268 ELEMENTS OF GENERAL SCIENCE 



time after feeding. In such cases the chief activities of the 

 body are for a time centered about digestion and the circu- 

 latory distribution of food. Animals which feed oftener and 

 eat smaller quantities man, for example may safely pro- 

 ceed with some sort of mild exercise after eating. 



295. Breathing and nutrition. Experiments similar to those 

 suggested in connection with circulation will enable one to 

 determine that the normal breathing rate is about sixteen times 

 per minute, and also that violent exercise increases the rate. 

 The tissues demand oxygen, which is secured from the blood. 

 The blood's supply of oxygen is replenished in the lungs, 

 where the waste carbon dioxide is disposed of. When an in- 

 creased amount of blood needing replenishment and purifica- 

 tion is hastened through the lungs, there is need of more air 

 in the lungs. It is the demand for oxygen in the tissues which 

 stimulates an increase in the rate of breathing. Any boy or 

 girl who has run until exhausted knows that at such a time 

 the body needs more air than usual. Similarly, in a room in 

 which there is a deficiency of oxygen or an abnormally high 

 per cent of carbon dioxide, the breathing rate may increase 

 unconsciously, owing to the body's call for more or better air. 

 If a mouse or other animal is placed under a bell jar and the 

 air is exhausted, the animal breathes rapidly and with the 

 same violent efforts to get air that are shown by an animal 

 that is exhausted from exercise. 



Any exercise which tends to increase the air capacity of 

 the lungs increases one's ability to supply his blood with 

 oxygen and to remove waste products from the blood. Deep- 

 breathing exercises are of great importance in increasing the 

 lung capacity. Vigorous exercise is helpful to many persons, 

 since such exercise often compels them to breathe deeply a 

 thing which they should practice whether compelled to or not. 



It needs no argument to show that pure air is better for the 

 body than impure air. The nasal passages are so constructed 

 as to assist in screening out floating particles, as bacteria and 



