In some of the lower organized plants every part of their surface is probably equally efficient in the performance of the function of respiration ; while in the higher plants, though the whole of the external surface may still aid, the leaves are the chief organs of respira tion. Botanists are not agreed as to what ex tent the spiral tubes, usually regarded as ana logous to the trachere of insects, act as organs of respiration. These spiral vessels do not form continuous canals, and do not open upon the stomata, so that the air cannot enter them without having. previously permeated a greater or less thickness of vegetable tissue covering them. Their share in the perform ance of the function of respiration cannot, probably, be great.
Respiration in animals. — The function of respiration varies greatly in activity-, and in the external form and position of the ap paratus by which it is effected, in the different divisions of the animal kingdom. In all ani mals, except some Infusoria, the nature of the chemical changes between the atmospheric air and the nutritious juices is pretty uniform, and essentially consists in the evolution of car bonic acid gas and the absorption of oxygen. A zote may be exhaled by, or absorbed at, the re spiratory organs in small quantities ; but these changes seem to be of secondary- importance in the function of respiration, do not appear to be uniform in the same animals at different times, and occasionally- cannot be detected. The evidence, however, preponderates in fa vour of the opinion that a small quan tity or azote is exhaled at the respiratory organs.
The function of respiration in animals in cludes two distinct processes—the evolution of one gas from the nutritious juices, and the absorption of another ; and while the former is an act of excretion necessary for the main tenance of the purity of the nutritious juices, the latter is an act of absorption necessary for their proper elaboration. These two acts are of equal importance in supporting the vitality of the organism, are so closely linked together, and are so reciprocally dependent for their continued action, that they have been regarded as belonging to the same function, though in a logical point of view they are parts of two distinct functions, viz. 1st, the absorption by the organism of new materials from the sur rounding media for completing the elaboration of the nutritious juices ; and, 2dly, the excretion from the organism of those substances which are of no further use, and would even prove prejudicial if retained. Many of the defini tions given of the respiratory process are liable to strong objections in consequence of its corn pound character not having been kept strictly in view. These mutual actions between the nutritious juices and atmospheric air are purely chemico-physical, take place wherever the air and the fluids are brought into con tact, and do not require the agency of vitality for their manifestation. When a urinary
bladder has been filled with venous blood and placed in atmospheric air, the oxygen of the atmospheric air, and the free carbonic acid in the blood, mutually permeate the coats of the bladder, the oxygen gas being absorbed by the blood, and the carbonic acid escaping into the surrounding atmosphere. This interchange depends upon the strong tendency that dif ferent gases have to intermix or diffuse them selves through each other, and as this action in this particular case takes place through a permeable membrane, it may be regarded as a kind of endosmose and exosmose.
It necessarily follows, that wherever the nutritious juices of organized bodies are se parated from the atmospheric air by tissues permeable by oxygen and carbonic acid gas, the function of respiration may be performed. The energy of this function will be regulated by the following conditions :— the greater or less thickness and permeability of the tissues interposed between the atmosphetic air and the nutritious fluids ; the quantities and con stitution of these substances thus brought into action ; the extent of surface over which they operate ; and the rapidity with which fresh portions of both are brought into contact, in the place of those whose mutual actions have been completed. In the higher animals, where this function is performed in greatest perfec tion, the apparatus for effecting it is very complicated and extensive, and consists, lst, of a special organ — the lungs, affording an immense extent of surface where the blood flows in innumerable minute streamlets only sepa rated by very thin membranes from the atmo spheric air ; 2dly, of an assemblage of muscles, bones, and nerves, for efficiently renewing the air in the lungs ; and, 3dly, of a series of vessels with a contractile propelling organ attached to them— the pulmonary arteries and veins and right side of the heart, — for rapidly changing the blood in the lungs, and bringing successive portions of it into contact with the atmospheric air. On the other hand, in some of the most simple forms of animal life, which, with the exception of some of the entozoa, are all aquatic, the function of respiration is effected by the external surface, and they have no special organ for exposing their nutritious juices to the action of the atmospheric air, no apparatus for bringing fresh supplies of the surrounding fluid into contact with their bodies, and no canals or tubes for securing a more rapid change of those portions of the nutritious juices exposed to the action of the atmospheric air.