ORGANS OF RESPIRATION.
The incessant continuation of the great chemical process, by which oxygen is ex changed for hydrogen and carbon, is es sentially necessary to the well being of the greater part of animals. Yet the or gans and mechanism, by which this won derful function is carried on, vary very considerably. In the mammalia, after birth ; in birds, when they have left the egg ; and in amphibia,' when complete ly formed ; the chief organ of this func tion is the lungs : in fish it is perform ed in the gills ; in most insects in their trachea ; in the vermes, in analogous, but at the same time very different parts.
The respiratory organs of birds consti tute one of the most singular structures in the animal economy, on account of se veral peculiarities which they possess ; but more particularly in consequence of their connection with the numerous air cells, which are expanded over the whole body.
The lungs themselves are compara tively small, flattened, and adhering above to the chest, where they seem to be placed in the intervals of the ribs ; they are only covered by the pleura on their under surface, so that they are in fact on the outside of the cavity of the chest, if we consider that cavity as being defined by the pleura : a great part of the tho rax, as well as the abdomen, is occupied by the membranous air-cells, into which the lungs open by considerable apertures. Those of the thorax are divided, at least in the larger birds, by membranous trans verse septa, into smaller portions ; each of which, as well as the abdominal cells, has a particular opening of communica tion with the air-cells of the lungs, and consequently with the trachea. The membranes of these cells, in the larger birds, are provided here and there with considerable fiisciculi of muscular fibres, which have been regarded as a substitute for the diaphragm, which is wanting in this class of animals. They also serve very principally, as we may ascertain by examining large birds in a living state, to drive back again into the lungs the air which they receive in inspiration; whence the repletion and depletion of the thora cic cells must alternate with those of the abdominal cavities.
Brides these cells, a considerable por tion of the skeleton is formed into recep tacles for air in most birds, (for there are indeed exceptions and considerable variations in the different genera and spe cies.) This structure is particularly mark ed in the larger cylindrical bones, as the scapula, clavicle, and femur. It is also found in most of the broad and multangu lar bones of the trunk, as the sternum, ossa innominata, dorsal vertebra, &c. All these are destitute of marrow in the adult bird, at least in their middle ; so that the cylindrical bones form large tubes, which are only interrupted to wards the extremities by a sort of trans verse bony fibres : the broad bones are filled with a reticulated bony texture, the cells of which are empty. They have considerable apertures, (most easily shewn in those extremities of the cylin drical bones which are turned towards the sternum) communicating with the lungs by small air-cells; which facts may be shewn by various experiments on liv ing and dead birds.
These receptacles of air probably serve the purpose of lightening the body of the bird in order to facilitate its motions. This effect is produced in most birds to assist their flight ; in some aquatic spe cies for the purpose of swimming ; in the ostrich and some others, for running. Hence we find the largest and most nu merous bony cells in birds which have the highest and most rapid flight, as the eagle, &c. And hence also the bones of the bird which has just left the egg are filled with a bloody marrow, which is absorbed soon after birth, entirely in some, in others, particularly among the aquatic species, at least for the greater part.
Besides the uses which have been al ready pointed out, these receptacles of air diminish the necessity of breathing frequently, in the rapid and long con tinued motions of several birds, and in the great vocal exertions of the singing birds.