Page:George Archdall Reid 1896 The present evolution of man.djvu/223

Rh from the blood-vessels into the tissues. When a solid organ is injured—e.g. the skin by a cut or a bruise—these cells appear in great numbers at the damaged spot, infiltrate the surrounding tissues, proliferate rapidly, adhere together, and undergo gradual differentiation, till a greater or lesser mass of dense connective tissue, a scar, is formed and healing is completed. Should the injury be severe or prolonged, more or less of them perish from various causes,—e.g. the poisons secreted by bacteria,—and the clear lymph is rendered turbid by their dead bodies; that is, pus is formed as in an abscess or on the surface of a sore. Should the blood be injured by the presence of solid impurities non-living—e.g. artificially introduced particles of vermilion or carbon—or living—e.g. bacteria or protozoa—the leucocytes, called phagocytes from their function, here also perform reparative functions. The impurities, living or non-living, are taken up by them into their substance, and an attempt (successful or unsuccessful as the case may be) is made at digestion; that is, to convert the impurities into food for the cells, or into soluble substances capable of being excreted by such organs as the skin and kidneys. Solid non-living foreign impurities, from the nature of the case, must under normal circumstances be rare in the blood. Living foreign impurities occur in many of the zymotic diseases—e.g. anthrax—when the bacteria producing these diseases may often be seen enclosed in the cell-substance of the phagocytes.

The pathogenic micro-organisms of some zymotic diseases—e.g. diphtheria and tetanus—do not enter the blood stream, and so pervade the whole body, but remain limited to a definite area of the inner or outer