12 GENERAL STRUCTURE OF THE BODY. 



are the peptones, the final result of protein digestion, 

 from which the albuminoids differ in that they contain 

 no sulphur. Ferments containing nitrogen exist in all 

 the cells of the body, though more particularly in those of 

 the digestive organs, and the coloring matters, as the 

 bilirubin of the bile, are nitrogenous. 



The organic substances that do not contain nitrogen 

 are the carbohydrates or starches, the hydrocarbons or 

 fats, and the acids, of which the most important is cnr- 

 bon dioxide, given off by the lungs. 



The inorganic substances are water, which forms a 

 large percentage of all the tissues and from one-fourth 

 to one-third of the whole body weight, sodium chloride 

 or common salt, which plays an important part in keep- 

 ing substances in solution, potassium and magnesium 

 chloride, and hydrochloric acid, found in the stomach. 



The Cell. Although the body is a very complex organ- 

 ism, the cell is its unit or foundation. In fact, the body 

 begins life as a single protoplasmic cell, the ovum, which 

 is frequently compared to the amoeba, a microscopic 

 animal consisting of a single cell of protoplasm or living 

 substance a substance not well understood as yet but 

 possessing practically all the functions of the human body. 

 For, although it has no organs and is homogeneous in 

 structure, the amoeba can move by throwing out a pro- 

 cess, and can surround and absorb food, which it builds 

 up into new tissue, discarding the waste. The ovum, 

 however, differs' from the amceba in that it has a trans- 

 parent limiting membrane and contains a darker spot, 

 the nucleus. This in turn contains another smaller spot, 

 the nucleolus, while through the protoplasm, which is 

 semi-fluid, extends a fine network that seems to hold it 

 in place. 



The ovum is very small, about y^-y inch in diameter, 

 and after fertilization grows by segmentation, the nucleus 

 dividing in two and the protoplasm grouping itself anc\v 

 about the two nuclei. This division continues, each cell 

 dividing and forming two, or sometimes four, new cells, 



