Page:A Handbook of the Theory and Practice of Medicine - Volume I - Frederick T. Roberts.djvu/88

72 is derived from the colouring matter of the blood, which undergoes certain changes. This colouring matter may be present in some localized part, either from its mere transudation through the coats of the vessels; from the migration of red corpuscles; from actual hæmorrhage having taken place; or from capillary stagnation. At first it is diffused, and stains the tissues which it involves, especially the cells, colouring their contents but leaving the nucleus and envelope unaffected. After a while the colour changes, assuming a variety of tints, such as yellow, yellowish-brown, brown, reddish-brown, dark-brown, grey, or black; the exact hue depends much upon the length of time the pigment has been formed, and the tissues which it occupies. At the same time it separates into minute molecules, or crystals, or both, which are found within or outside the cells. The molecules may aggregate into larger granules; they are round or irregular in shape, well-defined, opaque or sometimes glistening in appearance. The crystals are in the form of minute oblique prisms, needles, or plates, which present various tints of yellow, red, brown, or black, and have a shining aspect. These particles are very persistent, and are not easily destroyed. They resist the action of acids; water, alcohol, and ether do not affect them, but they are dissolved by strong alkalies, forming a red solution, while concentrated mineral acids bring out a series of changes of colour. Chemically the pigment is supposed to consist of hœmatoidine, but when it becomes black it is named melanin. Examples of these changes of blood-pigments are frequently met with in connection with cerebral apoplexy; in pulmonary congestion or hæmorrhage; in subcutaneous extravasations; and in the formation of the corpus luteum in the ovary. The alterations in the colour of the "rusty expectoration" of pneumonia during the progress of this disease are due to the same cause. In certain affections the blood itself contains corpuscles enclosing a quantity of black pigment. This occurs in the rare disease called melanœmia; as well as after prolonged attacks of ague, when the spleen becomes enlarged and contains

an abundance of the same pigment, which is supposed to be absorbed into the blood. It is, however, originally derived from the blood-pigment. The same condition is observed in melanotic tumours. Occasionally the minute portal vessels of the liver become filled with black pigment.

In some instances pigment is supposed to be directly formed