Considering that these animals are truly amphibious, these results are very curious; and it is interesting in a physiological point of view, to know that frogs are able to respire the air contained in the dense medium of water for an indefinite period, and just as easily as they breathe the finer medium on land.
Respecting the action of aërated water on the skin, the conclusion drawn seems to be correct, that it must be from cutaneous absorption that the air contained in the water promoted the continuance of life in Dr. Edward’s experiments upon this point, since the animals were in a state of asphyxy regarding respiration by the lungs; and that no [p149] air entered in combination with water was shewn from Dr. Edwards never having seen water in the lungs. Therefore, unless the air acted on the blood through some other organ, the lives of these animals would be definite and shortened, even though the water be renewed from time to time, and their asphyxy would be complete and continued. And since the skin is the only organ in contact with the air, it is fair to conclude that it is the medium of aërial absorption.
When the webs were examined under water, these membranes indicated the action of air upon their blood-vessels, by the bright tint of the blood.
Spallanzani imagined that frogs perish sooner in running than in stagnant water; but Dr. Edwards having secured some of these animals in ten feet of the Seine, whilst others were simultaneously placed in unrenewed stagnant water, he found the latter did not survive many hours, and the former lived a long time.
In order to fix the limits of this kind of existence, frogs were placed in renewed aërated water, and with a temperature never forced beyond ten degrees they were found to live in all seasons of the year; but when the temperature was elevated from twelve to fourteen, they died in a few hours. In running streams they lived longest, and at twelve degrees they were thus more favourably placed than in stagnant water, at a lower temperature even, and taking the precaution to renew the water daily; and at seventeen degrees in running water they died prematurely. Toads exhibited the same comparative results, but they lived the longest.
It appears, therefore, that water contained in vessels is less favourable to the lives of these animals than running streams, although the water and the temperature were identical. Probably the great advantage of running water is its constant and unceasing renewal. The separate and comparative influence of air, water, and temperature, being thus investigated, the combined action of the three physical agents was next inquired into, and it is demonstrated that frogs submersed in water are influenced by three circumstances,—1. the presence of air in water; 2. the quantity of its renewal; 3. the temperature of the medium. If the manners of frogs be closely examined, they appear to live in water under very considerable influence from the atmosphere.
From circumstances developed in the foregoing experiments, cutaneous respiration seems to be pretty evidently indicated. A chapter is, therefore, devoted to this subject, one that is not well known, although pulmonary respiration is [p150] generally understood. In frogs, the function of pulmonary respiration is united with that of deglutition, and the air enters only by the nostrils, the mouth being closed during respiration. While the mouth remains open, the action of deglutition is stopped, and, therefore, the animal does not then breathe. Dr. Edwards availed himself of this circumstance by gagging the mouth so as to keep it open, and thus prevent the air from entering the lungs. The frogs were sufficiently exposed to moisture and renewal of air to their bodies: the results were, that, at twenty-four degrees, five frogs so placed died next day, and one lived a week.
Dr. Edwards immersed some frogs in wet sand, and adopted an improved method of excluding air from the lungs, and some of them lived twenty days. Hence it evidently appears that air influences the skin materially, and counterbalances the asphyxious state induced by obstructing the air’s passage to the lungs. By adopting other methods, the existence of frogs was prolonged to thirty or forty days. It is, therefore, sufficiently proved that the blood undergoes its necessary changes from atmospheric influence through the medium of the skin, although in a minor degree compared with those which it passes through from pulmonary respiration. Frogs are thus shewn to possess a double source of respiration.
By substituting oil for water, frogs immersed in this fluid died in a few hours, being at liberty to breath the air on its surface. And, when plunged into oil, with the means of breathing by the lungs arrested, they lived an equal time with frogs simultaneously placed in water without power to respire. A comparison was instituted with frogs in oil and in water, being allowed to breathe air, when the difference was found to be very considerable in favour of the aquatic bath. These circumstances shew, that, even with the feeble succour of the air through the skin, absorbed from the water, the respiratory function was far more prolonged, than in the case of the obstruction afforded by the oil. Thus we have abundant evidence of the double function by which frogs are maintained, from the action of the air on the skin and the lungs; and this appears to be the means of existence among amphibious animals generally.
It may be asked why these animals die in deep water when prevented from approaching the surface? It appears that, having expelled the respired air from their lungs, which is imperfectly renewed from the water, they become specifically heavier than the water, and unable to rise from the bottom; and thus placed, the duration of their lives depends upon [p151] the resistance offered by their constitutions to the depressing effects of a state of asphyxy while remaining submersed.