RESPIRATION. The essential process in respiration is the taking in of oxygen and the giving out of carbon dioxide by the living cells of a plant or animal. In higher animals special organs, the lungs (q.v.), and a special tissue, the blood (q.v.), are the main factors in this exchange; but in the lower animals various of structures are utilized. Thus in insects there are series of ramifying tubes into which the air circulates; in water animals there are gills, which are the precursors of lungs in land animals. Even those higher animals in which the major part of the oxygen exchange takes plate in the lungs (external respiration), there is also an internal respiration constantly taking place in the exchange between the blood, the lymph and other tissues. Respiration in plants is of the same general character as in animals. It is not to be confounded with the much more active process of photosynthesis, by which plants utilize carbon dioxide and give off oxygen, a process which was termed respira tion by the older plant phySiologists.
Historical earliest natural ists and philosophers, Aristotle and others, even down to the anatomists of the 15th century, taught that the function of respiration was to draw air into the arteries and heart, and thus to cool the blood. The old Galenic doctrine was that the °air introduced by breathing served to regulate, to maintain and at the same time to temper, to refrigerate the , innate heat of the heart.° Again the pumping action of the thorax introduced into the blood the air which was necessary to generate the vital spirits in the left side of the heart, and filially the same pumping action got rid of the foul vapors, the product of the innate fire burning in the heart. Harvey was one of the first to show that the same channel probably did not serve to carry these two different currents. But the chemical prob lems involved were too intricate for Harvey, although it was chiefly through the work of a group of Englishmen of the 17th century that the unraveling of the mystery of respiration came about. Von Helniont had taught a theory of fermentations, and he had notgd the differ ence in color in`blood passing through the Inng.s. Borelli approached the mechanical side of the problem and showed the truth regarding the mechanics of the chest, the action of which took in by atmospheric pressure and expelled the air by the muscular relaxation and the elasticity of the chest-walls. Malpighi had in the meantime discovered, with the aid of his crude microscopes, the minute structure of the lung, and Borelli, as well as others (Hooke, 1667; Croon and Boyle, 1666), working with the newly discovered principle of the air-primp and -vacuum,. had learned that air was absolutely essential to the life of the animal, and had ad vanced to the point that the particles of the air taken into the lungs enter into and become mixed with the blood. But Borelli was a physi cist, and had no sympathy with the new chemical learning, which was then a very vague subject. Hooks and Lower made the next steps possible, the former by his artificial-respiration experi ments, proving the necessity for air, and the hitter by his careful transfusion experiments, demonstrating that the difference between dark venous blood and lighter arterial blood was due to the admixture of air, thus explaining this color change, which had been recorded for thousands of years and referred to thousands of causes. Up to this time it had always been
thought that the air was a simple substance, and not a mixture. John Mayow (1643) first showed that not all of the air was used by the lungs in influencing the blood, but only a certain part; and although he called it by a different name (spiritus nitro-aerius), it was that part now known as oxygen. Mayow thus developed the first full adequate physico chemical theory of respiration. He saw that respiration was a process analogous to com bustion, and that the movements were to take in the nitro-aerial gas and give out the vapors arising from the blood. But he did not know at all what these vapors were. He had grasped at the meaning of the intake, but had no knowl edge of the exchange products. He said: In the first place then, I take it for granted that the air contains certain particles termed by us elsewhere nitro-aerial which are absolutely in dispensable for the production of fire, and that these in the burning of flame are drawn from the air and removed, so that the latter when deprived of these particles ceases to be fit for supporting fire.* ((With respect, then, to the use of respiration, it may be affirmed that an aerial something, whatever it may be, essential to life passes into the mass of the blood. And thus air driven out of the lungs, these vital particles having been drained from it, is no longer fit for breathing again.* This and other wonderful discoveries were made by a young man of 25, and who died at the age of 34. Mayow's work was then forgotten for almost 100 years. As Foster expresses it, that which in the first years of the latter half of the 17th century as igneo-aerial particles shone out in a flash and then died away again into darkness, in the last years of the 18th century, as oxygen, lit a light which has burned, and which has lighted the world with increasing steadiness up to the present day." The names of Stahl, Hal ler, Boerhaave appear as keeping alive the study of respiration. Stephen Hales, an active parish priest, and the earliest sanitarian, was an ardent student of chemistry. He first enunciated the principle of free and combined gases, and led the way to the work of Joseph Black, who re discovered Von Helmont's "fixed air,* which was given off by the lungs, and that animals placed in the air" died much as if they were deprived of fresh air. The next step was made by Priestley, who in 1772 rediscovered Mayow's spirit and isolated oxy gen. Priestley's views, however, were permeated by the century-old phlogiston theory, and it remained for Lavoisier clearly to demonstrate what oxygen really was and to overthrow the phlogiston theory. Thus the problem of animal respiration was solved. The air, composed mostly of innocuous nitrogen and of small pro portions of active oxygen, passes into fine air vesicles, first seen by Malpighi, the blood there takes up the free oxygen, giving off carbon dioxide, which exchange constitutes the es sential feature of respiration. It should be added that it was not until Gustavus Magnus (1837) proved the presence of the blood-gases in different proportions in the blood that the present theory of respiration assumed a defi nite form. The study of the relation of the iron of the hemoglobin in the blood to this oxygen exchange constitutes the last chapter in the unraveling of this secret of nature.