12 CELLS. 



membranes, cartilages, ligaments, muscles, tendons, nerves, 

 etc., are tissues of the animal body. 



.Through researches made by the aid of the microscope, 

 it has been ascertained that all animal and vegetable tissues 

 are made up of cells, the cells in most cases being more or 

 less modified, according to the nature of the part of the 

 organism. Nay, in a vast majority of cases they are so 

 exceedingly modified, that to the uninstructed eye they 

 present little or nothing of their fundamental form and 

 structure. 



Cells then, or something nearly or quite equivalent to 

 them, are the ultimate structural elements of the animal 

 or vegetable body, as far as the subject is now understood. 

 That is, when the animal or vegetable body is analyzed 

 to the last degree as to the structure of its various parts 

 the microscopist comes to the cell, more or less modified, 

 as the final limit of his analysis (Figs. 3-11). 



A living, active cell, in its simplest form, is an exceed- 

 ingly minute vesicle or sac, filled with a more or less 

 viscid liquid named protoplasm from proton, first, and 

 plasma, from plasso, to form ; within the sac there, is a 

 central particle or portion called the nucleus, and within 

 the nucleus there is, in many cases, a still smaller particle 

 called the nucleolus. In many cases there are two or more 

 nuclei in a cell (Figs. 5, 7, 8). In the liquid of the cell 

 there are often seen many minute granules and globules, 

 besides the nucleus. As to the sac itself, or cell-wall, it 

 must here be stated that it has no structure, so far as is 

 known ; that is, it is structureless. Chemically considered, 

 the cell-wall in animals consists of a nitrogenous com- 

 pound ; in plants, it consists of the substance called cellu- 

 lose, and is composed of carbon, hydrogen, and oxygen. 



It should be remarked here that while our general state- 



