RESPIRATION, in animal economy. The absolute necessity of respiration, or of something analogous, is known to eve ry one ; and few are ignorant that in man, and hot blooded animals, the organ by which respiration is performed is the lungs. Now respiration consists in draw ing a certain quantity of air into the lungs, and throwing it out again alternately.
Whenever this function is suspended, even for a very short time, the animal dies. The fluid respired by animals is common atmospherical air ; and it has been ascertained by experiment, that no Other gaseous body with which we are acquainted can be substituted Ibr it. All the known gases have been tried ; but they alt prove fatal to the animal which is made to breathe them. Gaseous bo dies, a, Ell' as respiration is concerned, May re• uividAl into two classes :-1. Un ie gases. 2. Respirable gases. The g-0.•a belonging to the first class are of so:!.) a INV lire, that they cannot be drawn into the lungs of an animal at all ; the epiglottis closing spasmodically when ever they are applied to it. To this class bdoug carbonic acid, and probably all the other acid gases, as has been ascer tained by the experiments of Pilatre de Rozier, who went into a brewer's tub while full of carbonic acid gas evolved by fermentation. A gentle heat manifested itself in all parts of his body, and occa. :cloned a sensible perspiration. A slight itching sensation constrained him fre quently to shut his eyes. When he at tempted to breathe, a violent feeling of suffixation prevented him. He sought fir the steps to get out ; but not finding them readily, the necessity of breathing increased, he became giddy, and felt a tingling sensation in his ears. As soon as his mouth reached the air, he breathed freely ; but for some time he could not distinguish objects ; his face was purple, his limbs weak, and he understood with difficulty what was said to him. But
these symptoms soon left him. He re peated the experiment often ; and always ibund, that as long as he continued with. out breathing, he could speak and move about without inconvenience; but when ever he attempted to the feeling of suffbcation came on. For the lungs of animals suffocated by it were found by Muse not to give a green colour to ve getable blues. The gases belonging to the second class may be drawn into the lungs, and thrown out again without any opposition from the respiratory organs : of course the animal is capable of respir ing them. They may be divided into four subordinate classes :-1. The first set of gases occasion death immediately, but produce no visible change in the blood. They occasion the animal's death merely by depriving him of air, in the same way as he would be suffocated by being kept under water. The only gases which belong to this class are hydrogen and azotic. 2. The second set of gases occasion death immediately ; but at the same time they produce certain changes in the blood, and therefore kill, not mere ly by depriving the animal of air, but by certain specific properties. The gases belonging to this class are carburetted hydrogen, sulph uretted hydrogen, carbon ic oxide, and perhaps also nitrous gas. 3. The third set of gases may be breathed for some time without destroying the ani but death ensues at last, provided their action be long enough continued. To this class belong the nitrous oxide and oxygen gas. 4. The fourth set may be breathed any length of time without injuring the animal. Air is the only gase ous body belonging to this class. See PHYSIOLOGY, and Thomson's Chemis' try.