on the varied proportion of alkaline and acid matter mixed with the juices of the coloured parts of plants; that green and yellow, for example, are always produced by an excess of alkali in the colourable juices of the leaf or flower; and all the shades of red, by a predominance of acid; while a neutral mixture produces a white. And hence there is most green in the summer season, when the oxygene is parted with most freely, as drawn away by the rays of light; while in autumn, when there is less separation, the other colours of yellow and red are most frequent.

Mr. Ellis has also quoted a variety of experiments on different kinds of fishes, muscles, marine testacea, snails, leeches, zoophytes, and tadpoles, in which it was found that the water wherein these animals had been placed had lost a part of its oxygene, and received an addition of carbonic acid, while its nitrogene had remained unaffected.*

This hypothesis, however, requires confirmation, and is at present open to many objections. If caloric can permeate animal membranes, as Mr. Ellis admits it to do, and unite by chemical affinity with the blood in the blood-vessels, so also may oxygene in certain cases of combination. Mr. Porrett has shown that the Voltaic fluid, when operating upon water, is capable of carrying even water itself through a piece of bladder, and of raising it into a heap against the force of gravitation; and hence other affinities may not only introduce the oxygene of the respired air, or a part of it, into the blood of the blood-vessels in the lungs, through the tissue of the air-cells, but at the same time carry off the superabundant carbone in the form of carbonic acid, instead of its being thrown out in that of carhonic vapour. Nor have we any proof that carbone will dissolve in water, and produce such vapour; and hence such an idea is gratuitous.

Of the general operation, however, there is no doubt, whatever be the manner in which it is performed: and by such operation the new blood becomes assimilated to the nature of the system it has to nourish; and the old or exhausted blood both relieved from a material that may be said to suffocate it, and reinspirited for fresh action. In this state of perfection, produced from the matter of food introduced into the stomach, and elaborated by the gases of the atmosphere, received chiefly by the act of respiration, but perhaps partly also by the absorbing pores of the skin, the blood on its analysis is found to consist of the following nine parts, independently of its aerial materials :-first, a peculiar aroma, or odour, of which every one must be sensible who has been present at a slaughter-house on cutting up the fresh bodies of oxen; secondly, fibrine, or fibrous matter; thirdly, uncoagulable matter, but no gelatine, which is a subsequent secretion; fourthly, albumen; fifthly, red colouring matter; sixthly, iron; seventhly, sulphur; eighthly, soda; and lastly, water. The proportion of these parts vary almost infinitely, according to the age, temperament, and manner of living; each of these having a character that essentially belongs to it, with particular shades that are often difficult to be laid hold of.

Of these component parts, the most extraordinary are the red colouring matter, the iron, and the sulphur; nor are we by any means acquainted with the mode by which they obtain an existence in the blood. I have already had occasion to observe, that albumen and fibrine are substances formed by the action of the living principle out of the common materials of the food, and that it is probable the lime found in the bones and other

Inquiry into the Changes induced on Atmospheric Air by the Germination of Seeds, &c. Sve. 1807. As also, Further Inquiries into the Changes, &c. 8vo. 1811.

† Stud. of Med. edit. ii. vol. i. p. 474. Thomson's Annals of Philos. No. xliii. pp. 75, 76.

parts is produced in the same manner. Whether the iron and sulphur that are traced in the blood have a similar origin, or exist in the different articles of our diet, and are merely separated from the other materials with which they are combined, is a physical problem that yet remains to be solved. It should be observed, however, that the sulphur does not exist in a free state even in the blood itself, but is only a component part of its albumen. Considering the universality of these substances in the blood, and the uniformity of their proportion in similar ages, temperaments, and habits, whatever be the soil on which we reside; that those who live in a country in which these minerals are scarcely to be traced have not less, while those who live in a country that overflows with them have not more; it is perhaps most rational to conclude, that they are generated in the laboratory of the animal system itself, by the all-controlling influence of the living principle.

The exact proportion of sulphur contained in the system has been less accurately ascertained than that of the iron, which last in an adult, the weight of whose blood may be estimated at 28lbs.,* ought usually to amount to seventy scruples, or about three ounces: and hence the blood of about forty men contains iron enough to make a good plough-share, and might easily have its iron extracted from it, be reduced to a metallic state, and manufactured into such an instrument.

Iron is seldom found except in the red particles of the blood;† and it has hence been supposed by the French chemists to be the colouring material itself. The process of respiration, according to the theory of Lavoisier and Fourcroy, is a direct process of combustion in which the animal system finds the carbone, and the atmosphere the oxygene and caloric; and in consequence of the sensible heat which is set at liberty during the combustion, the iron of the blood is converted into a red oxyde, and hence necessarily becomes a pigment.

But it is impossible to ascribe the red colour to this principle: for, first, we are by no means certain that the air communicates any such substance as caloric to the blood; and, secondly, let the sensible heat of the blood arise from whatever quarter it may, it can never be sufficiently augmented by the most violent degree, either of local or general inflammation, to convert the iron of the blood into a red oxyde, which, indeed, is never produced without rapid combustion, flame, and intense heat. And hence, Sir Humphry Davy conjectures the carbon itself of the blood to be the real colouring material, and to be separated from the oxygene, with which it is necessarily united to constitute carbonic acid gas, by the matter of light, which he supposes to be introduced into the system in the act of respiration, instead of the matter of caloric; in consequence of which it immediately becomes a pigment. But the difficulties which attend this theory are almost, if not altogether, as numerous as those which attend the theory of combustion, and it is unnecessary to pursue the subject any farther.

In the Philosophical Transactions, and in several of the best established

* Blumenbach states the proportion in an adult and healthy man to be as 1 to 5 of the entire weight of the body. By experiments on the water-newt (lacerta palustris), he found the proportion in this animal to be only as 2 to $6.

† Mr. Brande denies that iron exists more in the red particles of the blood than in the other principles: according to his experiments, it exists but in a very inconsiderable quantity in any of them; but he has traced it in the chyle, in the serum, and in the fibrine, or washed crassament. Phil. Trans 1812, p. 112. Vauquelin has traced it as a constituent in eggshells and oyster-shells. Thomson's Annals of Philos. No. 1. p. 66. But Berzelius has proved Brande to be mistaken, and that iron exists largely in the blood, and is the cause of The red colour. See his Anim. Chemistry.

foreign Memoirs, we meet with a few very curious instances of spontaneous inflammation, or active combustion, having occurred in the human body. The accident has usually been detected by the penetrating smell of burning and sooty films, which have diffused themselves to a considerable distance; and the sufferers have in every instance been discovered dead, with the body more or less completely burnt up, and containing in the burnt parts nothing more than an oily, sooty, extremely fetid and crumbly matter. In one or two instances there has appeared, when the light was totally excluded, a faint lambent flame bickering over the limbs; but the general combustion was so feeble that the chairs and other furniture of the room within the reach of the burning body have in no instance been found more than scorched, and in most instances altogether uninjured.

It is by no means easy to explain these extraordinary facts, but they have been too frequent, and are too well authenticated in different countries, to justify our disbelief. In every instance but one the subjects have been females, somewhat advanced in life, and apparently much addicted to spirituous liquors. I shall hence only observe, in few words, that the animal body in itself consists of a variety of combustible materials; and that the process of respiration (though not completely established to be such) has a very near alliance to that of combustion itself: that the usual heat of the blood, taking that of man as our standard, is 98 of Fahrenheit, and under an inflammatory temperament may be 103° or 104°; and hence, though by no means sufficiently exalted for open or manifest combustion, may be more than sufficiently so for a slow or smothered combustion; since the combustion of a dung-hill seldom exceeds 81°, and is not often found higher in fermenting hay-stacks, when they first burst forth into flame. The use of ardent spirits may possibly, in the cases before us, have predisposed the system to so extraordinary an accident; though we all know that this is not a common result of such a habit, mischievous as it is in other respects. The lambent flame emitted from the body is probably phosphorescent, and hence little likely to set fire to the surrounding furniture. It is not certain whether this flame originates spontaneously, or is only spontaneously continued, after having been produced by a lighted substance coming too nearly in contact with a body thus surcharged with inflammable materials.

Such, then, are the circulatory and respiratory systems in the most perfect animals as mammals, birds, and amphibials. It should be observed, however, that in birds the hollow bones themselves, and a variety of aircells that are connected with them, constitute, as we have already had occasion to notice,* a part of the general respiratory organ, and endow them with that levity of form which so peculiarly characterizes them, and which ⚫ is so skilfully adapted to their intention. It should be remarked, also, that in most amphibious animals, and especially in the turtle, whose interior structure is the most perfect of the entire class, the two ventricles, or larger cavities of the heart, communicate something after the manner in which they do in the human fetus. The lungs of this class are for the most part unusually large; and they have a power of extracting oxygene from water as well as from air; whence their capability of existing in both elements. The oxygene, however, obtained from the water is not by a decomposition of the water into its elementary parts, but only by a separation of such air as is loosely combined with it; for if water be deprived of air or oxy

*Ser. 1. Lect. XI. p. 119.

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gene the animal soon expires. We have already observed that some amphibials appear to possess only a single heart, and even that of a very simple structure.

In fishes the heart is single, or consists only of two compartments instead of four, and hence the circulation is single also. The gills in this class answer the intention of lungs, and the blood is sent to them for this purpose from the heart, in order to be deprived of its excess of carbone, and supplied with its deficiency of oxygene. It is not returned to the heart, as in the case of the superior animals, but is immediately distributed over the body by an aorta or large artery issuing from the organ of the gills. The oxygene, in these animals is separated from the water instead of from the air and for this purpose the water usually passes through the mouth before it reaches the gills: yet in the ray-tribe there is a conducting aperture on each side of the head, through which the water travels instead of through the mouth. In the lamprey it is received by seven apertures opening on each side of the head into bags, which perform the office of gills, and passes out by the same orifices, and not as has been supposed, by a different opening said to constitute its nostril.

In the common leech there are sixteen of these orifices on each side of the belly, which answer the same purpose. In the sea-mouse (aphrodita aculeata) "the water passes through the lateral openings between the feet into the cavity under the muscles of the back."*

The siren possesses a singular construction, and exhibits both gills and lungs; thus uniting the class of fishes with that of amphibials. Linnéus did not know how to arrange this curious animal, and shortly before his death formed a new order of amphibials, which he called MEANTES, for the purpose of receiving it. It ranks usually in the class of fishes.

The only air-vessels of the winged insects have a resemblance to the apertures of the lamprey, and are called stigmata. In most instances these are placed on each side of the body; and each is regarded as a distinct trachea, conducting the air, as M. Cuvier elegantly expresses it, in search of the blood, as the blood has no means of travelling in search of the air. They are of various shapes and number, and are sometimes round, sometimes oval, but more generally elongated like a button-hole. In the grasshopper they are twenty-four, disposed in four distinct rows.

The membranous tube that runs along the back of insects is called by Cuvier the dorsal vessel. It discovers an alternate dilatation and contraction; and is supposed by many naturalists to be a heart, or to answer the purpose of a heart. Cuvier regards it as a mere vestige of a heart without contractions from its own exertion, and without ramifications of any kind the contractions being chiefly produced by the action of the muscles running along the back and sides, as also by the nerves and trachea, or stigmata. Scorpions and spiders have a proper heart: and as the term insects is now confined by M. Cuvier and M. Marcel de Serres to those that have only this dorsal vessel, or imperfect heart, the two former genera are struck out of the list of insects as given by Linnéus.§

This organ differs very considerably in its structure and degree of sim

Sir E. Home, Phil. Trans. 1815, p. 260.

Home's Life of Hunter, prefixed to Hunter's Treatise on the Blood, Inflammation, &c.

p. xi.

En un mot, le sang ne pouvant aller chercher l'air, c'est l'air qui va chercher le sang. Lecons d'Anat. Comp. I. 23. Sect. 2. Art. 5.

§ See M. Marcel de Serres' statement, Tilloch's Journal, vol. xliv. p. 148. ; and especially Thomson's Annala of Phil. No. XXIII. p. 847, 348. $50. 354.

plicity in molluscous animals. The heart of the teredo has two auricles and two ventricles; that of the oyster one auricle and one ventricle. In the muscle the heart is not, strictly speaking, divided into an auricle and ventricle, but rather consists of an oval bag, through the middle of which the lower portion of the intestine passes. Two veins from the gills open into the heart, one on each side, which may be considered as the auricles. In several of the crustaceous insects of Linnéus, as, for example, the monoculus and craw-fish, the stigmata converge into a cluster, so as to form gills; which in some species are found seated in the claws, and in other species under the tail. These have for the most part a small single heart, and consequently a single circulation, the course of which, however, is directly the reverse of that pursued in fishes; for the heart in the present instance propels the blood through the body, and the gills receive it, and propel it to the heart. This is also the case in the snail, slug, and many other soft-bodied worms, which possess a gill in the neck consisting of a single aperture, which it can open or shut at pleasure. Yet, with a singular kind of apparent sportiveness, the cuttle-fish is possest of three distinct hearts, which is one more than is allotted to mankind, in whom this organ is only double.

In zoophytes we are in great ignorance both as to their sanguineous and respiratory functions. That they stand in need of oxygene, and even of nitrogene, has been sufficiently determined by Sir H. Davy; as it has also that they absorb their oxygene and nitrogene, as fishes do, from the water which holds these gases in solution. Their nutrition appears to be effected by an immediate derivation of the nutritive fluid from their interior cavity into the gelatinous substance of their body.*

Hence, then, the respiratory organs of the animal kingdom may be divided into three classes; lungs, gills, and holes or stigmata: each of the three classes exhibits a great variety in its form, but the office in which they are employed is the same. Animals of every kind must be supplied with air, or rather with oxygene, however they may differ in other respects in tenacity of life; for a vacuum, or a medium deprived of oxygene, kills them equally. Snails and slugs corked up in small bottles have been found to live till they had exhausted the air of every particle of oxygene, and to die immediately afterwards: and frogs and land-turtles, which are well known to survive the loss of the spinal marrow for months, and that of the head or heart for several days, die almost instantly on exposure to a vacuum.†

Connected with this general subject, there is still an important question to be resolved, and which has greatly occupied the attention of physiologists for the last fifty years.

Mediately or immediately, almost all animal nutriment, and consequently almost all animal organization, is derived from a vegetable source. The blade of grass becomes a muscular fibre, and the root of a yam or a potato a human brain. What, then, is that wonderful process which assimilates substances in themselves so unlike? that converts the vegetable into an animal form, and endows it with animal powers?

Now, to be able to reply succinctly to this question, it is necessary first of all to inquire into the chief feature in which animal and vegetable substances agree, and the chief feature in which they diffier.

Animals and vegetables, then, agree in their equal necessity of extract

* Blumenbach, § 167.

† See Encyclop. Brit. art. Physiol. p. 679.