290 THE ANIMALS AND MAN 



food and prepares the food for the use of the tissues. A 

 circulatory system, consisting of heart, blood-vessels, capil- 

 laries, and lymphatics, transports the prepared food and 

 oxygen to the cells of the body. The muscular and skeletal 

 systems, consisting of the muscles and bones of the body and 

 their attachments, enable the body to do all the things re- 

 quiring motion. The respiratory system, consisting of the 

 air passages of the nose, throat or pharynx, larynx, the 

 bronchial tubes and minute air sacs forming the large respira- 

 tory surface, is employed in supplying the blood with oxygen 

 which is to be carried to the cells. An excretory system, con- 

 sisting of the kidneys and their ducts, and certain glands in the 

 skin known as sweat glands, take up the waste from the 

 blood and remove it from the body. The nervous system, 

 consisting of the brain, spinal column and innumerable 

 nerves, puts all the parts of the body into communication 

 so that they may work in harmony. The sense-organ sys- 

 tem, intimately connected with the nervous system and 

 functioning with it, comprises the organs for seeing, hearing, 

 smelling, tasting, feeling, etc. They put the inside of the 

 body into communication with the outside world. 



All of these systems work together to maintain life, that 

 is, to maintain the metabolism of the cells. If any system 

 fails to fulfill its function the whole body suffers and disease 

 sets in. It is our business to pro\'ide the body with good 

 food, fresh air, pure water and daily exercise so that each 

 system may be kept in the best condition possible for its work. 



The chemistry of the body. — The chemist tells us that, 

 in all the world, there are only about seventy simple or ele- 

 mentary substances. All the gases, liquids and solids that 

 we know of, are formed by uniting these simple elements in 

 many ways. Thus the simple element oxygen united with 

 the simple element hydrogen forms the water we drink. A 

 mixture of pure oxygen, nitrogen, hydrogen and a few other 

 gases forms the air we breathe. 



