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stituents, which is necessary for the wants of the system, the excess being rejected? Could it be shown that such a decomposition is actually effected at the point of contact between the pulmonary vessels and the air in the lungs, it would go far to prove the notion of Ellis, and of Chaussier and Adelon, that an action of selection, or of vitality is exerted; but we have no evidence in favour of this. Sir Humphry Davy, indeed, is of opinion, that the whole of the air is absorbed, and that the surplus quantity of each of the constituents is subsequently discharged. In favour of this view, he remarks, that air has the power of acting upon blood through a stratum of serum; and he thinks, that the undecomposed air must be absorbed before it can arrive at the blood in the vessels. This is extremely probable; for we have already seen, that air disappears during respiration, and consequently, it must have been taken into the system.

It has been remarked, that when oxygen is applied to venous blood it changes it to a florid colour. On what part of the blood, then, does the oxygen act? The general belief is, upon the red globules. The facts we have stated in the description of venous blood, have shown, that these globules appear to consist of a colourless nucleus, surrounded by a coloured envelope; that both of these are devoid of colour, whilst they exist as chyle and lymph; but that, in the lungs, the contact of air changes the envelope to a florid red. Some, indeed, have believed, that both the envelope and its colour are added in the lungs. The coloration of the blood, consequently, seems to be effected in the lungs; but whether this change is of any importance in hæmatosis is doubtful. In many animals, the red colour does not exist; and, in all, it can perhaps only be esteemed an evidence, that the other important changes have been accomplished in the lungs. Recently, the opinion has been revived, that the oxygen of the air acts upon the iron which Engelhart and Rose have detected in the colouring matter, but how we know not. It is asserted, that if the iron be separated, the rest of the colouring matter, which is of a venous red colour, loses the property of becoming scarlet by the contact of oxygen.

The slight diminution, if it exist, in the specific gravity of arterial blood, is considered, but we know not on what grounds, to be owing to the transpiration which takes place in the air-cells, and which was formerly thought to be owing to the combustion of oxygen and hydrogen. This will engage us in another place, as well as the changes produced in its capacity for heat, and on which several ingenious speculations have been founded, to account for animal temperature. The other changes are at present inexplicable, and can only be understood hereafter by minute chymical analysis, and by an accurate comparison of the two kinds of blood,—venous and arterial.

It is manifest, from the preceding detail, that our knowledge regarding the precise changes, effected upon the air and the blood by respiration, are by no means definite. In the first place:—the air VOL. II.

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loses a part of its oxygen, but this loss varies according to numerous circumstances. 2dly, It is found to have acquired carbonic acid, the quantity of which is also variable; but as a general principle it is less than the oxygen consumed. 3dly, The bulk of the air is diminished; but the quantity of this likewise differs. 4thly, Azote is both absorbed and exhaled by the lungs, to a variable amount. 5thly, The blood, when it attains the left side of the heart, has a more florid colour. 6thly, This change appears to be caused by the contact of oxygen. 7thly, The blood in the lungs gets rid of a quantity of carbon, united with oxygen in the form of carbonic acid. 8thly, It absorbs oxygen, and more than is necessary for the carbonic acid formed. 9thly, The blood, as it passes through the lungs, probably both absorbs and exhales azote;—the proportion which these processes bear to each other being extremely variable. 10thly, The air passes directly through the coats of the pulmonary vessels, and certain portions of each of its constituents are discharged or retained, according to circumstances; and, lastly, a quantity of aqueous vapour, containing albumen, is discharged from the lungs; but this is a true secretion, and not a consequence of respiration.

A question, again, has arisen, whether any absorption and exhalation of air and conversion of blood from venous to arterial takes place in any other part of the body than the lungs. The reasons urged in favour of the affirmative of this view, are:—that, in the lower classes of animals, the skin is manifestly the organ for the reception of the air; that the mucous membrane of the lungs evidently absorbs air, and is simply a prolongation of the skin, and resembles it in texture; and, lastly, that when a limited quantity of air has been placed in contact with the skin of a living animal, it has been absorbed and experienced the same changes, as are effected in the lungs. Mr. Cruikshank and Mr. Abernethy analyzed the air, in which the hand or foot had been confined for a certain length of time, and detected in it a considerable quantity of carbonic acid. Jurine, having placed his arm in a cylinder hermetrically closed, found, after it had remained there two hours, that oxygen had disappeared, and that 0.08 of carbonic acid had been formed. These results were confirmed by Gattoni. On the other hand, Drs. Priestley, Klapp, and Gordon could never perceive the least change in the air under such circumstances. Perhaps in these, as in all cases, where the respectability of testimony is equal, the positive, should be adopted rather than the negative evidence. It is probable, however, that, in all cases, the absorption must be effected with difficulty; and that the cuticle is placed on the outer surface to obviate the bad effects, which would be induced by heterogeneous, gaseous, miasmatic, or other absorption. We have seen, that some of the deleterious gases, as the sulphuretted hydrogen, are most powerfully penetrant, and, if they could enter the absorbents of the surface of the body with readiness, unfortunate results might supervene. It is probable, however, that in those parts, where the cuticle is ex

EFFECTS OF THE SECTION OF THE EIGHTH PAIR OF NERVES. 107

tremely delicate, as in the lips, some conversion of the venous blood into arterial may be effected, and be a great cause of their florid colour. According to this view, the arterialization of the blood occurs in the lungs chiefly, owing to their formation being so admirably adapted to the purpose, and it is not effected in other parts, owing to their arrangement being unfavourable for such result.

It remains for us to inquire into the effect, produced on the lungs by the cerebral nerves distributed to them,—or rather into what is the effect of depriving the respiratory organs of their nervous influence from the brain. The only cerebral or encephalic nerves, distributed to them, are the pneumogastric or eighth pair of Willis, which we have seen are sent, as their name imports, to both the lungs and the stomach. The section of these nerves early suggested itself to physiologists, but it is only in recent times that the phenomena resulting from it have been clearly comprehended. The operation appears to have been performed as long ago as the time of Rufus of Ephesus, and was afterwards repeated by Chirac, Bohn, Duverney, Vieussens, Schrader, Valsalva, Morgagni, Haller, and numerous other distinguished physiologists. It is chiefly, however, in very recent times, and especially by the labours of Dupuytren, Dumas, De Blainville, Provencal, Legallois, Magendie, Breschet, Hastings, Broughton, Brodie, and Wilson Philip, that the precise effects upon the respiratory and digestive function have been appreciated.

When these nerves are divided in a living animal, on both sides at once, the animal dies more or less promptly; at times, immediately after their division; but sometimes it lives for a few days; Magendie says never beyond three or four.

The effects produced upon the voice, by the division of the pneumogastric nerves above the origin of the recurrents, have been referred to, under another head. Such division, however, does not simply implicate the larynx, but necessarily affects the lungs, as well as the stomach. As regards the larynx, precisely the same result would be produced by dividing the trunk of the pneumogastric above the origin of the recurrents, as by the division of the recurrents themselves: the muscles, whose function it is to dilate the glottis, are paralyzed; and, consequently, during inspiration, no dilatation takes place; whilst the constrictors, which receive their nerves from the superior laryngeal, preserve all their action, and close the glottis, at times so completely, that the animal dies immediately from suffocation. But if the division of these nerves should not induce instant death in this manner, a series of symptoms follows, considerably alike in all cases, which go on, until the death of the animal. These phenomena, according to Magendie, are the following: the respiration is, at first, difficult; the inspiratory movements are more extensive and rapid; and the animal's attention appears to be particularly directed to it; the locomotive movements

are less frequent, and evidently fatigue; frequently the animal remains entirely at rest: the formation of arterial blood is not prevented at first; but soon, in the second day for instance, the difficulty of breathing augments, and the inspiratory effects become gradually greater. The arterial blood has now no longer the vermillion hue, which is proper to it. It is darker than it ought to be. Its temperature falls. Respiration requires the exertion of all the respiratory powers. At length, the arterial blood is almost like the venous, and the arteries contain but little of it; the body gradually becomes cold, and the animal dies. On opening the chest, the aircells, the bronchi, and, frequently, even the trachea, are found filled by a frothy fluid, which is sometimes bloody; the substance of the lung is tumid; the divisions and even the trunk of the pulmonary artery are greatly distended with dark, almost black, blood; and extensive effusions of serum and even of blood are found in the parenchyma of the lungs. Experiments have, likewise, shown, that, in proportion as these symptoms appeared, the animals consumed less and less oxygen, and gave off a progressively diminishing amount of carbonic acid.

From the phenomena that occur after the section of these nerves on both sides, it would seem to follow, that the first effect is exerted upon the tissue of the lungs, which, being deprived of the nervous influence they receive from the brain, are no longer capable of exerting their ordinary elasticity or muscularity, whichsoever it may be. Respiration, consequently, becomes difficult; the blood no longer circulates freely through the capillary vessels of the lungs; the consequence of this is, that transudation of its serous portions, and occasionally effusion of blood, owing to rupture of small vessels, takes place, filling the air-cells more or less; until, ultimately, all communication is prevented between the inspired air and the blood-vessels of the lungs, and the conversion of the venous into arterial blood is completely precluded. Death is, then, the inevitable and immediate consequence.

The division of the nerve of one side affects merely the lung of the corresponding side; life can be continued by the action of one only. It is, indeed, a matter of astonishment how long some individuals have lived, when the lungs have been almost wholly obstructed. Every morbid anatomist must have had repeated opportunities for observing, that, in cases of pulmonary consumption, for a length of time prior to dissolution, the process of respiration must have been wholly effected by a very small portion of lung.

The experiments of Dr. Wilson Philip and others moreover show, -what has been more than once inculcated,—the great similarity between the nervous and galvanic fluids. When the state of dyspnoea was induced by the division of the pneumogastric nerves, the galvanic current was passed from one divided extremity to the other, and, in numerous cases, the dyspnoea entirely ceased.

The results of these experiments induced him to try the effect of galvanism in cases of asthma. By transmitting its influence from the nape of the neck to the pit of the stomach, he gave decided relief in every one of twenty-two cases, four of which occurred in private practice, and eighteen in the Worcester Infirmary.

There is one other topic, which, although not directly belonging to physiology, has been so much the subject of experiment with physiologists, that it is worthy of observation. We allude to the

Respiration of different Gases.

Experience has sufficiently proved, that no combination of gases, except that which exists in the atmosphere, is adapted for the prolonged existence of animals, or even of plants. Of the other gases, there are some, which are entirely irrespirable, producing a spasmodic closure of the glottis, and thus inducing suffocation; others that are negatively deleterious, by depriving the animal of its due supply of oxygen; and others, again, which act on the body in a positively noxious manner.

Soon after the gases were discovered, their effects upon the respiration of animals were tested; but the most accurate and extensive information, which we possess on the subject, was afforded by the labours of Beddoes, and his distinguished pupil Sir Humphry Davy.

The gases, which have been chiefly subjected to experiment, are:oxygen, protoxide of azote, hydrogen, azote, carburetted hydrogen, carbonic acid, carbonic oxide, sulphuretted hydrogen, arsenuretted hydrogen, ammoniacal gas, muriatic acid gas, nitrous acid gas, nitric oxide, and chlorine.

Oxygen. This gas, which we have seen to be so essential to respiration, and which has hence acquired the name vital air, has been subjected to numerous experiments, and the general result appears to be, a belief, that it acts in a positively deleterious manner; and that, although an animal may live in a limited portion of it a considerable time longer than in the same quantity of atmospheric air, its respiration becomes hurried and laborious before the whole is consumed, and it dies, although a fresh animal of the same kind is capable of sustaining life for some time in the residuary air. The belief is not perhaps legitimate. A part, if not the whole, of the dyspnoea and death may be produced by the evolution of carbonic acid, which is unfavourable to animal life; whilst a fresh animal may be enabled to resist its action for a time and take up some of the residuary oxygen. Oxygen is one of the gases, which has been regarded, on very insufficient evidence however, to exert a stimulant effect upon the blood, by which the left side of the heart, to which the blood is returned from the lungs, and the arterial system are excited to action; and it was accordingly respired, at one

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