8 VITAL FUNCTIONS OF PROTOPLASM. 



4. Excretion. — Movement implies a loss or expenditure 

 of energy which is furnished by the chemical decomposi- 

 tion of protoplasm or its constituents, resulting in its 

 turn in the formation of waste products or excreta. These 

 products have to be removed, and, in the simplest organisms, 

 they are extruded at the limiting surface. The carbon of 

 proteids is removed in combination with oxygen, as 

 carbonic acid gas, and the hydrogen and oxygen as water. 



For this purpose oxygen is taken into the interior of 

 the body. This form of excretion is often called Respira- 

 tion. It involves the introduction of oxygen and the 

 extrusion of carbonic acid gas. In addition, the nitrogen 

 and sulphur of the proteids leave the body, in combination 

 with other elements, as complex nitrogenous compounds, 

 such as urea. Thus the waste products are of two kinds, 

 non-nitrogenous and nitrogenous, removed by Respiration 

 and Nitrogenous Excretion respectively. 



The taking-in of oxygen during respiration should be 

 carefully distinguished from the ingestion of *' food," as also 

 should excretion from the egestion of waste residue or 

 faeces. Ingestion and egestion are processes of alimenta- 

 tion, which itself is part of the building-up of fresh 

 protoplasm, whereas respiration and excretion are processes 

 essentially connected with the breaking-down or consump- 

 tion of protoplasm. A starving man will, unfortunately 

 for himself, continue to respire and excr.ete though the 

 alimentary function be in abeyance. 



Secondary Vital Functions of Protoplasm. 



I. Growth. — It is quite conceivable that protoplasm 

 might carry on the above functions in such a manner that 

 the waste and repair were exactly balanced, in which case 

 the original protoplasm would remain the same in size 

 and other relations. This, however, is not the natural 

 state of matters. Given suitable conditions, an organism 

 will acquire a credit account with nature, and the result is 

 a continued production of fresh protoplasm and increase 

 in bulk or growth. In the case of living organisms growth 

 takes place by addition throughout the bulk of the body, 

 and is called growth by intussusception to distinguish it 



