PHYSIOLOGY, the science which treats of the processes which go on in the bodies of living beings under nor mal conditions, and of the use of their various parts or organs. It is divided into plant physiology, animal physiology, and human physiology. One of the simplest forms of animal life is seen in the amoeba of pond water or in the white corpuscle or leucocyte of the blood. These simple organisms are composed each of a single physiological unit, which is termed a cell, that is to say, there are no organs for the carrying on of the various functions of life, but all take place in the same microscopic semi fluid mass of complex organic nature termed the cell protoplasm. All animals, no matter how complex be their struc ture, resemble this primitive type, for they consist of an aggregation of micro scopic cells. In the leucocyte or amoeba all kinds of labor proceed in the same cell which imbibes its own food, prepares its own nutriment from the food so taken in, is capable of moving from place to place, and in the end is capable of re producing its like by division. In the higher animal by the evolution of dif ferent types of cell, which start from the same parent cell initially but de velop afterward along different lines, there come to be formed the various dif ferent tissues, glands and organs, each with its own specific function to perform for the general welfare of the whole mass or colony of cells constituting the animal or individual. Thus the skeletal system develops for the support and maintenance of form of the whole, the muscular system to bring about the movements of the animal as a whole and of its various parts with respect to one another; the digestive system for pre paring the nutriment for the whole sys tem; the vascular or circulatory system with its contained blood for carrying the nutriment so prepared and distribut ing it to all the various parts; the re spiratory system for taking in oxygen from the air, necessary for the combus tion of the foodstuffs and for carrying off the carbon dioxide formed by such combustion in the various tissues; the excretory system for the removal of chemical waste products formed in the tissues; and the nervous system for set ting the animal tissues in communica tion with what is passing both in the body and in the external world.
The source of all the energy of an animal lies in its food, and this is either burned as it were within the tissues, used as fuel for the protoplasmic ma chinery, or used to keep that machinery in repair; in either case the foodstuffs have to be prepared before they can be used. Such preparation is called diges
tion, which consists in making the solid foodstuffs soluble. The digested food is absorbed into the blood, and all of it, except the fat, is carried direct to the liver. The liver, among other functions, regulates the composition of the blood, thus it stores the sugar in its cells and gives it out as the other tissues require. Muscular tissue is the great consumer of sugar. An important foodstuff requir ing no digestion is oxygen, needed by the protoplasm for its life, and also for the burning of fuel within the living machin ery to get heat and energy of motion. The oxygen is held in the BLOOD (q. v.) by means of a special substance which greedily absorbs it from the air in the lungs, and yet gives it up readily to the protoplasm of the tissues. The blood as is well known circulates round and round the body pumped by the heart. It is a stream of food material by which each cell of the tissue is fed. For each cell is close to a capillary (a thin blood vessel) along which the fluid food flows and as it flows the fluid part or plasma exudes and bathes the tissues. The plas ma which has thus passed out of the blood vessels is collected into another system of vessels, the lymphatics, and eventually emptied into one of the great veins. The lymph stream is also the drain into which is thrown by each cell the waste products of its activity. The carbonic acid that is formed in the tis sues is carried away by the blood, and escapes out of the system from the lungs. Some of the useless water is also got rid of in the same way, and some more of it is sweated out by the glands in the skin; the rest is filtered out of the blood by the kidneys. There are many other waste matters besides carbonic acid and water. These are to a large extent prepared for excretion in the liver, and to some extent actually taken out of the blood by that organ, being poured into the intestines, mixed with other matters, dissolved in a. fluid called BILE (q. v.) They are all taken out of the blood by the kidneys, and cast out of the body along with the water fil tered out by the same organs, as urine. For comparative physiology, see the ar ticles on the various functions and groups of animals. Consult works by Loeb, Piersol, Lyle, Cannon, Lamarck, etc.