Excretion (Lat. excemere, excretum, to purge), the elimination of waste or effete matters from the living body. There is evidence that during the vital processes every exertion of activity by a living tissue or or-gan is necessarily accompanied by a molecular change in its chemical constitution. So intimate is this connection between the alteration of substance in a living organ and its physiological action, that it is impossible to say with certainty which of these two is the cause and which the effect. The fact is however that, as we have said above, every manifestation of vital activity involves a change in the immedi-ate constitution of the active organ. The consequence of this is that, in the living body, new substances, the result of its internal disintegration, are constantly making their appearance. These substances, termed excre-mentitious matters, must not be allowed to remain and accumulate; for in that case the constitution of the organs would become so changed from their original condition that they would be no longer capable of performing their proper functions. These matters must therefore be gotten rid of, or eliminated from the body, as fast as they are produced; and the process by which this is accomplished is called excretion.

The mechanism of this process is as follows: The excrementitious matters produced in the solid tissues are absorbed from them by the blood, carried by the circulation to some organ adapted to the purpose, exhaled or exuded in the gaseous, fluid, or semi-fluid form, and thus discharged from the body. The two principal excretory organs are the lungs and the kidneys. The venous blood in passing through the lungs discharges the carbonic acid which it has absorbed from all the vascular parts of the body, and returns to the left side of the heart purified and renovated. The blood which passes through the circulation of the kidneys exhales, together with its watery parts, urea, creatine, creatinine, and the compounds of uric acid; nitrogenous crystallizable matters produced in various parts of the system, and which form the important ingredients of the urine. Thus the blood constantly relieves the solid tissues of the excrementitious matters produced in their substance, and is itself relieved of them by passing through the excretory organs. Should this process from any cause be suspended or retarded, the accumulation of excrementitious matters in the body would soon make itself felt by a derangement of the health, and especially by its injurious effects upon the nervous system.

Pain, loss of appetite, confusion of mind, disturbance of the special senses, and in extreme cases convulsions, coma, and death, result from the arrest of excretion, which is therefore no less important to life than nutrition.