The lungs of amphibia are distinguish ed from those of warm-blooded animals, both by a great superiority in point of size, as well as by a greater' looseness of texture, arising from the great size of their air-vessels. In frogs, lizards, and serpents, the lungs consist of a cavity, whose sides are cellular. The poste rior part of the organ either forms a mere membranous bag, or else the cells are larger there than elsewhere. In the turtle the vesicles are very large, but the texture is uniform through out.
In the tadpole, and the young of such lizards as bring forth in water, there are two organs, which somewhat resemble the gills of a fish (appendices fimbriatx, Swammerdam.) These serve for the purposes of respiration while the animal lives in the water. They are connected to the sides of the neck, and hang loose from the animal ; they are not permanent, but are gradually withdrawn into the chest, (within a tew da)s, in the reptiles of this country,)where their remains may still be perceived for some time near to the true lungs. Instead of the branchial opening, by which fishes again discharge the water which they have taken in at the mouth, some, tadpoles have for this purpose a canal on the left side of the head, near the eye; which must be dis tinguished from the small tube on the lower lip, by which they attach them selves to aquatic plants.
Instead of lungs, fishes are furnished with gills or branchix ; which are placed behind the head, on both sides, and have a moveable gill cover (operculum bran chiale,) which is wanting in the order of pisces chondropterygii only. By means of these organs, which are connected with the throat, the animal receives its oxyen from the air contained in the wa ter; as those animals which breathe de rive it immediately from the atmosphere. They afterwards discharge the water through the branchial openings (apertu re branchiales ; ) and therefore they are distinguished from animals of the three preceding classes by this circumstance ; viz. that they do not respire by the same way that they inspire.
'We have already shewn, in speaking of the organs of circulation, how the gills receive the venous blood by means of the branchial artery, and how this blood is sent into the aorta after its conversion into the arterial state. The distribution of these vessels on the folds and divisions of the gills consti tutes one of the most delicate and mi nute pieces of structure in the animal economy.
EaCh of the gills consists in most fishes of four divisions, resting on the same number of arched portions of bone or cartilage, connected to the os hyoides. Generally there is only a single opening for the discharge of the water; but in many cases, particularly among the cartilaginous fishes, there Are several.
Many animals of this order possess a single or double swimming bladder ; which has been found in different in stances to contain azote, hydrogen, and oxygen. It has not been hitherto deter mined, whether it be subservient to any other functions, besides that well known one from which its name is derived. In the mean time, like the air receptacles of birds, it may he considered, without impropriety, in the present division of the work.
It is placed in the abdomen, and close ly attached to the spine. It communi cates generally with the eesophagus, and sometimes with the stomach, by a canal (ductus pneumaticus,) containing in some instances, as the carp, valves which seem to allow the passage of air from the blad der, but not to admit its entrance from without.
That white.blooded animals indispensa bly require a species of respiration would have been inferred, by analogy, from the wonderful apparatus of gills or trachea:, which have been discovered in most or ders of both classes of these beings. But in many cases direct proof has been ob tained on this point : experiment has ac tually proved the exchange of carbon for oxygen.
White-blooded animals are moreover distinguished from those which have red blood by this circumstance : that none of the former, as far as we hitherto know, take in air through the mouth.
Many aquatic insects, as the genus can cer, have a species of gills near the attach ment of their legs. The others, and par ticularly the land-insects, which consti tute, as is well known, by far the greatest number of this class of animals, are fur nished with air-vessels, or trachese, which ramify over most of their body. These trachea are much larger and more nume rous in the larva state of such insects as undergo a metamorphosis, (in which state also the process of nutrition is carried on to the greatest extent) than after the last, or, as it is called, the perfect change has taken place.