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Rh to their weight far higher in the smaller ones. In a mouse-or small bird, for instance, the rate is about twenty times as great as in a man. The difference is in part due to the fact that the smaller an animal is the greater is its surface relatively to its mass, and consequently the more heat does it require to keep up its temperature. The smaller animal must therefore produce more heat. Even in cold-blooded animals, however, oxidation appears to be more rapid the smaller the animal. In the case of man, oxidation is relatively more than twice as rapid in children than in adults, and the difference is greater than would be accounted for by the difference in the ratio of surface to mass. Allowing for differences in size, oxidation is about equally rapid in men and women.

It was for long believed that the special function of respiratory oxidation was (1) the production of heat, and (2) the destruction of the supposed “waste products.” Further investigation has, however, tended to show more and more clearly that in reality respiratory oxidation is an essential and intimate accompaniment of all vital activity. To take one example, secretion and absorption, which were formerly explained as simple processes of filtration and diffusion, are now known to bel accompanied, and necessarily so, by respiratory oxidation in the tissues concerned. The respiratory oxidation of an animal is thus a very direct index of the activity of its vital processes as a whole. Looking at what is known with regard to respiratory oxidation, we see that what is most striking and most characteristic in it is its tendency to persist-to remain on the whole at about a normal level for each animal, or each stage of development of an animal. The significance of this cannot be over-estimated. It indicates clearly that just as an organism differentiates itself from any non-living material system by the manner in which it actually asserts and maintains its specific anatomical structure, so does it' differentiate itself from any mere mechanism by the manner in which it asserts and maintains its specific physiological activities.

.—For further general information the reader may be referred to the sections by Pembrey and by Gamgee in Schafer's Handbook of Physiology, vol. i., and by Bohr in Nagel's Handbuch der Physiologie, vol. i. The following additional references are to recent investigations: Regulation of Breathing, Haldane and Priestley, Journal of Physiology, xxxii. 225 (1905). Respiration at High Altitudes and Effects of Want of Oxygen, Zuntz, Loewy, Caspari, and Müller, Das Höhenklima (1905); Boycott and Haldane, Ward, and Haldane and Poulton, Journal of Physiology, xxxvii. (1908). Respiration at High Pressures, “Report to the Admiralty of the Committee on Deep Diving" (1907). Respiratory Exchange and Secretion, Barcroft, Journal of Physiology, xxvii.-31 (1901); Barcroft and Brodie, Journal of Physiology, xxvii. 18, and xxxiii.,52 (1905). Excretion of CO2 by the Lung Epithelium, Bohr, Zentralblatt fur Physiologie. 337 (2907). “Normal Alveolar CO2 Pressure in Man, ” Mabel Fitzgerald and J. Haldane in Physiological Journal (1905).

Normal Respiration.—If the naked body of a person asleep or in perfect inactivity be carefully watched, it will be found that the anterior and lateral walls of the chest move rhythmically up and down, while air passes into and out of the nostrils (and mouth also if this be open) in correspondence with the movement. If we look more closely we shall find that with every uprising of the chest walls the membranous intercostal portions sink slightly as if sucked in, while at the same time the flexible walls of the abdomen bulge as if protruded by some internal force. If respiration be in the slightest degree hurried, these motions become so marked as to escape the attention of no one. The elevation of the chest walls is called inspiration, their depression, expiration. Inspiration is slightly shorter than expiration, and usually there is a slight pause or momentary inaction of the chest between expiration and the following inspiration. Apparatuses for measuring the excursion of a given point of the chest wall during respiration are called thoracometers or stethometers. Apparatuses for recording the movements of the chest are called stethographs or pneumographs.

Frequency of Respiration.—The frequency of respiration during perfect rest of the body is 16 to 24 per minute, the pulse rate being usually four times the rate of respiration; but the respiratory rhythm varies in various conditions of life. The following are the means of many observations made by Lambert Adolphe Quételet (1796–1874): at the age of one year the number of respirations is 44 per minute; at 5 years, 26; from 15 to 20 years, 20; from 25 to 30, 16; from 30 to 50, 18.1. Muscular exertion always increases the frequency of respiration. The higher the temperature of the environment the more frequent is the respiration. Paul Bert (1833–1886) has shown that with higher atmospheric pressures than the normal the frequency of respiration is diminished while the depth of each inspiration is increased. The frequency of respiration diminishes until dinner-time, reaches its maximum within an hour of feeding, and thereafter falls again; if dinner is omitted, no rise of frequency occurs. The respiratory act can be interrupted at any stage, reversed, quickened, slowed and variously modified at will, so long as respiration is not stopped entirely for more than a short space of time; beyond this limit the will is incapable of suppressing respiration., Depth of Respiration.-The depth of respiration is measured by the quantity of air inspired or expired in the act; but the deepest expiration possible fdoes not suffice to expel all the air the lungs contain. The following measurements have been ascertained, and are here classified according to the convenient terminology proposed by John Hutchinson (1811–1861). (1) Residual air, the volume of air remaining in the chest after the most complete expiratory effort, ranges from 100 to 130 cub. in. (2) Reserve or supplemental air, the volume of air which can be expelled from the chest after an ordinary quiet expiration, measures about 100 cub. in. (3) Tidal air, the volume of air taken in and given out at each ordinary respiration may be stated at about 20 cub. in. (4) Complemental air, the volume of air that can be forcibly inspired over and above what is taken in at a normal inspiration, ranges from about Ioo to 130 cub. in. By vital capacity, which once had an exaggerated importance attached to it, is meant the quantity of air which can be expelled from the lungs by the deepest possible expiration after the deepest possible inspiration; it obviously includes the complemental, tidal and reserve airs, and measures about 230 cub. in. in the Englishman of average height, i.e. 5 ft. 8 in. (Hutchinson). It varies according to the height, body weight, age, sex, position of the body and condition as to health of the subject of observation.

Vital capacity is estimated by means of a spirometer, a graduated gasometer into which air may be blown from the lungs. The residual air, which for obvious reasons cannot be actually measured, may be estimated in the following way (Emil Harless, 1820–1862; Louis Gréhant, b. 1838). At the end of ordinary expiration, apply the mouth to a mouthpiece communicating with a vessel filled with pure hydrogen, and breathe into and out of this vessel half a dozen times-until, in fact, there is reason to suppose that the air in the lungs at the time of the experiment has become evenly mixed with hydrogen. Then ascertain by analysis the proportion of. hydrogen to expired air in the vessel and estimate the amount of the air which the lungs Contained by the following formula:—

v : V+v = p : 100;

V=$v(100−p)⁄p$;

where V=volume of air in the lungs at the time of experiment, v=volume of the vessel containing hydrogen, p=proportion of air to hydrogen in the vessel at the end of the experiment. V, then, is the volume of air in the lungs after an ordinary expiration; that is, it includes the residual and the reserve air; if we subtract from this the amount of reserve air ascertained by direct measurement, we obtain the 100–130 cub. in. which Hutchinson arrived at by a study of the dead body.

Volume of Respiration.—It is clear that the ventilation of the lungs in ordinary breathing does not merely depend on