-LQ PHYSIOLOGY. 



occasionally found indigestible bodies, such as grains of sand, indi- 

 gestible residue of foodstuffs, and excretory substances, waiting to 

 be expelled from the body. 



Other substances found within the protoplasm and supposed to 

 be of great importance to cell-life are drops of liquid vacuoles, as 

 they are commonly called. 



Specific Gravity of Living Protoplasm. 



Living protoplasm has the physical property of having a greater 

 specific gravity than water. When cells of the most varied kinds are 

 allowed to fall into water they sink to the bottom. In some cases the 

 protoplasm contains a considerable quantity of fat; so that, although 

 the substratum of protoplasm is heavier than water, the floating of 

 the cell is due to the lighter specific gravity of the fat-particles over- 

 coming the heavier specific gravity of the protoplasm. 



The chemical composition of protoplasm (a living substance) 

 can be obtained only after it has been killed. However paradoxical 

 this may seem, it is found impossible to apply the methods of chem- 

 istry without killing it. Every reagent that comes in contact with it 

 disturbs and changes it and eventually kills it. Thus, our ideas of 

 the chemical composition of living protoplasms are the ideas we get 

 from 'the chemical composition of dead protoplasm. 



The substances of which it is composed are: 



1. Water. Water is that element in a living substance that gives 

 .it its liquid nature, allowing its particles to move about with a cer- 

 tain degree of freedom. In the cell, water occurs, either chemically 

 combined with other constituents or in the free state. Salts occur 

 dissolved in the water. Protoplasm is semifluid, and about three- 

 fourths of its weight is due to water. The molecules of protoplasm 

 are thought to be separated from one another by layers of water. 



2. Proteids. The proteids take a very active and essential part 

 in the functions of all cells. The proteids consist of the elements 

 carbon, hydrogen, sulphur, nitrogen, and oxygen. Proteids occur 

 both in the protoplasm and in the nucleus, but with this difference: 

 that found in the nucleus has combined with it phosphoric acid, 

 forming the so-called nucleins. To show this fact is very easy, for 

 the nuclein of cells resists the action of digestion by the gastric juice. 

 All kinds of cells in artificial gastric juice have their protoplasm 

 digested and only the nuclei remain; that is, nuclein. If, now, this 

 nucleus is treated with stains, it shows that the nuclear bodies consist 



