The Carbohydrates are formed of carbon, hydrogen, and oxygen, the last two in the proportion to form water.  Thus we have animal starch, or glycogen, stored up in the liver.  Sugar, as grape sugar, is also found in the liver.  The body of an average man contains about 10 per cent of Fats.  These are formed of carbon, hydrogen, and oxygen, in which the latter two are not in the proportion to form water.  The fat of the body consists of a mixture which is liquid at the ordinary temperature.

Now it must not for one moment be supposed that the various chemical elements, as the proteids, the salts, the fats, etc., exist in the body in a condition to be easily separated one from another.  Thus a piece of muscle contains all the various organic compounds just mentioned, but they are combined, and in different cases the amount will vary.  Again, fat may exist in the muscles even though it is not visible to the naked eye, and a microscope is required to show the minute fat cells.

10.  Protoplasm.  The ultimate elements of which the body is composed consist of “masses of living matter,” microscopic in size, of a material commonly called protoplasm.[2] In its simplest form protoplasm appears to be a homogeneous, structureless material, somewhat resembling the raw white of an egg.  It is a mixture of several chemical substances and differs in appearance and composition in different parts of the body.

Protoplasm has the power of appropriating nutrient material, of dividing and subdividing, so as to form new masses like itself.  When not built into a tissue, it has the power of changing its shape and of moving from place to place, by means of the delicate processes which it puts forth.  Now, while there are found in the lowest realm of animal life, organisms like the amoeba of stagnant pools, consisting of nothing more than minute masses of protoplasm, there are others like them which possess a small central body called a nucleus.  This is known as nucleated protoplasm.

[Illustration:  Fig. 1.—­Diagram of a Cell.

  A, nucleus;
  B, nucleolus;
  C, protoplasm. (Highly magnified)
]

11.  Cells.  When we carry back the analysis of an organized body as far as we can, we find every part of it made up of masses of nucleated protoplasm of various sizes and shapes.  In all essential features these masses conform to the type of protoplasmic matter just described.  Such bodies are called cells.  In many cells the nucleus is finely granular or reticulated in appearance, and on the threads of the meshwork may be one or more enlargements, called nucleoli.  In some cases the protoplasm at the circumference is so modified as to give the appearance of a limiting membrane called the cell wall.  In brief, then, a cell is a mass of nucleated protoplasm; the nucleus may have a nucleolus, and the cell may be limited by a cell wall.  Every tissue of the human body is formed through the agency of protoplasmic cells, although in most cases the changes they undergo are so great that little evidence remains of their existence.