20 PHYSIOLOGY 



starch they not only provide him with a large proportion 

 of his food, but by taking in carbon dioxide and giving 

 off oxygen, they purify the air, so that it is fit for him to 

 breathe. 



Air, its Composition and Pressure. — Air is a physical 

 mixture, not a chemical compound. When pure, it is 

 composed largely of two gases, oxygen and nitrogen, 

 with small traces of water vapor and carbon dioxide. 

 Besides furnishing gases that living organisms need, it 

 affects them in many other ways, chiefly through its 

 weight, which is enormous. It presses upon all sides of 

 us in every direction with a force of fifteen pounds for 

 every square inch of surface. This force represents the 

 weight of a column of air with a base of one square inch 

 stretching from the earth to the limit of the atmosphere. 

 One would think this weight enough to crush a delicate 

 little organism, but the external pressure is so nicely ad- 

 justed to the pressure within the body that it does no 

 harm, and it regulates many functions. In the hair cell 

 the streams of protoplasm are separated by large, trans- 

 parent spaces which seem empty. They are, however, 

 filled with a transparent liquid which by pressing back 

 upon the wall with a force of fifteen pounds to the 

 square inch neutralizes the air pressure, which would 

 otherwise crush out the spaces and prevent their exist- 

 ence. This liquid is sap, or water containing dissolved 

 substances, one of which is sugar. 



Summary. — Every organism has a definite cellular 

 structure, that is, it is made up of one or more cells, or 

 little masses of protoplasm, each of which contains a 

 nucleus and is surrounded by a cell-wall. It is further 



