driven away from the surface to 

 the internal organs of the body, and a condition of conges- 

 tion in these organs is the result. We describe this condition 

 as a " cold." The real cause of colds is not yet fully under- 

 stood. It is probable, however, that frequently the conges- 

 tion of the internal organs, which may follow an exposure, 

 favors the growth of disease producing bacteria, should these 

 be present in the air passages or in the alimentary canal. 



Those who are accustomed to taking vigorous exercise 

 followed by cold baths are less liable to colds. The wearing 

 of woolen underclothing is another means of prevention, 

 since this material, unlike cotton and linen, does not allow 

 the skin to be acted upon quickly by sudden changes of 

 temperature. It is 'without doubt unwise to keep the neck 

 and throat muffled with furs or other wrappings ; for when 

 they are removed the sensitive skin is more easily affected 

 by a slight draught. 



Effect of Exercise on the Heart. 1 The pulse rate is slowest 

 when we are asleep. As the activities of the day begin, 

 the heart beat is quickened, and after violent exercise this 

 organ may beat as often as twice to three times a second. 

 Exercise, when properly regulated, is undoubtedly bene- 

 ficial to every organ in the body; for a higher pulse rate 

 means that the blood is renewed in each tissue so much the 

 oftener, that more oxygen is received from the lungs, and 

 that more waste matters are excreted. Heart muscle itself, 

 as well as other organs, profits by this increased activity. 



Effect of Exercise on the Size of the Blood Vessels. When 

 one is using one's muscles, greater metabolism of the tissues 

 goes on, and a larger amount of blood is needed to supply 

 material for repairing the waste. The sympathetic nervous 

 system therefore causes the muscular walls of the arteries to 

 1 See " Laboratory Exercises," No. 29. 



