A moist condition of the respiratory mem brane appears to be essential to the proper performance of its functions, and this is obtained in those animals which breathe atmo spheric air, by its deep position, and by the fluid secretions poured out upon its surface.
The structure of that portion of the respi ratory apparatus which acts in bringing fresh supplies of atmospheric air into contact with the respiratory surface, is chiefly regulated by the animal being terrestrial or aquatic, and by the amount of respiration required. In many of the lower aquatic tribes the respiratory surface is external and floats in the water, and any movements on the part of the animal, and currents in the water, must change, more or less rapidly, the fluid in contact with the respiratory surface. In such cases, the only structural provision for promoting such move ments in the water, is the presence of nume rous cilia on the surface of the respiratory membrane, which by their incessant action produce currents in their neighbourhood. In those aquatic animals, where the respiratory orran assumes the form of branched tubes or of:Cavities, the water in their interior is con stantly undergoing a gradual renevval by the incessant action of the cilia upon the inner surface, and it is at other times expelled or renewed much more rapidly by the action of the surrounding contractile tissues. In some of the Crustacea where the branchice are lodged in a cavity placed under the lateral portions of the carapace, the renewal of the water is effected by the movements of distinct appen dages, belonaing more especially to the masti catory and 'locomotive organs ; and in the Cephalopoda, where the branchim are placed in a cavity beneath the mantle into which the rectum and generative organs also open, the water is chiefly renewed by the contractions of the mantle. In fishes, whose demand for atmospheric air is greater, a complicated appa ratus of muscles, bones, and nerves is arranged around the branchise, for keeping up a con stant stream of water over the respiratory membrane. In insects the air in the trachew is chiefly renewed by the contractions and dilatations of the abdominal segments of the body.
In all the vertebrata that breathe by lungs, the muscles for renewino. the air in contact
with the respiratory surface are numerous, are called into action involuntarily and by ex citations conveyed through the nervous sys tem, and contract more or less frequently according to the wants of the organism. In batrachian reptiles, where the ribs are want ing, and in chelonian reptiles whose ribs are soldered together and immovable, the air is not drawn into the lungs, as in birds and the Mamtnalia, by the dilatations of' the walls of the cavity enclosing them, but it is forced into the lungs chiefly by the action of the muscles attached to the hyoid bone as by a forcing pump, from which ft is again expelled chiefly by the abdominal muscles, as in the MammaIia and birds.* The manner in which the nutritious juices are carried to and from the respiratory mem brane is usually regarded as a part of the function of the circulation, and has already been described in the article on that function. The position and extent of the respiratory membrane, and the degree of activity required of it, are the circumstances that chiefly in fluence the arrangement of this portion of the circulatory apparatus, and the quantity and velocity with which the nutritious juices are circulated through it. When the respira tory membrane is closely packed in a par ticular part of the organism, and the function of respiration is at the same time energetic as in the Mammalia, the blood is circulated with great activity, and in great quantity, through vessels distributed in this membrane, and appropriated solely to this purpose. When the respiratory membrane is extensively dif fused, as in insects, throughout the organism, and the atmospheric air is brought into contact with it in the different tissues, a particular set of canals for carrying the nutritious juices to and from the respiratory rnembrane is not required ; and were we, in such animals, to examine the circulatory apparatus without any reference to the nature of' the respiratory apparatus, we could not understand how a circulatory apparatus, apparently so imperfect, is yet equally efficient in carrying on the nutri tive processes as in other animals where its mechanism is much more complicated.