All the accessaries of the assimilating cavity or stomach which we find in animals, from the organs of sense that guide them in. their choice of aliment, to the lips that seize it, the teeth or jaws that bruise it or destroy its vitality, the muscular actions by which it is swallowed, and the chemico-vital processes by which it is dissolved, and the purely vital sen sibilities by which such parts as are proper for nourishment are retained, and such as are irn proper for this purpose are expelled,—all of these are wanting among vegetables.
There are yet other processes which form an essential item in the acts by which organized beings universally continue their existence, which it is necessary we should include in this summary of the common, particular, and dis tinguishing attributes of vegetables and ani mals. One of the most important of these is Respiration.—The leaves in the more per fect vegetables are the instruments of respi ration ; their place is supplied by the general surface in those plants that are aphyllous. Vege tables that live in air act immediately by means of their respiratory organs upon the ambient medium ; those that live in water, upon the air held in solution by the fluid around them.
Vegetables are well known to exhale abun dantly from the surface of their leaves, or stems, in case they have no leaves. The mat ter exhaled is principally water. They have also the farther property of decomposing one of the elements of atmospheric air, namely, carbonic acid gas. In the sunshine the leaves of vegetables fix the carbon which enters into the composition of this gas, and set the oxy gen at liberty ; in the dark, however, a very different process goes forward ; they then actually absorb oxygen and exhale carbonic acid gas; the balance, however, in the aggre gate is not equal between these opposite pro cesses, a much larger quantity of carbon being fixed by the decomposition of carbonic acid gas and oxygen set at liberty, than there is of oxygen absorbed and carbonic acid gas set free. These acts are essential to the life and health of vegetables ; their end and object appear to be the preparation of their proper nutritive fluids or cambium : the sap which reached the leaves, colourless, not coagulable, without glo bules, mere water holding carbonic acid, acetic acid, a muco-saccharine matter, and various salts in solution, is in them converted into a greenish fluid, partly coagulable, and full of globules, which special vessels then 'distribute for the growth and maintenance of the different parts.
The respiratory act is necessary, and goes on without the aid or concurrence of the indi vidual among vegetables.
Animals are no less dependent than vege tables on communication with the air of the atmosphere, either immediately or mediately, for a continuance of their existence, or the manifestation of those acts whose sum con stitutes their lives. In the very lowest tribes the communication between them and the air of the atmosphere takes place over the surface of the body generally, without the intermede of any particular organ or organs for the pur pose. The fluids absorbed into their bodies are brought into contact with the atmospheric air in those points where they approach the external surface, and there appear to undergo the changes necessary to fit them for being converted into the substance of the animals themselves. Simple as this process may ap pear, slender as the means of accomplishing it may seem to be, it is nevertheless essential : interrupted for any length of time, the animal inevitably perishes. A process of such im portance, as may be imagined, is not long left without its appropriate and special apparatus. This varies extremely in its structure, in the different tribes of animals, and according to the circumstances surrounded by which they live. Some have lungs, branchia or gills, and trachea opening by spiracula, of infinitely va ried construction.
Respiration is also carried on vicariously in a very large proportion of animals, if not perhaps in all to a certain extent, by means of the skin, and in some even by the instru mentality of the alimentary canal.
The changes effected in the atmospheric air by the respiratory apparatus of all animals are similar, but they differ from those that are produced by the corresponding implements in vegetables : the proportion of oxygen it con tains universally diminishes, and the quantity of carbonic acid gas it holds in solution as invariably increases in amount. A quantity of water or of watery vapour is at the same time thrown off. This is exactly the opposite of what we have seen to be the effect of respi ration among vegetables ; in these the quantity of oxygen is augmented, whilst that of car bonic acid gas is diminished. The nutritive fluids newly prepared by the apparatus of digestion, or that have already gone the round of the system, are by a variety of means ex posed, in the special or common apparatuses mentioned, to the influence of the atmospheric air, from the contact of which they undergo certain important and often manifest changes that fit them for their ultimate office in the animal economy, — the maintenance of its parts, with their inherent capacities to execute the various functions imposed upon them.