INTRODUCTION 3 



point of view are the complex nitrogenous compounds called Proteins* 

 So far as is at present known, protein material is never absent from 

 living substance, and is never present in anything else than that 

 which is alive or has been formed by the agency of living cells. It 

 may therefore be stated that Protein Metabolism is the most essential 

 characteristic of vitality. 



The chemical structure of protoplasm can only be investigated 

 after the protoplasm has been killed. The substances it yields are 

 (1) Water ; protoplasm is semi-fluid, and at least three-quarters of 

 its weight, often more, are due to water. (2) Proteins. These are 

 the most constant and abundant of the solids. A protein or albu- 

 minous substance consists of carbon, hydrogen, nitrogen, oxygen, 

 with sulphur and phosphorus in small quantities only. In nuclein, 

 a protein-like substance obtained from the nuclei of cells, phosphorus 

 is more abundant. The protein obtained in greatest abundance from 

 the cell-protoplasm is nucleo -protein : that is, a compound of protein 

 with varying amounts of nuclein. White of egg is a familiar instance 

 of an albuminous substance or protein, and the fact (which is also 

 familiar) that this sets into a solid on boiling will serve as a reminder 

 that the greater number of the proteins found in nature have a 

 similar tendency to coagulate under the influence of heat and other 

 agencies. (3) Various other substances occur in smaller proportions, 

 the most constant of which are lecithin, a phosphorised fat ; chole- 

 sterin, a monatomic alcohol : and inorganic salts, especially phos- 

 phates and chlorides of calcium, sodium, and potassium. 



It will be seen from this rapid survey of the composition of the 

 body how many are the substances which it is necessary we should 

 study ; the food from which it is built up is also complex, for animals 

 do not possess, to such an extent as plants do, the power of building 

 up complex from simple materials. 



The substances out of which the body is built consist of 

 chemical elements and of chemical compounds, or unions of these 

 elements. 



The elements found in the body are carbon, hydrogen, nitrogen, 

 oxygen, sulphur, phosphorus, fluorine, chlorine, iodine, silicon, 

 sodium, potassium, calcium, magnesium, lithium, iron, and occa- 

 sionally manganese, copper, and lead. 



Of these very few occur in the free state. Oxygen and nitrogen (to 

 a small extent) are found dissolved in the blood-plasma ; hydrogen is 



1 In most English text-books these substances have hitherto been called 

 Proteids. The change to Protein brings English, American, and German usage 

 into harmony. 



B2 



