28 



STUDIES IN PHYSIOLOGY 



other cells. A bone section, therefore, looks as though it 

 were filled with a multitude of amcebas, each of which 

 has extended several branching false feet. Between these 

 irregular bone cells is the hard intercellular substance that 

 gives rigidity to this kind of tissue. 



Definition of a Tissue. A tissue may now be defined as a 

 building material of an organ, composed of cells of the same 

 kind, together with more or less intercellular substance. 



3. SOME OF THE PROPERTIES OF PROTOPLASM 



Microscopic Appearance. Protoplasm, when examined with 

 the highest powers of the micro- 

 scope, appears as a colorless, semi- 

 fluid substance, in which are usually 

 seen solid particles or granules. 

 The nucleus is commonly found 

 near the center of the cell and is 

 composed of protoplasm denser 

 than that of the cell body. The 

 appearance and consistency of the 

 protoplasm surrounding the nu- 

 cleus, that is in the cell body, may 

 be well represented by the raw 

 white of an egg ; but in making this comparison one should 

 bear in mind that the white of egg is not living substance. 



Chemical Composition. Living substance contains a large 

 amount of water, which keeps it semi-fluid. If we dry pro- 

 toplasm, we kill it. While the amount of water in the 

 different tissues, as we have seen, varies considerably, it con- 

 stitutes on the average nearly two thirds of the body weight. 

 The most important constituents of living cells, however, are 

 the proteids. These complex substances are always present 

 in protoplasm, and, so far as we know, they can be made only 

 by protoplasm. Particles of sugar, fat, and other food sub- 

 stances, are usually found in protoplasm and help give to 

 the cell body its granular appearance. 



FIG. 9. Parts of a Cell. 



p = protoplasm of cell body. 

 n = nucleus. 



