effects that would be induced by heterogeneous gaseous, miasmatic, or other absorption. We have seen that some of the deleterious gases, as sulphuretted hydrogen, are most powerfully penetrant, and, if they could enter the surface of the body with readiness, unfortunate results might supervene. In those parts where the cuticle is extremely delicate, as in the lips, some conversion of venous into arterial blood may be effected, and this may 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, because their arrangement is unfavourable for such a result.

d. Effects of the Section of certain Nerves on Respiration.

It remains to inquire into the effect produced on the lungs by the cerebro-spinal and spinal nerves distributed to them, or rather, into what is the effect of depriving the respiratory organs of their nervous influence from the brain and spinal marrow. The only 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 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, Schräder, Valsalva, Morgagni, Haller, and numerous other distinguished physiologists. It is chiefly, however, in recent times, and especially from the labours of Dupuytren, Dumas, De Blainville, Provençal, Legallois, Magendie, Breschet, Hastings, Broughton, Sir Benjamin Brodie, Wilson Philip, Longet, John Reid, and others, that the precise effects upon the respiratory and digestive functions 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 it sometimes lives for a few days;-M. Magendie says never beyond three or four. The effects produced upon the voice, by their division above the origin of the recurrents, will be referred to under another head. Such division, however, does not simply implicate the larynx; it necessarily affects the lungs, as well as the stomach. As regards the larynx, the same results, according to M. Magendie,' are 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 at once from suffocation. But if the division of those nerves should not induce instant death in this manner, phenomena follow, considerably alike in all cases, which go on until the death of the animal. These are the fol

1 Précis, &c., 2de édit., ii. 355.

lowing:-respiration is, at first, difficult; the inspiratory movements are more extensive and rapid, and the animal's attention appears to be particularly directed to them; 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, on the second day for instance, the difficulty of breathing augments, and the inspiratory efforts become gradually greater. The arterial blood has now no longer the vermilion hue proper to it. It is darker than it ought to be: its temperature falls; respiration requires the exertion of all the respiratory powers; the body gradually becomes cold, and the animal dies. On opening the chest, the air-cells, bronchi, and frequently 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 phenomena appeared, the animal consumed less and less oxygen, and gave off a progressively diminishing amount of carbonic acid.

From the phenomena that occur after the section of the nerves on both sides, it would seem to follow, that the first effect is exerted upon the tissues of the lungs, which, being deprived of nervous influence, are no longer capable of exerting their ordinary tonicity and muscularity. Respiration, consequently, becomes difficult; the blood no longer circulates freely through the capillary vessels of the lungs; the consequence 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 bloodvessels, and the conversion of venous into arterial blood is completely precluded. Death is then the inevitable and immediate consequence. The division of the nerve on one side affects merely the lung of the corresponding side. Life can be continued by the action of one lung 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 has had repeated opportunities of observing, that for a length of time prior to dissolution, in cases of pulmonary consumption, the process of respiration must have been carried on by a very small portion of lung.

From his experiments on this subject, Sir Astley Cooper infers, that the pneumogastric nerve is most important;-1st, in assisting in the maintenance of the function of the lungs, by contributing to the change of venous into arterial blood; 2dly, in being necessary to the act of swallowing; and 3dly, in being essential to the digestive process. Dr. John Reid is of opinion, that the pulmonary branches seem to be nerves concerned chiefly in transmitting to the medulla oblongata the impressions that excite respiratory movements, and are thus principally afferent nerves; but it is possible, he thinks, that they containmotor filaments also."

Edinb. Med. and Surg. Journ., April, 1839; and art. Par Vagum in Cyclop. of Anat. and Physiol., Part xxvii. p. 896, March, 1846.

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

Sir A. Cooper instituted similar experiments on the phrenic nerves. As soon as they were tied, the most determined asthma was produced; breathing went on by means of the intercostal muscles; the chest was elevated to the utmost by them; and in expiration the chest was as remarkably drawn in. The animals did not live an hour; but they did not die suddenly, as they do from pressure on the carotid and vertebral arteries. The lungs appeared healthy, but the chest contained more than its natural exhalation. He also tied the great sympathetic; which produced little effect; the heart appeared to beat more quickly and feebly than usual. The animal was kept seven days, when one nerve was found ulcerated through; the other nearly so at the situation of the ligatures. On examination, no particular alteration of any organ was observed. Lastly, Sir Astley tied all three nerves on each side, the pneumogastric, phrenic, and great sympathetic: the animal lived little more than a quarter of an hour, and died of dyspnoea. From these experiments, he infers, that the sudden death, which he found to follow pressure on the sides of the neck, cannot be attributed to any injury of the nerves, but to an impediment to the due supply of blood to the great centres of nervous influence.

The nervous centre of the respiratory movements is the vesicular neurine in the upper part of the medulla oblongata. Into it the pneumogastric nerves, which appear to be the chief excitors of respiration, may be traced; and from it the different motor or efferent nerves proceed either directly or indirectly. Of these, the most important is the phrenic. The vesicular neurine of the medulla receives the impression of the besoin de respirer or necessity of breathing; and thence it is reflected along the appropriate nerves to the muscles concerned in inspiration.3

e. Respiration of Animals.

In concluding the subject of respiration, we may briefly advert to the different modes in which the process is effected in the classes of animals, and especially in birds, the respiratory organs of which constitute one of the most singular structures of the animal economy. The lungs themselves are comparatively small, and adherent to the chest, where they seem to be placed in the intervals of the ribs.

1 Experimental Inquiry into the Laws of the Vital Functions, &c., 2d edit., p. 223, Lond., 1818; also, Journal of Science and Arts, viii. 72.

2 Op. cit., p. 475.

3 See, on all this.subject, Longet, Traité de Physiologie, ii. 328, Paris, 1850.

They are covered by the pleura on their under surface only, so that they are, in fact, on the outside of the cavity of the chest. A great part of the thorax, as well as of the abdomen, is occupied by membranous air-cells, into which the lungs open by considerable apertures. Besides these cells, a considerable portion of the skeleton in many birds forms receptacles for air, and if we break a long bone of a bird of flight, and blow into it when the body of the animal is immersed in water, bubbles of air will escape from the bill. The object, of course, of all this arrangement is to render the body light, and thus to facilitate its motions. Hence, the largest and most numerous bony cells are found in such birds as have the highest and most rapid flight, as the eagle. The barrels of the quills are likewise hollow, and can be filled with air, or emptied at pleasure. In addition to the uses just mentioned, these air receptacles diminish the necessity for breathing so frequently in the rapid and long-continued motions of certain birds, and in the great vocal exertions of those that sing.

2

In fishes, in the place of lungs we find branchiae or gills, which are placed behind the head on each side, and have a movable gill-cover. By the throat, which is connected with the gills, the water is conveyed to, and distributed through them: in this way, the air, contained in the water, which, according to Biot, Von Humboldt' and Provençal, Configliachi, and Thomson, is richer in oxygen than that of the atmosphere, having from 29 to 32 parts in the 100, instead of 20 or 21, comes in contact with the blood circulating through the gills. The water is afterwards discharged through the branchial openings,-aperturæ branchiales, and, consequently, they do not expire along the same channel as they inspire.

Lastly, in the insect tribe,-in the white-blooded animal,-we find the function of respiration effected altogether by the surface of the body; at least, so far as regards the reception of air, which enters through apertures termed stigmata, the external terminations of trachea or air tubes, whose office it is to convey air to different parts of the system.

In all these cases, we find precisely the same changes effected upon the inspired air;-and especially, that oxygen has disappeared; and that carbonic acid of a bulk nearly equal to that of the organ is met with in the residuary air.3

CHAPTER IV.

CIRCULATION.

THE next function to be considered is that by which the products of the various absorptions, converted into arterial blood in the lungs, are

Mémoires de la Société d'Arcueil i. 252, and ii. 400.

2 Dr. Thomson found that 100 cubic inches of the water of the river Clyde contained 3.113 inches of air; and that the air contained 29 per cent. of oxygen. Edinb. New Philosoph. Journal, xxi. 370, Edinb., 1836.

3 See Carpenter's Principles of Comparative Physiology, Amer. edit., Philad. 1854.

distributed to every part of the body,-a function most important to the physiologist and the pathologist, and without a knowledge of which it is impossible for the latter to comprehend the doctrine of disease.

Assuming the heart to be the great organ of the function, the circulatory fluid must set out from it, be distributed through the lungs, undergo aeration there. be sent to the opposite side of the heart, whence it is distributed to every part of the system by efferent vessels, and be returned by veins or afferent vessels to the right side, from which it set out, thus performing a complete circuit.

D. Right auricle. E. Right ventricle. K. Left auricle. L. Left ventricle. F.

Pulmonary artery. A. Aorta.

The lower classes of animals differ essentially, as we shall find hereafter, in their organs of circulation: whilst in some, the apparatus appears to be confounded with the digestive; in others, the blood is propelled without any great central organ; and in others, again, the heart is but a single organ. In man, and in the upper classes of animals, the heart is double; consisting of two sides, or really two hearts, separated from each other by a septum. In the dugong, the two ventricles are almost entirely detached from each other.

As all the blood of the body has to be emptied into this central organ, and to be subsequently sent from it; and as its flow is continuous, two cavities are required in each heart, the one to receive the blood, the other to propel it. The latter distinctly contracts and dilates alternately. The cavity or chamber of each heart, that receives the blood, is called auricle, and the vessels that transport it thither are veins; the cavity by which the blood is projected forwards is called ventricle, and the vessels, along which the blood is sent, are arteries. One of these hearts is entirely appropriated to the circulation of venous blood, and hence has been called venous heart,-also right or anterior heart, from its situation,-and pulmonary, from the pulmonary artery arising from it. The other is for the circulation of arterial blood, and is hence called arterial heart, also left or posterior, from its situation,-aortic heart, because the aorta arises from it; and systemic, because the blood is sent from it to the general system.

The whole of the vessels communicating with the right heart contain venous blood; those of the left side arterial blood,

Fig. 92.

Diagram of the Circulating Ap

paratus in Mammals and Birds.

a. The heart, containing four cavinous blood into c, the right auricle.

ties. 3. Vena cava, delivering ved. The right ventricle, propelling nary artery, to, the capillaries of ceiving the aerated blood from the

venous blood through e, the pulmo

the lungs. g. The left auricle, re

pulmonary vein, and delivering it to the left ventricle, h, which pro

pels it through the aorta, i, to the

systemic capillaries, 5, whence it is

collected by the veins, and carried

back to the heart through the vena cava, b.