240 FIRST YEAR SCIENCE 



of the sun and the soil, they live upon the food prepared 

 by the green plants. 



Probably the simplest plants are the bacteria, single-cell 

 plants, which multiply very rapidly. Bacteria arid fungi 

 cause many diseases as well as most of the spoiling of 

 food. Disease bacteria are usually called germs. Their 

 effects may be counteracted by the use of disinfectants 

 and antitoxins. 



Animals take their energy indirectly from the foods 

 prepared by green plants or by other animals. They are 

 usually classed as invertebrate and vertebrate. The lowest 

 form of invertebrate is the protozoon. Worms and in- 

 sects are other forms of invertebrates, the importance of 

 which is seldom realized. 



Vertebrates usually have a backbone, skull, ribs and 

 appendages. In the skull is the brain, connected to the ( 

 various parts of the body by nerves. Vertebrates breathe 

 by " drawing " air through the windpipe into the lungs. 

 This is done by the muscles of the chest and diaphragm. 

 The lungs purify the blood, which circulates from the 

 heart through the arteries and capillaries and back through 

 the veins. 



The five senses are sight, hearing, taste, smell and 

 touch. These sensations are carried to the brain by the 

 nerves, and they come from the eye, the ear, the nose, the 

 mouth and the skin, respectively. 



For all the activities of body and brain, food is required. 

 As food passes through the alimentary canal, various 

 juices are mixed with it and certain parts of it are 

 digested arid absorbed into the body. Foods are of three 

 kinds, proteins, carbohydrates and fats, and we need a 

 certain percentage of each for proper nourishment. 

 Usually foods 'are most nourishing and most appetizing 

 when properly cooked. 



