In a former part of this article we have de tailed several observations, both upon the human species and the lower animals, to prove that a quantity' of azote is frequently exhaled in respiration. The experiments of Allen and Pepys ***, and Nysten -III, show that the exhalation of azote is considerably increased by breathing oxygen or hydrogen, or a mixture of these two gases, and thus afford additional evidence that free azote exists in the blood. Marchand concludes from his experiments on frogs, that when they are made to breathe pure oxygen gas, azote is evolved from the blood, and that when made to breathe pure hydrogen, both oxygen and azote are evolved from the blood.tIt conditions of the red corpuscles can be changed by the action of various agents, such as pure water, and solutions of certain neutral salts. By the action of the former, the corpuscles swell, become more globular, and reflect less light ; by the action of the latter, they become smaller, thinner, somewhat bent and notched, and reflect more light. These changes are apparently dependent upon endosmotic and exosinotic currents, between the fluid contents of the red corpuscles and the surrounding fluid. It has been maintained that the red cor puscles of venous and arterial blood differ in their external form,— the former approaching in their shape those acted upon by water, the latter those subjected to the action of solutions of the neutral salts ; and this change in the form of the corpuscles has been adduced as the cause of the difference in colour between arterial and venous blood. Kaltenbrunner *, Schultz t, H. Nasse Scherer §, Reuter I], Mr. Gulliver 1, and Harless **, have de scribed various differences in the external form of the red corpuscles of the two kinds of blood, as observed by them under the microscope, from which some of them infer an increase in their power of reflect ing light ; while Burdach tT, Muller §§, Bruch II II, and Marchand IfIr, have failed in de tecting by the microscope any difference in their external form in the two kinds of blood.*** Those observers who have described differ ences in the shape of the red corpuscles in arterial and venous blood do not quite agree in their account of these. They agree, how ever, in this, that the red corpuscles are more turgid and less clear in venous than in arterial blood. Scherer describes the red corpuscles in arterial blood as biconcave, and those in venous blood as biconvex and de cidedly swollen. Mr. Gulliver states that in all his experiments " the red corpuscles were reduced in size, both in breadth and thickness, by neutral salts, and in a less degree by sugar and oxygen ; while the first effect of water and of carbonic acid was to swell the cor puscles and make them more globular." Nasse says that the red corpuscles of the arterial blood in the mammalia, on the contact of car bonic acid gas, become muddy in the middle, the ring formed by the colouring matter be comes broader, they become darker and some what thicker, at least on one side, and they adhere closer together. Harlass gives measure ments of the corpuscles of the blood of the frog, when brought into contact with oxygen and carbonic acid, to show that they becoine somewhat broader and thicker when exposed to the action of the latter gas. He also states that while the corpuscles in the former are finely granulated on the external surface, those in the latter are smooth.
Theory of respiration. — The actions be: tween the blood and the atmospheric air in the performance of the function of respiration are regulated entirely by chemico-physical laws. No doubt the blood and air are con veyed to and from the lungs through the in: strumentality of the vital properties of the nervous and muscular tissues, but the changes they there undergo do not appear to be in fluenced by vitality. When venous blood and atmospheric air are brought into contact out of the body, the same actions apparently occur as in the lungs during life, viz., the atmospheric air loses part of its oxygen, acquires in its place a quantity of carbonic acid gas, and the blood assumes the arterial hue. The distri bution of the blood in innumerable minute streamlets upon the surface of the air-cells, filled with atmospheric air, affords much more advantageous means than can be obtained in experiments out of the body, for facilitating the mutual actions of the blood and atmo spheric air. From the known rapidity with which gases permeate both living and dead animal membranes, the moist delicate mem branes that intervene between the blood con tained in the capillaries of the lungs, and the atmospheric air in the air-cells, will readily permit the endosmose of a portion of the at mospheric air, and the exosmose of a portion of the gases held in solution in the blood.
The rest of our remarks on the theory of respiration may be arranged under three heads : viz. 1st, the manner in which the air in the upper and in the lower parts of the respiratory apparatus is intermixed ; 2dly, the nature of the immediate actions between the blood and atmospheric air in the lungs, in which a quan tity of carbonic acid gas appears in the expired, and a quantity of oxygen disappears from the inspired air ; 3dly, the nature of the changes the blood undergoes in passing from the venous to the arterial condition.
On the manner in which the air in the upper and lower parts of the respiratory apparatus beconies intermixed.—The respiratory qualities of the other parts of the inner surface of the air-passages must be very feeble when com pared with the membrane of the air-cells of the lungs ; and there can be no doubt that almost all the carbonic acid present in the txpired air is derived from the blood circula ting in the capillary blood-vessels of the air cells ; and that this evolution of carbonic acid gas is continuous, going on during expiration as well as during inspiration. As a portion only of the atmospheric air, probably not much more than a fourth or a fifth part, is re newed at each ordinary respiratory movement when the body is in a state of rest, the air expelled during expiration will chiefly consist of that occupying the larynx, trachea, and the larger bronchial tubes ; so in the same man ner, the air drawn in by inspiration will chiefly occupy the same parts of the respira tory apparatus. It is well known that the air expelled in the first part of an expiration contains less carbonic acid than that expelled towards its close ; thus the air in the deeper parts of the respiratory apparatus must be richer in carbonic acid and poorer in oxygen than that in the upper parts. The amount of intermixture of the gases in the different parts of the respiratory apparatus effected by the muscular movements of the chest would, in all probability, be too imperfect for the proper arterialisation of the blood, were this not aided by the well-known tendency of gases to diffuse themselves through each other. As the air in the air-cells differs from that in the higher parts of the respiratory apparatus in containing more carbonic acid anti less oxy gen, the nitrogen being nearly the same in both, this diffusion of gases is probably chiefly confined to the two former. From the oxy gen being of lighter specific gravity than the carbonic acid gas, the descending current of oxygen gas will exceed the ascending current of carbonic acid, and 81 parts of carbonic acid will be replaced by 95 of oxygen, for according to the law regulating the diffusion volumes of gases under such circumstances, established by Graham, in the case of each gas this is inversely proportional to the square root of its density.* On the nature of the actions between the blood and the atmospheric air in the lungs, by which a quantity of oxygen is removed from the inspired air, and a quantity of carbonic acid gas added to the expired air. —Four views have been maintained on this point.— I. That of Lavoisier, La Place, and others ; that the oxygen which disappears from the inspired air unites directly in the lungs with hydro carbon furnished by the venous blood, and forms the carbonic acid gas and watery vapour that escape along with the expired air.-1 2. That of La Grange and llassenfratz; that free carbonic acid gas is present in a state of solution in the venous blood before it arrives at the lungs, where this gas is ex haled ; that nearly the whole of the oxygen gas abstracted from the inspired air is absorbed at the lungs, and held in solution by the arterial blood ; and that the combination of the oxygen with the carbon and formation of carbonic acid chiefly take place when the blood is passing through the capillaries of the systemic circulation.* 3. That the oxygen that disappears from the inspired air enters into chemical combina tion with one or more of the constituent parts of the blood in its course through the lungs, that in the passage of the blood through the capillaries of the systemic circulation this oxygen leaves the substance or substances to which it had united itself, and combines with carbon to form carbonic acid, or with carbon and hydrogen to form carbonic acid and vvater, and that the carbonic acid thus formed does not combine chemically with any of the constituent parts of the venous blood, but is held in solution by it, and is evolved while passing through the capillaries of the lungs.