PHYSIOLOGY FOR BEGINNERS 



as one wall may differ from another wall in its bricks and in 

 the way in which the bricks are laid. 



The cells of the tissues of a living animal consist of living 

 material, and this is sometimes called protoplasm. 



When a tissue is examined chemically we find that many 

 different chemical substances can be got out of it. The 

 chemical substances make up the material of which it is com- 

 posed. Albumin, sugar, fat, common salt, water, are examples 

 of substances occurring in tissues. 



Substances are^either simple or compound. Compound 

 substances are those that can be split up into, or can be shown 

 to consist of, simple substances. Simple substances cannot 

 be so split up. They are called elements. The body of an 

 animal, then, is ultimately made up of elements for the most 

 part united to one another to form compound substances. 



About seventy elements are known to chemists, but only a 

 few of these are found in the substances forming animal tissues. 

 The chief elements found in the body are : oxygen, hydrogen, 

 nitrogen, carbon, sulphur, phosphorus, chlorine, sodium, potas- 

 sium, calcium, magnesium, iron. 



Oxygen (O) is, under ordinary conditions, an invisible 

 gas, and forms about one-fifth of the volume of the atmosphere. 

 It supports combustion, and is necessary for the support of 

 life. It forms eight-ninths of the weight of water, and is 

 abundantly present in nature in combination with other ele- 

 ments. Oxygen is found as an element in the blood, and 

 combined with other elements in a large number of the sub- 

 stances of the body. 



Hydrogen (H) is a very light, invisible, combustible gas. 

 When it burns in air it unites with oxygen, and water is 

 formed. It forms one-ninth of the weight of water. It is 

 found in combination with other elements in all animal and 

 vegetable substances. 



Nitrogen (N) is an invisible gas forming four-fifths of the 

 volume of the atmosphere. A very small amount of nitrogen is 

 found as an element in the blood. It exists in combination 

 with other elements in very many animal and vegetable 

 substances, and, as we shall see, then possesses its great 

 importance. 



Carbon (C) is a solid element existing in different condi- 



