INTRODUCTION 9 



Fats, constituting about 12 per cent of dry protoplasm, contain 

 carbon, hydrogen, and oxygen in such proportions that there is 

 much less oxygen than in the carbohydrates. Fats of the body are 

 derived from fatty substances consumed and are also formed from 

 carbohydrates and proteins. Fats are essential in maintaining the 

 proper body temperature. 



Water is most essential to life and constitutes over 50 per cent 

 of the weight of most animals. Even the most ardent anti-pro- 

 hibitionist is made up of about 60 per cent water. It is necessary 

 to bathe tissues and to furnish the adequate liquid for blood, lymph 

 and cerebro-spinal fluid. 



Chemical Elements of Protoplasm. — Carbon compounds are the 

 primary materials of protoplasm. About 18 per cent of protoplasm 

 consists of carbon. Carbon unites with oxygen to form carbon 

 dioxide |B|HflH| and to liberate energy. 



Hyaroger^^hout 1 1 per cent of protoplasm) is taken into the 

 bodies of plants and animals in combination with oxygen as water, 

 and is also excreted in this form. 



Oxygen is found in the free state and unites with various com- 

 pounds of protoplasm, the process of oxidation releasing energy. 

 In combination with living tissues we find that oxygen makes up 

 about 65 per cent of protoplasm. 



Nitrogen is essential to protoplasm of which it comprises about 

 1 per cent. It forms 79 per cent of the atmosphere. Taken into 

 plant bodies usually in the form of nitrates, the plants utilize it in 

 the manufacture of proteins. Jfjimonia, a nitrogen compound, 

 formed in the katabolism of plants and animals, is changed by 

 certain bacteria into nitrates which are then absorbed by plants. 



Sulphur, usually found in the soil as calcium sulphate, is ab- 

 sorbed by plants and used in the manufacture of some amino-acids. 



Mineral Salts, Enzymes, Hormones and Vitamins. — The above 

 elements are said to comprise about 99 per cent of the weight of an 

 animal, but there are a number of other elements present in various 

 chemical combinations in extremely minute quantities. Some of 

 these are absolutely necessary to life, while others influence the 

 glands of internal secretion and thus aflfect growth. Among the 

 most important of these elements are iodine, iron, calcium, phos- 

 phorus, chlorine, magnesium, potassium, and sodium. Other 

 elements assuming greater importance every day are arsenic, 

 manganese, copper and zinc. Suffice it to say here that mineral 



