On Respiralion. 207 
rise in the scale of animals. In Fishes, the gills form a conside- 
rable portion of the system, and their office appears to be more 
essential to life than in the Mollusca. The situation and struc- 
ture of these organs were minutely described, together with the 
mechanism by which their action is maintained. The air -con- 
tained in the water is equally vitiated by the respiration of fishes, 
and requires an equally constant renewal as in terrestrial animals. 
Fishes are, therefore, killed in a short time, if confined in a li- 
mited portion of water which has no access to fresh air. When 
many fish are iuclosed in a narrow vessel, they all struggle for 
the uppermost place, where the atmospheric air is first absorbed, 
like the unfortunate men imprisoned in the black hole at Cal- 
eutta. In Humboldt and Provencal’s experiments, a tench was 
found to be able to breathe when the quantity of oxygen in the 
water was reduced to the five-thousandth part of its bulk, though 
it is in this way brought into a state of extreme debility: but the 
fact itself shows the great perfection of the organs in this fish, 
that can extract so minute a quantity of air from water, to which 
the last portions always adhere with great tenacity. 
The respiration of air in its gaseousstate is performed by breath- 
ing terrestrial animals in two ways: first, by means of trachee, 
a mode peculiar to insects; and secondly, by pulmanary cavities, 
which constitute the essential structure of lungs. The trachee 
of insects are tubes which take their rise by open orifices, called 
spiracles or stigmata, from the surface of the body, and are dis- 
tributed by extensive ramifications to every part. They extend 
even to the wings, to the sudden expansion of which they appear 
to contribute. In the higher classes of articulated animals, as 
soon as blood-vessels are met with, the whole® apparatus of 
trachez is found to disappear; their necessity being superseded 
by the power, derived from the possession of circulating vessels, 
of transmitting the juices to particular organs, where their expo- 
sure to the influence of the air may be conveniently effected. 
The pulmonary cavities of spiders, and of some gasteropodous 
Mollusca, such as the snail and slug, which breathe atmospheric 
air, are of this description. 
The structure of the pulmonary organs becomes more refined 
and complex as we proceed to the higher classes of animals, Dr. 
Roget entered into a description of these various structures, and 
of the diversified modes in which the air was received, and made 
to act upon them, and afterwards expelled, in the different or- 
ders of reptiles, of mammalia, and of birds. The singular mode in 
which the frog swallows its air, and inflates its lungs at pleasure, 
was pointed out. The dilatation of the chest ia man, and the 
other mammalia, by the muscular action of the diaphragm, and 
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