however, considerations of a still weightier character, which must compel us to adopt it. The affinity between the blood and the parts with which it is brought in contact is a chemical fact beyond contradiction. The pressures and motions I have been speaking of follow as the inevitable consequences of that affinity. We can not, therefore, gainsay their existence in the living mechanism, and the only doubt we can entertain is as to whether they are of competent power to produce all the effects before us. But after what has been already said respecting the energy of endosmotic movements displayed against pressures of many atmospheres, we may abandon those doubts; and since we have here a force of universality enough, and intensity enough, and in every instance acting in the right direction, it would be unphilosophical to look farther, since such a force must, under these conditions, exist in the physical necessity of the case.

CHAPTER IX.

OF RESPIRATION.

Respiration introduces and removes aerial Substances.- Coalescence of Respiratory and Urinary Organs in Fishes.-Physical and chemical Conditions of Respiration.-Interstitial Movements of Solids, Liquids, and Gases.—Condition of Equilibrium in the Diffusion of Gases.—Con- ` densing Action of Membranes.-Forms of Respiratory Mechanism.-The Lungs of Man.— Three Stages in the Introduction of Air: Atmospheric Pressure, Diffusion of Gases, and Condensation by Membranes.-Exchange of Carbonic Acid for Oxygen.-Divisions of the Contents of the Lungs.— Variations in the expired Air.-Removal of Water.-Effect of irrespirable Gases.-Experiments of Regnault and Reiset.—Nervous Influence concerned in Respiration. -Results of Respiration.

SINCE it is essentially necessary to the life of all animals that the blood should pass to every part of the system, provision must Objects of be made for securing aeration. The breathing apparatus is the respiration. skin, or some extension, reflection, or modification of it.

Besides the great duty of originating the circulation, respiration is connected with others of equal importance. The functional activity of the nervous and muscular tissues is dependent on their oxidation, and this implies the introduction of air. In each tribe, moreover, it is necessary to keep the temperature up to a specific point. This also is accomplished by oxidation, either of the disintegrating material which is passing to waste, or of combustible substances, such as sugar or fat.

All organic material, at its death, eventually gives origin, Final products under the action of the air, to two products with which the of tissue metafunction of respiration is mainly concerned. These products morphosis. are carbonic acid and water. With the exception of gelatin, the other

150

NATURE OF RESPIRATION.

respiratory elements of food-fat, sugar, starch, &c., yield these two products alone. The nutritive elements give rise to nitrogenized compounds in addition. The conditions of life are such that carbonic acid can not be permitted to accumulate in the system, and means have therefore to be resorted to for its removal. The introduction of oxygen and excretion of carbonic acid are accomplished by the same mechanism, the lungs, the action of which is dependent on a physical principle.

aerial and va

Under its simplest condition, respiration consists in the passing of carbonic acid with the vapor of water from the system, and the Respiration is connected with reception of oxygen in exchange. The construction of the porous matter apparatus which accomplishes this double duty in atmosonly. pheric animals is such that it can deal with substances in the aerial state alone. Nothing can be introduced through the lungs or escape therefrom except it be in the gaseous or vaporous form. All those products of disorganization which are not presented under this condition must therefore be removed by other organs, and this is more particularly done by the kidneys.

and urinary or

But in aquatic animals, as in fishes generally, there is not this restricCoalescence of tion or concentration of function, for the gill, being in contact the respiratory with water, offers a channel for the passing away of many gans in fishes. products of waste which, from their non-aerial state, could never escape through a lung, and so I regard this organ, the gill, as in a measure sharing the duty of a kidney in eliminating nitrogenized and perhaps saline matters. Comparative anatomists have long recognized that the so-called kidney in fishes approaches in character the Wolffian bodies largely developed in the foetal condition of man. I am disposed to believe that the physical interpretation of this depends on the fact now before us, and that the gill in fishes, and the placenta, in part, in mammals, discharge at once the double office of a respiratory and urinary organ. It is consistent with the scheme of organic design that there should be this separation and concentration of function as development takes place.

These considerations would therefore lead us to expect that we should find in the respiration of air-breathing animals that function in its purest and least complicated form, and this is accordingly the case. If it be merely the skin that is relied on, as in the low orders of aerial life, or if the mechanism be constructed on the type of carrying the air to the blood, as in insects, or that of carrying the blood to the air, as in man, the operation consists essentially in the escape of carbonic acid and steam, and the reception of oxygen in return.

Respiration, like circulation, furnishes us with a signal instance of the employment of purely physical principles for the accomplishment of physiological purposes. It is with the pressure of the atmosphere, the

INTERSTITIAL MOVEMENTS OF SOLIDS, LIQUIDS, GASES.

151

Physical prin

ciples alone resorted to in con

diffusion of gases, and the condensing action of membranes, that we have now to deal. These give us so precise and perspicuous an explanation of the act of breathing that it is needless to look beyond them; yet on that act depend the respiratory enhighest operations of life. In this particular the Scriptures gine.

structing the

have summed up the deductions of modern physiology in a single line— no metaphorical expression, but the simple assertion of a truth: He "breathed into his nostrils the breath of life, and man became a living soul."

Of the physical principles now to be dealt with, it is unnecessary to say any thing respecting the pressure of the atmosphere, since that is well understood; but not so with the phenomena of the diffusion of gases, and the condensing action of membranes. Though these are subjects which have been particularly examined by American physicians, the facts they have elicited are little known abroad. For example, the error of Valentin's statement respecting the diffusion exchanges of carbonic acid and oxygen, and the uselessness of the elaborate discussions which have originated therefrom, would at once have been recognized, had attention been directed to the facts developed here almost twenty years ago. Interstitial motions are exhibited by solids, liquids, and gases. I have had occasion to examine Roman silver coins, from the interior of which the copper originally present had made its way out to the surface, forming the greenish incrustation known as patina by antiquarians, the silver being left almost pure. In speaking of absorption by the blood-vessels in Chapter VI., we had occasion to dwell upon the same propensity as shown by liquids, the endosmosis of Dutrochet being an example of it. The ready mobility of this group of bodies, arising from their diminished cohesion, greatly promotes these effects. Mr. Boyle collected a number of cases of solid movements in his tract on the languid motions of bodies.

Interstitial

movements of solids and liq

uids.

the endosmose

Gases and vapors, by reason of their total want of cohesion, present the most striking examples of these effects. Their propensity to intermingle with each other is manifested, even though they be obliged to pass through crevices or winding passages. One of the first instances to which attention was directed occurred under the observation of Dr. Priestley's obPriestley, who found, on passing steam through an earthen servation on tube placed in a furnace, that air would be delivered at the of gases. farther end. For some time he supposed that this experiment demonstrated the conversion of water into air by a great heat, but eventually traced it to its proper cause-the escape of the steam outward through the pores of the earthen tube, and the intrusion in the opposite direction of air from the furnace. This singular experiment may be well shown by attempting to pass steam through a red-hot tobacco-pipe, the

152

EXPERIMENTS OF DALTON AND GRAHAM.

end of which dips beneath some water. A torrent of gas bubbles will

escape.

iment on the diffusion of gases.

Mr. Dalton demonstrated that if a light gas be placed above a heavy Dalton's exper- gas in a suitable apparatus, the former, notwithstanding its levity, will descend, and the latter, notwithstanding its weight, will rise, and a complete and uniform intermixture will result. By such experiments he was led to believe that gases act as vacua to one another, and correctly explained the uniform composition of the atmosphere on this property of diffusion, or tendency of its constituents to intermix.

Thus, if a vial filled with hydrogen be placed with its mouth downward over the mouth of a vial of the same size containing carbonic acid gas, as shown at h, c, Fig. 67, in the course of a few moments the diffusion will be complete, and if the mixture in either vial be examined, it will be found to contain equal quantities of the gases.

Graham's ex

stucco.

Diffusion

Fig. 67.

C

Linusion of

gases.

Fig. 68.

Professor Graham extended Dr. Priestley's observations on the passage through porous barriers. The subperiments with stance he chiefly employed was a mass of dry plaster of Paris. This enabled him to prove that in the case of different gases diffusion takes place at different rates, which are dependent on the density of the gas. Perhaps the most satisfactory method of illustrating this class of results is by taking a porous earthenware cup, through porous a a, Fig. 68, such as is used in Grove's voltaic earthenware. battery, drying it perfectly, and cementing into its mouth an open glass tube, b, three quarters of an inch in diameter, and a foot or more long. A wide-mouthed bottle, c c, being placed as a temporary cover over the porous cup, it may be filled with hydrogen gas by displacement; and if the end of the glass tube be put into water contained in a reservoir, d, the water will rush up the moment the bottle is removed. When this motion is completed, if a jar of hydrogen be held over the porous cup, the water will be driven down with great rapidity, and a number of air-bubbles quickly escape. The extraordinary speed with which a gas will flow in and out of pores could not be better displayed. This rapidity of motion is an element with which Difusion through the physiologist has to deal, as we shall presently find.

Diffusion

earthenware.

Even when the texture of the substance is much closer, and the pores of extreme minuteness, similar results can be obtained, as was through In- shown in the experiments of Dr. Mitchell, of Philadelphia, who dia-rubber. employed thin sheets of India-rubber. If, over the mouth of

PASSAGE OF GASES THROUGH POROUS FILMS.

153

a glass bottle, such a thin tissue be tightly tied, and the bottle placed in an atmosphere of carbonic acid gas, movement at once takes place, a little air flowing out of the bottle into the carbonic acid, and so large a

ठ

Fig. 69.

a

quantity of the acid passing the opposite way that the India-rubber soon swells outward, and eventually caps the bottle like a dome, as in Fig. 69, at b. Or, if the conditions be reversed, the bottle being filled with carbonic acid, and then exposed to the atmosphere, the India-rubber will be depressed, as at a, and stretch so as almost to sink to the bottom. Such experiments therefore prove that, even though barriers of a very close texture should intervene, gases will pass through them, and with so much force, as Dr. Mitchell showed, that many inches of mercury may be lifted, nor does the movement cease until the gases on both sides of the membrane have the same composition.

Diffusion through India-rubber.

Fig. 70.

bles and liq

Other substances having a close texture may be thus readily permeated. I found that a little bladder of shellac, blown on the Experiments end of a glass tube, permitted the passage of the vapor aris- with soap-bubing from water of ammonia. The uids. instantaneousness of these motions is, however, most beautifully illustrated by employing soap-bubbles, the liquid nature of which excludes the idea of pores in the strict acceptation of that term. If a bottle, a a, Fig. 70, be rinsed out with ammonia, and then, by means of a piece of glass tube, bb, a soap-bubble, c, be blown therein, the air from the bubble being immediately drawn into the mouth without a moment's delay, the strong taste of the ammonia is perceived. Or if a rod, dipped in hydrochloric acid, be presented to the projecting end of the glass tube, copious white fumes arise. This therefore shows that vapors will pass through barriers having no proper pores, the transit taking place instantaneously.

Soap films enable us to demonstrate the endosmosis of gases in a very advantageous manner, owing to their cohesiveness and thinness. If the finger be dipped in soap-water, and then rapidly passed over the mouth of an empty bottle, so as to leave a horizontal film attached across, on exposing the bottle to carbonic acid gas, the horizontality of the film is immediately disturbed, and it soon swells up into an almost spherical dome. Or if the bottle be filled with carbonic acid, and then exposed