INTRODUCTION 3 



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

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

 may therefore be stated that Proteid 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) Proteids. These are 

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

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

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

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

 is more abundant. The proteid obtained in greatest abundance from 

 the cell-protoplasm is nucleo-proteid, that is a compound of proteid 

 with var\-ing amounts of niiclein. White of egg is a familiar instance 

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

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

 that the greater number of the proteids 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 ; clioles- 

 terin, 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 

 l^ody 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. 



We may now proceed to an enumeration of the chemical con- 

 stituents of the animal body, and group them in a systematic way. 



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, n;trogen, 

 oxygen, sulphur, phosphorus, fluorine, chlorine, iodine, siUcon, 

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

 sionally manganese, copper, and lead. 



Of these very few occur in the free state. Oxygen (to a small extent) 

 and nitrogen are found dissolved in the blood-plasma ; hydrogen is 

 formed by putrefaction in the ahmentary canal. With some few ex- 

 ceptions such as these, the elements enumerated above are found 

 combined with one another to form compounds. 



b2 



