The distinctive character of warm-blooded animals to preserve an uniformity of heat has no reference to bulk. The eagle maintains the same temperature as the wren or the tom-tit, taking them at the same age, and placing them under the same circumstances; but if cooling measures be adopted, the lesser body parts with its heat faster than the larger, though ultimately they arrive at the same point. The dimensions of animals are infinitely varied; but the giant reaches no higher standard than the dwarf, nor sinks to a lower temperature.
In estimating the temperature of young animals, it must be taken into account that they are born at different periods of organic developement. Some come earlier into the world than others, and some are more perfectly formed than others at their birth, and more capable of helping themselves. This variation produces a different standard of heat after birth, and especially creates a variety of temperature among birds when tested at the same epochs of their existence. The season in which animals are produced also modifies their temperature.
The influence of age in modifying temperature is common both to mammiferæ and birds. Young and healthy sucking pigs cooled faster than their parent, their generating means of heat being more feeble. Animals of warm blood possess [p300] the power of supplying heat at its maximum when first born; they then part with it by degrees, and, as they advance in age, their heat becomes gradually augmented again till it reaches the adult standard.
Dr. Edwards next proceeds to discuss the phenomena of animal temperature more exclusively regarding adults, and especially among those singular creatures of the mammiferæ which form an exception to the general law of nature respecting the uniformity of temperature as to warm-blooded animals. These beings are what are termed hybernants, such as the dormouse, the hedgehog, the bat, the marmot, &c., natives of Europe; which remain dormant during winter without any external signs of life and motion. The change which these undergo reduces them from the state of warm-blooded animals to that of cold. Unlike the rest of their class, the autumnal season lowers their temperature by degrees, till in winter it reaches so low as to be scarcely higher than the surrounding air. Their powers fail gradually, and their losses of heat are not repaired, till at length their respirations become slow and feeble, and the heart languidly urges the blood through the arteries. In this state there is an imperfect aërification of the blood, and a partial state of asphyxia, producing continued repose of the nervous and muscular system. But the temperature of these animals sinks no lower than the air, and remains sufficient to maintain a passive existence, till the returning spring raises their heat again, and they become lively and active till autumn; but even in spring these animals are characterised by producing less heat than others of their class.
If we seek to know the cause of this curious variety, we can only refer it to peculiarity of constitution, which is instituted by nature as adapted to animals placed in situations of rigorous cold, and where they cannot procure sustenance but in spring and summer.
Our author imitated the process of hybernation by artificial cold, and produced the same effects; and when he restored animation by gradual warmth, he found the animals as lively as before.
John Hunter and others have written on the natural history of hybernants, and Dr. Edwards regards only their temperature. The experiments on hybernants by artificial cold prove this fact, that hybernation is attributable to other causes than to the reduction and deprivation of nutriment; for the animals submitted to the ordeal of cold were well [p301] fed, and in the lively season of advanced spring. The deprivation of food seems to be a local consequence provided for by the phenomenon of hybernation, and not its exciting cause. Nor does there appear to be any change of organization in these cases, but a state of constitution exists which we are unable to account for further.
We have, in the next place, a series of experiments showing the influence of the seasons upon animal temperature with the warm-blooded; by which it seems that they produce a variety of results: and it is demonstrated that animals of warm blood in general undergo some constitutional changes with the periodical returns of the seasons. When, for example, the highest degree of temperature is attained, animals no longer produce heat; so that their temperature continues below that of the air in the hot season. And, in the cold season, if the cold be not too rigorous, the animal’s age offers a proportionate resistance to the cooling effects of the air as the approach to maturity is attained. An elevated and a depressed temperature thus produce contrary effects upon the internal powers of generating animal heat, a high temperature arresting them, and a low one promoting them. Thus we cannot fail to observe the beautiful adaptation of means to final causes.
Upon the subject of asphyxia in warm-blooded animals, Dr. Edwards found a great dependence between animal heat and the faculty of living without contact with the air, a state in which the blood is not aërated by respiration, and which is sustained by hybernants while in the dormant condition. Having submersed animals in water of various temperatures successively, so as to bring them under the influence of variable temperature, he found the descending scale of temperature the most hurtful. The ascending heat was that which prolonged life most. Between 20° and 10° the results were similar to those between 20° and 40°.
Animals, then, of warm blood in a state of asphyxia hold their existence on two principal conditions relative to heat; one regarding the different measures by which some develope their heat, and the other the degree of external temperature. The first is proper to animals naturally, the second fortuitous.