When classified according to their essential structure, the tissues fall into four main groups: epithelial and glandular tissue, muscular tissue, nervous tissue, and connective tissue. According to this system the osseous, cartilaginous, and adipose tissues are classed as varieties of connective tissue. See page [18].
The properties of substances are the qualities or characteristics (color, weight, etc.) by means of which they are recognized.
Certain of these cells also form deposits of fat, giving rise to the adipose, or fatty, tissue.
Any organized structure, such as the body, whose parts are pervaded by a common life, is known as an organism. The term "organism" is frequently applied to the body.
In birds, reptiles, amphibians, and fishes the red corpuscles have nuclei (Fig. 9).
The micron is the unit of microscopical measurements. It is equal to 1/1000 of a millimeter and is indicated by the symbol μ.
The peculiar shape of the red corpuscle has no doubt some relation to its work. Its circular form is of advantage in getting through the small blood vessels, while its extreme thinness brings all of its contents very near the surface—a condition which aids the hemoglobin in taking up oxygen. If the corpuscles were spherical in shape, some of the hemoglobin could not, on account of the distance from the surface, so readily unite with the oxygen.
The coloring matter of the bile consists of compounds formed by the breaking down of the hemoglobin; the spleen contains many large cells that seem to have the power first of "engulfing" and later of decomposing red corpuscles. A further evidence that the spleen aids in the removal of worn-out corpuscles is found in the fact that during diseases that cause a destruction of the red corpuscles, such as the different forms of malaria, the spleen becomes enlarged.
An infected part of the body, such as a boil or abscess, should never be bruised or squeezed until the time of opening. Pressure tends to break down the wall of white corpuscles and to spread the infection. Pus from a sore contains germs and should not, on this account, come in contact with any part of the skin. (See treatment of skin wounds, Chapter XVI.)
Coagulation is not confined to the blood. The white of an egg coagulates when heated and when acted upon by certain chemicals, and the clabbering of milk also is a coagulation.