But the new-formed fluid, even at the time it has reached the heart, has by no means undergone a sufficient elaboration to become genuine blood, or to support the living action of the different organs. It has yet to be operated upon by the air, and must for this purpose be sent to the lungs, and again returned to the heart, before it is fitted to be thrown into the general circulation.

This is the rule that takes place in all the more perfect animals, as mammals, birds, and most of the amphibials ;* and hence these classes are said to have a double circulation. And as the heart itself consists of four cavities, a pair belonging to each of the two circulations, and each pair is divided from the other by a strong membrane, they are also said to have not only a double circulation, but a double heart—a pulmonary and a corporeal heart.

The blood is first received into the heart on the pulmonary side, and is conveyed to the lungs by an artery which is hence called the pulmonary artery, that soon divides into two branches, one for each of the lungs; in which organs they still further divide into innumerable ramifications, and form a beautiful net-work of vessels upon the air-vesicles of which the substance of the lungs consists; and by this mean every particle of blood is exposed in its turn to the full influence of the vital gases of the atmosphere, and becomes thoroughly assimilated to the nature of the animal system it is to support. The invisibly minute arteries now terminate in equally minute veins, which progressively unite till they centre in four common trunks, which carry back the blood, now thoroughly ventilated and of a florid hue, to the left side or corporeal department of the heart.

From this quarter the corporeal circulation commences: the stimulus of the blood itself excites the heart to that alternate contraction which constitutes pulsation, and which is continued through the whole course of the arteries; and by this very contraction the blood is impelled to the remotest part of the body, the arterial vessels continuing to divide and to subdivide, and to branch out in every possible direction, till the eye can no longer follow them, even when aided by the best glasses.

The arterial blood having thus visited every portion of every organ, and supplied it with the food of life, is now returned, faint, exhausted, and of a purple hue, by the veins, as in the pulmonary circulation; it receives, a short space before it reaches the heart, its regular recruit of new matter from the digestive organs, and then empties itself into the right side or pulmonary department of the heart, whence it is again sent to the lungs, as before, for a new supply of vital power.

The circulation of the blood, therefore, depends upon two distinct sets of vessels, arteries, and veins, the former of which carry it forward to every part of the system, and the latter of which return it to its central source. Both sets of vessels are generally considered as consisting of three distinct layers or tunics; an external, which in the arteries is peculiarly elastic; a middle, which is muscular in both, but whose existence is doubted by some physiologists; and an internal, which may be regarded as the common covering or cuticle. The projectile power exercised over the arteries is unquestionably the contraction to which the muscular tunic of the heart is

* Cuvier seems to ascribe a double heart to the class of amphibia, without any limitation. See Lawrence's additional note E. chap. xii. of his translation of Blumenbach's System of Comparative Anatomy. Blumenbach himself has remarked, that many of the frogs, lizards, and serpents have a simple heart, consisting of a single auricle and ventricle, like that of fishes. Sect. 162.

excited by the stimulus of the blood itself; and which contraction would be permanent, but that the heart appears to become exhausted in a considerable degree of its muscular irritability by the exertion that produces the contraction, and hence speedily returns to its prior state of relaxation, exhibiting that alternating succession of systole and diastole which constitutes pulsation.*

In the venal system, however, we meet with even fewer proofs of muscular fibre than in the arterial, and no such force of the heart as to produce pulsation on the pressure of the finger; and hence, to this moment, we are in a greater degree of ignorance as to the projectile power by which this system is actuated. The theories that have been chiefly advanced upon the subject are, first, that of a vis à tergo, or an impetus given to the blood by the arterial contraction, which is supposed by its supporters to be sufficient to operate through the whole length of the venal canals; secondly, that of capillary attraction, the nature of which we explained in a former lecture; and lastly, a theory of a much more complicated kind than either, and which supposes the projectile power to result jointly from the impetus communicated by the heart and arteries, from the pressure of the surrounding organs, and especially from the elasticity of the lungs, and the play of the diaphragm, in conjunction with the natural irritability of the delicate membrane that lines the interior of the veins. It is unnecessary to enter into a consideration of any of these theories; for they all stand self-convicted of incompetency; and the last, which is the most operose of the whole, has been only invented to supply the acknowledged inefficacy of the other two. Whatever this projectile power consists of, it appears to have some resemblance to that of the vegetable system; and, like many of the vessels in the latter, is assisted by the artifice of numerous valves inserted in different parts of the venal tubes.

'The most important process which takes place in the circulation of the blood is that of its ventilation in the lungs. It is this process which constitutes the economy of RESPIRATION, and has till of late been involved in more than Cimmerian darkness.

We see the blood conveyed to the lungs of a deep purple hue, faint and exhausted by being drained in a considerable degree of its vital power, or immature and unassimilated to the nature of the system it is about to support, in consequence of its being received fresh from the lacteal trunk. We behold it returned from the lungs spirited with newness of life, perfect in its conformation, more readily disposed to coagulate, and the dead purple hue transformed into a bright scarlet. How has this wonderful change been accomplished? What has it parted with? what has it received? and by what means has so beneficial a barter been produced?

These are questions which have occupied the attention of physiologists

* Physiological experiments have sufficiently proved, of late, that the same alternation of contraction and dilatation does not take place in the arteries in a free or natural state; for where there is no resistance to the flow of the blood along their canals, there is no variation in their diameter; and that it is only the pressure of the finger or some other substance against the side of an artery that produces its pulse. Stud. of Med. ii. p. 16. Experimental Inquiry into the Nature, &c. of the Arterial Pulse, by C. H. Parry, M. D. 1816.

† It has lately been pretty clearly established, that by far the most active power in the return of the blood to the heart from the veins is the comparative vacuum which takes place in the ventricles of the heart, when exhausted of blood by the systole or alternating contraction of this organ; in consequence of which, the venous blood is, as it were, sucked up into the right ventricle from the vena cava, or venous system at large. So that the heart, upon this beautiful principle of simplification, becomes alternately a forcing and a suction pump. By its contraction it forces the blood into the arterial system, and by its vacuum it sucks it up from the venous. See Stud. of Med. ii. p, 19. 2d edit. 1825.

in almost all ages; and though we have not yet attained to any thing like demonstration, or even universally acceded to any common theory, the experiments of modern times have established a variety of very important facts which may ultimately lead to such a theory, and clear away the difficulties by which we are still encumbered.

These facts I shall proceed to examine into in language as familiar as I can employ : I must nevertheless presume upon a general acquaintance with the elementary principles and nomenclature of modern chemistry, since a summary survey of zoonomy is not designed to enter into a detail of its mere alphabet or rudiments, but to apply and harmonize detached facts that relate to it, and to condense the materials that have been collected by others into a narrow but regular compass.

The chief substance which has been ascertained to be introduced from the atmosphere into the air-vesicles of the lungs during the act of respiration, and from these into the blood, is oxygene, of which the atmosphere, when pure, consists of about twenty-eight parts in a hundred, the remaining seventy-two being nitrogene.

That this gaseous fluid enters into the lungs is rendered highly probable from a multiplicity of experiments, which concur in proving that a larger portion of oxygene is received by every act of inspiration than is returned by every correspondent act of expiration; and that it passes from the air-vesicles of the lungs into the blood we have also reason to believe from the change of colour which immediately takes place in the latter, and from other experiments made out of the body, as well as in the body, which abundantly ascertain that oxygene has a power of producing this change, and of converting the deep purple of the blood into a bright scarlet.

It is also supposed very generally, that a considerable portion of caloric or the matter of heat, in its elementary form, is communicated to the blood at the same time and in conjunction with the oxygene; but as this substance has hitherto proved imponderable to every scheme that has been devised to ascertain its weight, this continues at present a point avowedly undetermined. That an increase of sensible heat at all times accompanies an increase of respiration is admitted by every one; but since caloric may be obtained by other means, if obtainable at all, and since a denial of its existence as a distinct substance has of late years been as strenuously urged as it was in former times by the Peripatetic school, and upon experiments inaccessible to those philosophers, we are at present in a state of darkness upon this subject, from which I am much afraid we are not likely to be extricated very soon.

I have already observed that nitrogene, or azote as it is also called, is the other gaseous fluid that constitutes the respirable air of the atmosphere. And from a variety of well-conducted experiments by Mr., now Sir Humphry, Davy, it appears also that a certain quantity of this gas is imbibed by the lungs in the same manner they imbibe oxygene, and that like oxygene it is also communicated from the lungs to the blood while circulating through its substance; for, in the experiments adverted to, he found that, as in the case of the oxygene, a smaller quantity was always returned by every successive act of expiration, than had been inhaled by every previous act of inspiration.*

The only gas that seems to have been thrown out from the lungs in

* Priestley had before shown that nitrogene is absorbed. See Phil, Trans. 1790. p. 106.

the course of these experiments is carbonic acid; a very minute proportion of which appears also to be almost always contained in the atmospheric air, though altogether a foreign material, probably eliminated from the decomposition of animal and vegetable bodies that is perpetually taking place, and certainly unnecessary to healthful respiration.

The general result of these experiments was as follows; the natural inspirations were about twenty-six or twenty-seven in a minute; thirteen cubic inches of air were in every instance taken in, and about twelve and three-quarters thrown out by the expiration that succeeded.

The atmospheric or inspired air contained in the thirteen cubic inches, -nine and a half of nitrogene, three and four-tenths of oxygene, and onetenth of an inch of carbonic acid. The twelve inches and three-quarters of returned air contained nine and three-tenths of nitrogene, two and twotenths of oxygene, and one and two-tenths of carbonic acid.

This inhalation, however, varies in persons of different sized chests from 26 to 32 cubic inches, at a temperature of 55°; but these, by the heat of the lungs, and saturated with moisture, become forty or forty-one cubic inches.

Taking, therefore, forty cubic inches as the quantity of air equally inhaled and exhaled about twenty times in a minute, it will follow that a full-grown person respires 48,000 cubic inches in an hour, or 1,152,000 cubic inches in the course of a day; a quantity equal to about 79 hogɛ heads.

A similar train of experiments has more lately been pursued by Messrs. Allen and Pepys, and will be found fully detailed in the Transactions of the Royal Society for 1808. They confirm the preceding proportions, excepting in the retention of nitrogene; this substance having been found by Messrs. Allen and Pepys to have been returned, in every respiration, in the precise proportion in which it was received. It is highly probable, however, that the diet of these two sets of ingenious experimenters had not previously consisted of the same proportion of animal and vegetable materials; and that the blood in the former instance was less charged with nitrogene than in the latter; which would at once account for the difference.

Upon Sir Humphry Davy's experiments, however, the quantity of nitro gene received by the lungs is very inconsiderable, not amounting to more than two-tenths of a cubic inch in an inspiration. And omitting the con sideration of this gas, as also that of caloric, on account of the unsettled state of the question, respiration, from this view of the subject, consists merely in the act of receiving oxygene, and throwing out carbonic acid gas; the lungs imbibing and communicating to the system not less than 32.4 cubic inches of the former, and parting with not less than 26.5 of the latter, every minute. So that, taking the gravity of carbonic acid gas, as calculated by Lavoisier, eleven ounces of solid carbon or charcoal arc emitted from the lungs every twenty-four hours.*

The whole of the theory and some of the supposed facts here advanced, however, have of late been very considerably disputed by Mr. Ellis, in his Inquiry into the Changes induced on Atmospheric Air by the Germination of Seeds. He concurs with Messrs. Allen and Pepys, in ascertaining that precisely the same quantity of nitrogene is expired as is inspired; but he objects to their conclusion, that the whole of any constituent ele

* Phil. Traps. 1808. part. 249;

iment of respired air introduced into the air-vesicles, and not returned by the alternate expiration, is necessarily conveyed into the blood-vessels, believing that much of this may remain unascertained, in consequence of an increased, but not sensibly increased, expansion of the chest. He admits that carbonic vapour is thrown forth in the quantity usually alleged with every act of expiration; but he offers evidence to prove that it is the carbone only that is discharged from the animal system, in connexion with the exhaling vapour; contending that the carbone thus existing is separated from the vapour by its union with the whole of the oxygene introduced by the previous act of inspiration by which alone it is converted into carbonic acid gas: for he found the same decomposition of atmospheric air produced by introducing a small bladder, moistened, and filled with any substance, or perfectly empty, and introduced into an inverted glass containing a certain portion of atmospheric air standing upon quicksilver. He denies, therefore, that the air-vessels are in any degree porous to gases of any kind, excepting caloric; and, consequently, denies that the blood is converted from a deep modena hue into a bright scarlet by its union with oxygene; believing, or seeming to believe, that this result is entirely produced by the action of the caloric separated in the air-vesicles upon the union of the carbone of the vapour exhaled from their surfaces, with the oxygene introduced-by inspiration. So that, according to this theory, respiration is nothing more than an introduction of caloric into the system, and the conversion of a portion of oxygene (the whole received by the act of inspiration) into an equal bulk of carbonic acid by the carbone exhaled from the living organized body. Air, therefore, examined after respiration, is found to differ from the same air before it is breathed in having lost a portion of oxygene, gained an equal volume of carbonic acid, and in being loaded with pure watery vapour, the vapour thrown off from the lungs; and he has offered an additional proof that the oxygene of the carbonic acid is that introduced in the act of inspiration, by showing, as in the case of breathing hydrogene gas, that no carbonic acid is returned, and apparently none produced.

In opposition to the hypothesis of Dr. Priestley, he seems to show, and plausibly to establish, that all terrestrial plants, whether growing in absolute darkness, in the shade, or exposed to the direct rays of the sun, are constantly removing a quantity of oxygene from the atmosphere, and substituting an exactly equal volume of carbonic acid; that they produce this change by emitting from their leaves, flowers, fruits, stems, and roots, and by a process like animal exhalation, carbonaceous matter, which combines with the oxygene of the surrounding air; and that such a function is essentially necessary to their vital existence. In doing this, however, the carbonaceous matter is given forth more freely from the green parts than from any other, especially when exposed to the direct rays of the sun, by means of its affinity for the calorific rays; in consequence of which the oxygene of the carbone is set at liberty, and escapes from the cellular texture of the green parts through the external pores; an action, however, which is not necessary to life, for a plant does not die when this has ceased, while it is equally found to occur in a dead as in a living plant. It was probably this occasional escape of oxygene that induced Priestley to regard it as an invariable and constant process, affording a compensation for the animal carbone thrown into the, air, and thus taking from and giving to the animal world what seemed to be mutually demanded.

Mr. Ellis also affirms that all the various colours of vegetables depend