332. Whatever the medium breathed, respiration in the animal is energetic in proportion to the extent of the respiratory surface exposed to the surrounding element. As the water-breathing animals successively rise in organization, their respiratory surface becomes more and more extended, and a proportionally larger quantity of water is made to flow over it. It is the same in aërial respiration: the higher the animal, the greater the extent of its respiratory surface; and the larger the bulk of air that acts upon it.
333. Whatever the medium breathed, respiration is effected by the contact of fresh strata of the surrounding element with the respiratory surface. The mode in which this constant renewal of the strata is effected, is either by the motion of the body to and fro in the element; or by the creation of currents in it, which flow to the respiratory surface. A main part of the apparatus of respiration consists of the expedients necessary to accomplish these two objects; and that apparatus is simple, or complex, chiefly according to the extent of the mechanism requisite to effect them.
334. Whatever the medium breathed, the organic tissue which constitutes the essential part of the immediate organ of respiration is the skin. The primary tissue of which the skin is composed is the cellular (23 et seq.), which, organized into mucous membrane (33 et seq.), forms the essential constituent of the skin (34). In all animals the skin covers both the external and the internal surfaces of the body (34). When forming the external envelop, this organ commonly retains the name of skin; when forming the internal lining, it is generally called mucous membrane; and in all animals, from the monad to man, either in the form of an external envelop, or an internal lining, or by both in conjunction, or by some localization and modification of both, the skin constitutes the immediate organ of respiration. In different classes of animals it is variously arranged, assumes various forms, and is placed in various situations, according to the medium breathed, and the facility of bringing its entire surface into contact with the surrounding element; but in all, the organ and its office are the same: it is the modification only—that modification being invariably and strictly adaptation, which constitutes the whole diversity of the immediate organ of respiration.
335. At the commencement of the animal scale, in the countless tribes of the polygastrica (vol. i. p. 34, et seq.), respiration is effected through the delicate membrane which envelops the soft substance of which their body is composed. The air contained in the water in which they live, penetrating the porous external envelop, permeates every part of their body; aërates their nutritive juices; and converts them immediately into the very substance of their body. They are not yet covered with solid shells, nor with dense impervious scales, nor with any hard material which would exclude the general respiratory influence of water, or render necessary any special expedient to bring their respiratory surface into contact with the element.
336. But in some tribes even of these simple creatures there is visible by the microscope an afflux of their nutritive juices to the delicate pellicle that envelops them, in the form of a vascular net-work, in which there appears to be a motion of fluids, probably the nutritive juices flowing in the only position of the body in which they could come into direct contact with the surrounding element. In some more highly advanced tribes, as in wheel animalcules, there is an obvious circulating system in vessels near the surface of the skin. In other tribes, the internal surface constituting the alimentary canal, is of great extent and width, and forms numerous cavities which are often distended with water. In this manner a portion of the internal, as well as the external surface is made contributary to the function of respiration, and this extended respiration is conducive to their great and continued activity, to their rapid development, and to the extraordinary fertility of their races.
1. The mouth; 2. the stomach; 3. large canals going from the stomach; 4. smaller canals which form; 5. a plexus of vessels at the margin of the disc serving for respiration; 6. margin of the disc.
337. In creatures somewhat higher in the scale, a portion of the external surface is reflected inwards in the form of a sac, with an external opening (fig. [CXXV]. 1). In some medusæ there are numerous sacs of this kind, which pass inwards until they are separated only by thin septa from the cavities of the stomach. The water permeating and filling these sacs comes into contact with an interior portion of the body, not to be reached through the external surface. At the margin of the disk (fig. [CXXV]. 6) there is spread out a delicate net-work of vessels (fig. [CXXV]. 5); these vessels communicate with small canals (fig. [CXXV]. 4) which open into larger canals (fig. [CXXV]. 3) that proceed directly from the stomach (fig. [CXXV]. 2). As the aliment is prepared by the stomach, it is transmitted thence by these communicating canals to the exterior net-work of vessels where it is aërated.
338. As organization advances, as the component tissues of the body become more dense, and are moulded into more complex structures, when, moreover, these structures are placed deep in the interior of the body, far from the external envelop, and proportionally distant from the surrounding element, the respiratory apparatus necessarily increases in complexity. The first complication consists in the formation of minute, delicate, transparent tubes (fig. [CXXVI]. 5), which communicate with the external surface by a special organ (fig. CXXVI. 4) that conveys water into the interior of the body (fig. [CXXVI]. 5). By means of these ramifying water-tubes, upon the delicate walls of which the blood-vessels are spread out in minute and beautiful capillaries, the water is brought into immediate contact with the vascular system.