700. Thus it is evident, that a part of the change effected in the Blood consists in an alteration in the proportion of the gases which always exist in it, either entirely free, or in a state of such loose combination that they can be removed by the air-pump. But it may be suspected, that a portion of the effect consists in the oxidation of the proteine of the fibrinous constituent; since the fibrine of arterial blood possesses properties that distinguish it from that of venous. It has been usually supposed that the hematosine of the red corpuscles undergoes a change under the influence of oxygen in the lungs, and a converse change in the systemic capillaries, where it is subjected to the influence of carbonic acid; this change being indicated by the alteration in the color of the red corpuscles. The alteration in question, however, seems due rather to a physical than to a chemical change (§ 222); and we have no direct evidence, though much that it is indirect, of the special influence of the aeration of the blood upon the contents of the red corpuscles. It appears tolerably certain that a part of the oxygen imbibed in the lungs, is appropriated to the oxidation of the matters set free by the decomposition of the solid tissues; whilst another part enters into combination with fatty, saccharine, farinaceous, and other matters, existing in the blood itself, and destined to be carried off in the form of carbonic acid and water, without ever entering into the composition of the solid fabric. The relative amounts of carbonic acid formed in these two modes, vary in different animals and in different states of the same individual: for a man in a warm atmosphere, taking a moderate amount of exercise, may thus set free, by the waste of his muscular and other tissues, a sufficient quantity of carbon for the maintenance of his animal heat by its union with oxygen; but this is far from being sufficient, when a larger amount of heat must be evolved, to sustain the temperature of the body in a colder climate.

701. The blood parts in the lungs with a very large amount of moisture; for the inspired air is always saturated with fluid, as soon as it reaches the air-cells; and, as it is heated at the same time to about 98°, it thus receives a considerable addition, even if it were previously charged with as much as it could contain at a lower temperature. The total quantity of fluid thus disengaged will vary, therefore, with the amount previously contained in the atmosphere, being greater as this was less, and vice versa; but the quantity that usually passes off seems to be from 16 to 20 ounces in the twenty-four hours. It cannot be doubted, that a great part of this water is a simple exhalation of that which has been absorbed; but, on the other hand, it seems probable that a portion of it may be actually formed in the system, by the union of a portion of the oxygen absorbed in the lungs, with the hydrogen of the combustible matters of the blood. In the various forms of saccharine and farinaceous aliments, the proportion of hydrogen and oxygen are such as would of themselves form water, when the carbon is withdrawn; but in oily and fatty matters, the proportion of oxygen is far too small thus to neutralize the hydrogen; and it seems likely that by their oxidation in the blood, as by their combustion elsewhere, water is actually generated by the union of atmospheric oxygen

with their hydrogen, at the same time that carbonic acid is produced by its union with their carbon.

702. Along with the water thus extricated from the lungs, a certain amount of organic matter is set free. If the fluid be collected in a closed vessel, and be exposed to warmth, a very evident putrid odor is exhaled from it; and if the expired air be made to pass through sulphuric acid, that liquid is colored red. Every one knows that the breath itself possesses, occasionally in some persons, and constantly in others, a fœtid taint; when this does not proceed from carious teeth, ulceration in the air-passages or lungs, or other similar causes, it must result from the excretion of the odorous matter, in combination with watery vapor, from the pulmonary surface. That this is the true account of it seems evident, from the analogous phenomenon of the exhalation of turpentine, camphor, alcohol, and other odorous substances, which have been introduced into the venous system, either by natural absorption, or by direct injection; and also from the suddenness with which the odor manifests itself, when the digestive apparatus is slightly disordered.

5. Effects of Insufficiency, or Suspension, of the Aerating Process.

703. The change which the Blood undergoes, by being brought into relation with atmospheric air in the respiratory organs, is so important to life, that the entire suspension of it inevitably produces a fatal termination, at no remote period; and if it be insufficiently performed, various disorders in the system are nearly sure to manifest themselves. The state which is induced by the entire suspension of the aerating process, is termed Asphyxia; a word which literally means the absence of pulse, and would be applicable therefore to the stoppage of the circulation from any cause; though it is usually employed to designate the particular condition resulting from suspended respiration. Asphyxia may be produced in aquatic animals, as well as in those which breathe air, by cutting them off from the influence of the atmosphere; for if a fish be placed in water from which the air has been expelled by boiling, it is precisely in the condition of an air-breathing animal placed in a vacuum, since it has no power of obtaining oxygen by decomposing the water it inhabits, and is entirely dependent for the aeration of its blood, upon the air that is absorbed by the liquid. Again, if a fish be placed in water impregnated with carbonic acid, its death is nearly as instantaneous as that of an air-breathing animal immersed in an atmosphere of that gas.

Thus there

704. Asphyxia may result from a great variety of causes. may be a mechanical obstruction to the entrance of air through the trachea; as in hanging, strangulation, or drowning; or as in occlusion of the aperture of the glottis, by oedema of its lips, or by the presence of a foreign body in the larynx. Or, again, the passage may be perfectly free, and yet no air may enter, in consequence of some obstacle to the performance of the respiratory movements. This obstacle may be mechanical; as when a quantity of earth has fallen round the body, in such a manner as completely to prevent the distension of the chest and abdomen. Or it may result (and this is a most frequent occurrence) from torpidity

or complete inactivity of the ganglionic centre, which is concerned in the respiratory actions; or from interruption to the transmission of its influence along the nervous trunks. Further, when there is no obstacle to the free ingress or egress of air, Asphyxia may be produced by the want of oxygen in the atmosphere that is respired, or by the presence of carbonic acid in too large an amount. And the presence of other gases, which exert a directly poisonous influence on the blood,-such as sulphuretted hydrogen,-produces a state, which may be included under the same general description.

705. Now when, from any of these causes, the free exchange of carbonic acid for oxygen in the pulmonary capillaries is checked, the first effect of the interruption appears to be, the stagnation of the blood in the pulmonary capillaries. This stagnation is evidently due, not to any deficiency of power in the heart; for that organ is not yet affected; but to the insufficiency of the heart's power acting alone, to drive the blood through the pulmonary capillaries: the force which should be generated by chemical changes in them (§ 598), being deficient. The stagnation is not, however, complete at first; since the quantity of oxygen contained in the lungs is sufficient to produce an imperfect arterialization of the blood; and the blood thus partially changed is transmitted to the left side of the heart, and is thence propelled to the system. Owing to its half-venous condition, it cannot exert its usual stimulating influence on the tissues, especially the muscular and nervous; and their powers are consequently weakened. For the same reason, it does not receive its usual auxiliary force in the systemic capillaries (§ 599); since the changes, which it ought to undergo in them, can only be partially performed.

706. As the air included in the lungs loses more and more of its oxy. gen, and becomes more and more charged with carbonic acid, the aeration of the blood in the pulmonary capillaries becomes more and more imperfect; the quantity of blood which is allowed to return to the heart is gradually diminished, and its condition become more and more venous; and at last, the pulmonary circulation is altogether suspended. From the relation which the respiratory circulation bears to the systemic, in all the higher classes of animals, save Reptiles, it follows that the systemic circulation must in like manner be brought to a stand. The venous blood accumulates in the pulmonary artery, in consequence of the obstruction of its capillaries; it distends the right cavities of the heart; and the accumulation extends to the venous system of the body in general, especially affecting those organs which naturally receive a large quantity of venous blood, such as the liver and spleen. The arterial system, on the other hand, is emptied in a corresponding degree; nearly all its blood having passed through the systemic capillaries; and no fresh supplies being received from the heart. From this deficiency, and from the venous character of the blood which the systemic arteries do contain, it results that the nervous and muscular systems lose their power; insensibility comes on, at first accompanied with irregular convulsive movements; but in a short time there is a total cessation of all movement except that of the heart; and the pulsations of that organ become feebler and feebler, until they cease altogether. The immediate cause of the cessation of the heart's action appears to be different on the

two sides. Both are equally affected by the want of arterial blood in the capillaries of their own substance; but the right side suffers from over distension, which produces a sort of paralysis of its muscular tissue; whilst the left side retains its contractility, but is not excited to contraction, for want of the stimulus of arterial blood in its cavities.

707. In those warm-blooded animals which are not endowed with any special provision for enabling them to sustain life during the prolonged suspension of the respiratory process, insensibility and loss of voluntary power almost invariably supervene within a minute and a half after the admission of air to the lungs has been entirely prevented; though the respiratory efforts and convulsive actions, which are dependent upon the medulla oblongata and spinal cord, may continue for a minute or two longer. The circulation generally comes to a complete stand within ten minutes at farthest.-The chief exceptions are in the case of diving animals, which are provided with large arterial and venous reservoirs, that serve to maintain the circulation during a prolonged suspension of the respiratory process; for the arterial plexuses being ordinarily filled, they afford a supply of aerated blood to the systemic capillaries, when other blood is wanting; and the reservoirs connected with the venous system, which were previously empty, receive this blood, and prevent it from exercising undue pressure on the heart. To such an extent is this provision carried in some animals, that the Whale has been known to remain under water for an hour. Another exception exists in the case of hibernating Mammals, which are reduced for a time to the condition of cold-blooded animals, and which can, like the latter, sustain a prolonged suspension of the aerating process. And there is reason to believe that, in the state of Syncope or fainting,-in which the circulation is already reduced to a very low amount, in consequence of a partial failure in the heart's power, all the functions of the body being nearly suspended, and the demand for aeration being consequently very small, the respiration may be suspended for a long period, even in the Human subject, without fatal results. Thus more than one case has been credibly recorded, in which recovery has taken place after complete submersion for more than three-quarters of an hour; and it is probable that, in these instances, a state of Syncope came on at the moment of the immersion, through the influence of mental emotion, or of concussion of the brain.

708. In the restoration of an animal from the state of Asphyxia, it is above all things of importance to renew the air in the lungs; for in this way the blood in the pulmonary capillaries will be aerated, the capillary circulation will be re-established, the right side of the heart will be relieved of its excessive load of venous blood, and the left side will receive the stimulus of a fresh supply of arterial blood; so that, if its movements have not ceased altogether, it may be speedily restored to due activity. At the same time, the temperature of the body should be kept up by artificial warmth; and the circulation in the skin should be excited by friction. Where no other means are at hand for introducing pure air into the lungs (of which means the application of galvanism along the course of the phrenic nerve, so as to produce contraction of the diaphragm, will probably be the most effectual), the object may

be attained by forciby compressing the trunk on all sides, so as to empty the lungs as much as possible, and then allowing the chest to dilate again, by the elasticity of its walls. In this manner, a large proportion of the carbonic acid may be expelled, and a considerable proportion of fresh air introduced, in the course of a few minutes. If air be blown into the lungs by the bellows, great care must be taken to prevent the employment of too much force, which is likely to produce rupture of the air-cells.

709. Now when, from the more prolonged action of various causes that impede the due performance of the respiratory function, the aeration of the blood in the lungs is insufficient for health, though not such as to produce a complete stagnation of the movement, a variety of results may follow; of which some, or others, will manifest themselves, according to the condition of the general system, and the peculiarities of the individual. Thus deficient respiration seems to favor the retention of fatty matter in the system; and this not merely in the condition of Adipose tissue, which (unless it accumulate to excess) may be regarded as a healthy product; but also in the place of the normal components of other tissues, as the muscular and glandular, giving rise to the condition which is termed "fatty degeneration."-Again, the due elaboration of the fibrine of the blood is undoubtedly prevented by an habitually-deficient respiration; and various diseases, which result from the imperfect performance of this elaboration, consequently mani fest themselves. The Scrofulous diathesis is thus frequently connected with an unusually small capacity of the chest.-Further, an habitual deficiency of respiration may impede, though it does not check, the circulation in the lungs; and thus a tendency arises, in various pulmonary diseases, to an overloading of the pulmonary arteries, to a dilatation of the right cavities of the heart, and to a congestion of the venous system in general, as marked by lividity of the surface, by venous pulsation, &c. This state may result, not merely from obstruction in the lungs themselves, but from deficiency of the respiratory movements, consequent upon torpidity of the medulla oblongata (as in apoplexy and narcotic poisoning) or upon partial interruption of the nervous circle requisite for all reflex movements. Thus when the par vagum is divided, the number of respiratory movements is greatly diminished, and a partial stagnation of the blood in the lungs is the result. The same happens in certain forms of typhoid fever, in which the respiratory movements are preternaturally slow, in consequence of torpidity of the medulla oblongata. Now in this state, an effusion of the watery part of the blood into the air-cells of the lungs (as in other cases of obstructed circulation) is very liable to occur; and when the lungs are thus loaded with fluid, the respiratory process is still more impeded, and the disorder has thus a tendency to increase itself.