CLASSIFICATION OF THE RESPIRATORY MOVEMENTS IN ANIMALS.—It is not easy to name any particular part in the perfect thorax of the higher vertebrate, which is equally destined for respiratory motion throughout the class. Commonly with ribs and intercostal muscles, we connect the idea of a thorax, or a breathing chamber for re spiration ; but a fish has ribs, and likewise intercostal muscles, yet not for any of the purposes of respiration, nor do we acknow ledge them to belong to its thorax. A frog has a thorax for respiration with internal lungs, but no ribs, nor, consequently, any intercostal muscles. Nor is a diaphragm necessary to thoracic respiration ; for it is mostly absent in birds and reptiles, and quite rudimentary in the few instances (such as the ostrich, crocodile, and chelonia), in which it is met with. In the chelonia neither ribs, spine, nor sternum are concerned in the respiratory movements.
The movements of respiration tend to bring before some surface, air or air contained in water ; or to bring a certain surface con tinually into a fresh medium, In whatever way this may be accomplished, whether by moving the whole body, or part of the body to and fro, such movements, likewise, are not uniformly for the mere purpose of re spiration, i. e. the mere purpose of aeration. All reptiles take more air into their capacious lungs than they require for oxygenating the blood, particularly in the aquatic kinds (as in the turtle), where the air serves to buoy up their heavy and slow-moving bodies in the dense element they inhabit. Serpents are provided with numerous highly-moveable ribs and powerful intercostal muscles, capable of rapid and extensive inspiration and expiration. They can perfectly distend their body with air. The same may be observed in the chameleon. These volumes of air cannot alone be subservient to respiration, as it cannot all come into contact with the simple undivided respiratory sacs. We see, therefore, there is no necessary rela tion between the quantity of air an animal may inspire and the extent of respiratory surface. The long hissing sound which serpents pro duce to alarm their prey, is effected by the expulsion of this great volume of air, by their ribs, through the narrow passages of the nos trils.
In the higher mammiferous animals, we find respiration more especially destined for the chemical purpose of oxygenating the blood.
Hence a more limited quantity is taken in, and it is speedily thrown out again. Large animals make fewer respirations than small ones. Ac cording to Scoresby *, the whale breathes four or five times a minute ; the dog, the cat, and rabbit, from twenty to thirty in the same period ; and in small birds the respirations are remarkably rapid.
Whatever be the form of the aerating or gan, " breathing " is accomplished either by, 1st, the weight of the atmosphere rushing into certain cavities, because certain parts of these cavities dilate and threaten a vacuum ; or, 2ndly, by the direct projectile or collapsing force of an organ throwing the ambient ele ment onward. These two ways are generally more or less combined in the same animal. Nor does there appear to be any relation be tween the grade of the animal and the order of respiratory movement obtained. We notice in the respiration of man a regular inspiration and expiration, two currents in different di rections ; and in the lowest animal, the connecting link with the vegetable kingdom, the porifcra, or sponge tribe, there are like wise two respiratory currents by distinct channels, which are as regular as the motion of rivers from their source to the ocean, or any other movement depending upon the established order of things. In some spe cies of medusm, there are peculiar sacs on the inferior surface of the body, which, during the expansion of the body, admit water through certain apertures, and again expel it during the succeeding contraction, re presenting a perfect inspiratory and expi ratory action. The complexity, therefore, of respiratory movements does not correspond with the increasing development of the breathing organs. Those animals which have an internal sacculated lung, always retain a certain quantity of the breathing element in "reserve" within their system ; whereas those animals which have external lungs, or gills, have no respirable medium. They need none, because where there is an exter nal lung, the ambient element answers the purpose of the " reserve" ; it is always in contact with the breathing surface. This " reserve," in tnammalia, &c., is not, pro bably, so necessary to aeration as for the purpose of ejecting any matter which may obstruct the air passages—or, in more po pular language, for " coughing" up any matter out of the throat.