38 PRINCIPLES OF PHYSIOLOGY 



sary to fix our attention on what is known as 

 a cel^ the first and smallest unit of structure 

 from which the tissues of the body are formed. 

 If we examine the body, say of a rabbit, we find 

 it is built up of various tissues. Thus we find 

 the flesh, consisting chiefly of muscular tissue, 

 the cartilages or gristles, the bones, the brain 

 and structures related to it (constituting the 

 nervous system), and the various glands and 

 internal organs, such as lungs, stomach and 

 liver. But if, by appropriate methods, such 

 as are used in the examination of tissues and 

 organs by the microscope, we pursue the 

 analysis farther, and more especially if we 

 study the tissues at various periods in their 

 development, we find that they all originate 

 from cells. A cell is a small bag or vesicle, 

 varying in size from the 1/6000 of an inch 

 to bodies just visible to the naked eye. Ante- 

 cedent even to cells, there is a still more 

 primitive substance known as protoplasm. 

 It is a jelly-like, colourless, or faintly yellow 

 substance, having often embedded in it 

 minute granules. It may either form large 

 masses, as in certain fungus-like forms, or it 



