8 VITAL FUNCTIONS OF PROTOPLASM. 
4. EXcrETION.—Movement implies a loss or expenditure 
of energy which is furnished by the chemical decompost- 
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 espiration 
and WVitrogenous 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 
feeces. 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 excrete though the 
alimentary function be in abeyance. 
Secondary Vital Functions of Protoplasm. 
1. GrowrH.—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 cutussusception to distinguish it 
