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 and shut at pleasure. Yet, with a singular appearance of capriciousness, the cuttle-fish is possessed 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

* Blumenbach, § 167.

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; one 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 may become part of 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 enquire into the chief feature in which animal and vegetable substances agree, and the chief feature in which they differ.

Animals and vegetables, then, agree in their equal necessity of extracting a certain sweet and saccharine fluid, as the basis of their support, from whatever substances may for this purpose be applied to their respective organs of digestion. Animal chyle and vegetable sap make a very close approach to each other in their constituent principles as well as in their external appearance. In this respect plants and animals agree. They disagree inasmuch as animal substances possess a very large proportion of azote, with a small comparative proportion of

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

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carbone; while vegetable substances, on the contrary, possess a very large proportion of carbone, with a small comparative proportion of azote. And it is hence obvious that vegetable matter can only be assimilated to animal by parting with its excess of carbone, and filling up its deficiency of azote.

Vegetable substances, then, part first of all with a considerable portion of their excess of carbone in the stomach and intestinal canal, during the process of digestion; a certain quantity of the carbone detaching a certain quantity of the oxygene existing in these organs, as an elementary part of the air or water they contain, in consequence of its closer affinity to oxygene, and producing carbonic acid gas; a fact which has been clearly ascertained by a variety of experiments by M. Jurine of Geneva. A surplus of carbone, however, still enters the animal system through the medium of the lacteals, and continues to circulate with the chyle, or the blood, till it reaches the lungs. Here, again, a certain portion of carbone is perpetually parted with upon every expiration, in the form of carbonic vapour, according to Mr. Ellis, but according to Sir H. Davy and others, in that of carbonic gas, in consequence of its union with a part of the oxygene introduced into the lungs with every returning inspiration*; while the excess that yet remains is carried off by the skin, in consequence of its contact with atmospheric air: a fact put beyond all doubt by the experiments and observations of M.

* See Sir H. Davy's Researches Chemical and Philosophical, &c.; and Mémoire sur la Chaleur, par MM. Lavoisier et De la Place. Mém. de l'Acad. De la Combustion, &c. J

Jurine, although, on a superficial view, opposed by a few experiments of Mr. Ingenhouz*, and obvious to every one, from the well known circumstance that the purest linen, upon the purest skin, in the purest atmosphere, soon becomes discoloured.

In this way, then, and by this triple co-operation of the stomach, the lungs, and the skin, vegetable matter, in its conversion into animal, parts with the whole of its excess of carbone.

Its deficiency of azote becomes supplied in a twofold method: first, at the lungs; also by the process of respiration, as should appear from the concurrent experiments of Dr. Priestley and Sir H. Davy, which agree in showing that a larger portion of azote is inhaled upon every inspiration than is returned by every succeeding expiration; in consequence of which the portion retained in the lungs seems to enter into the system, in the same manner as the retained oxygene, and perhaps in conjunction with it: while, in union with this economy of the lungs, the skin also absorbs a considerable quantity of azote, and thus completes the supply that is necessary for the animalization of vegetable food ‡;

* Essai de Théorie sur l'Animalisation et l'Assimilation des Alimens, &c. Annales de Chimie, tom. ii.

† See Davy's Researches Chemical and Philosophical, &c. and Priestley's Experiments and Observations on different Kinds of Air, vol. iii.

M. Jurine is chiefly entitled to the honour of this discovery: his experiments coincide with several of Dr. Priestley's results, and have been since confirmed by other experiments of MM. Lavoisier and Fourcroy. See Premier Mémoire sur la Transpiration des Animaux, par A. Seguin et Lavoisier, 1792; and compare with M. Hassenfratz's Mémoire sur la Combinaison de l'Oxygène, &c. Acad. des Scien. 1791.

evincing hereby a double consent of action in these two organs, and giving us some insight into the mode by which insects and worms, which are totally destitute of lungs, are capable of employing the skin as a substitute for lungs, by breathing through the spiracles existing in the skin for this purpose, or merely through the common pores of the skin, without any such additional mechanism. It is by this mode, also, that respiration takes place through the whole vegetable world, offering us another instance of resemblance to many parts of the animal; in consequence of which, insects, worms, and the leaves of vegetables, equally perish by being smeared over with oil, or any other viscous fluid that obstructs their cutaneous orifices.

But to complete the great circle of universal action, and to preserve the important balance of nature in a state of equipoise, it is necessary also to enquire by what means animal matter is reconverted into vegetable, so as to afford to plants the same basis of nutriment which plants have previously afforded to animals.

Now, this is for the most part obtained by the process of PUTREFACTION, or a return of the constituent principles of animal matter to their original affinities, from which they have been inflected by the superior control of the vital principle, so long as it inhabited the animal frame, and coerced into other combinations and productions.* Putrefaction is,

* It should hence appear, that putrefaction is the only positive criterion of death, or the total cessation of the principle of life. Galvanism has, indeed, been advanced as a decisive proof of the same by Behrends and Creeve; but Humboldt has sufficiently shown its insecurity as an infallible test.