Upon the respiration of both young and adult animals the author arrives at a conclusion opposite to that of common opinion, which is founded on the notion of the heat in young animals being higher than that of the matured. Finding, [p302] however, as already noticed, that the parent exceeds the temperature of its offspring after birth, it is naturally concluded that its consumption of air is also greatest. This was experimentally confirmed, and is in unison with other facts. In the first part of this work the vertebratæ of cold blood were also found to consume least air in proportion to their diminution of temperature. Temperature seems to act uniformly with all the vertebratæ, and their consumption of air is in proportion. The mammiferæ have a lower temperature than birds, and they consume less air than the latter. Fish and reptiles consume less air than the warm-blooded, and possess a lower temperature.
The influence of the seasons upon respiration is considered in the sixth chapter. Many changes occur in the atmosphere during the revolutions of the seasons, varieties in the temperature, and the pressure and density of the air. Dr. Edwards shows that the faculty of producing heat with warm-blooded animals is greater in winter than in summer, the constitution of animals being adapted to their individual climates; and in reference to the relation of this faculty to the consumption of air, it is presumable, all other circumstances being alike, that the consumption ought to be increased with the faculty of developing heat, and the experiments justify the presumption.
Upon the subject of transpiration, it is shown that the air not only exercises a vivifying effect upon the constitution, but one little less important in removing a vaporous substance from the surface of the body, and which is separated from the fluids before its conversion into vapour, and known by the name of perspiration or sweat, which transpires from the skin. The variations in the temperature of the air possess great influence over this function. Experiments on this subject were detailed most fully in our last Number, relative to cold-blooded animals; and therefore these need not now be repeated in respect to the warm-blooded, for the results are exactly similar, as to transpiration in equal and successive periods, the comparative influence of dry and moist states of the air, and the effects of air in motion and in repose. Inspection of the table annexed to the work displays the similarity of the effects produced by the same physical agents upon cold and warm blooded animals, and this accordance serves to afford mutual support to the different investigations.
We are now arrived at the fourth and last part of this [p303] work. Much, however, of this part appertains to what has been already detailed upon other animals. But the modifications of heat in the human being, from the period of birth to maturity, will be found highly interesting. They accord precisely with the results obtained among the lower animals and mammiferæ; and present analogical proofs of the general application of principles laid down in the preceding portions of our notices.
While, however, we trace analogy throughout the animal kingdom, it must be remembered that there are infinite sources of variation arising from the extensive variety of species modifying those principles, which are governed by a general harmony of effect. Of all animals, man exhibits this variety the most, possessing, as he does, attributes above all the groups of his class, from his intellectual properties, speech, &c., rendering his race unique and superior to all others. Our curiosity cannot, therefore, be allowed to rest satisfied with the general application of principles, until we have observed their modifications in the human being as well as in brutes. It is highly interesting to inquire into the conditions of human phenomena, and examine the forces which man opposes in his intelligent character to the physical agents around him. He is equally liable to their influence, exists by their contact, and yields, like other members of the animal kingdom, to their destructive tendency. The essential distinctions appertaining to his economy are thus the more necessary to be understood. His organization affords him no shelter from the operations of physical laws beyond that of brutes; but the superiority of his nature may be supposed to modify their influence from causes referrible to his sensibility. These have formed the subject of Dr. Edwards’s inquiry.
Man’s state and condition, at his birth, place him in very different circumstances from those at which he subsequently arrives. Here, therefore, we see an extensive field of inquiry; and it is suggested whether, in the infantile state, man generates less heat than in more matured existence. Dr. Edwards has shown that the young of mammiferæ generally, being born at the period when their eyes are open, produce less heat than adults. It is, therefore, presumable that the generating powers of heat differ in the two states of existence which man goes through, the infantile and mature.
But the power of producing heat differs among adult animals, and it is desirable to know the limits of this faculty, [p304] Moreover, this power differs in different parts of the body; so that, when experiments are made, we should always apply the thermometer to the same part of the body. Among twenty adult persons, Dr. Edwards found the average temperature 36°.12: in infants from a few hours to two days old, 34°.75 was the average. Thus we perceive that the temperature of human infants is inferior to that of adults. In infants born previous to the usual period, two or three hours after birth their heat was at 32° of Reaumur’s scale. So far we perceive a similarity in man to the mammiferæ in general.
We have next a chapter on the effects of cold upon mortality at different ages. It is highly interesting to observe the care of animals towards their offspring, in protecting them against the effects of cold instinctively at a period before their own powers of generating heat enable them to resist its baneful tendency.
Dr. Edwards endeavours to investigate the subject of cold, so as to discover its limit of action. He examined the young of mammiferæ and birds, the former born with closed eyes, and the latter unfledged. He exposed them separately and apart to the air, so as to be independent of each other’s warmth, and they exhibited a temperature below their natural standard at the period of birth, even when a degree of artificial heat was applied beyond that of adult birds. The final result of these experiments was, that the application of heat may be conducive to their developement, but is not indispensable to their preservation. The author discovered, that the diminution of temperature is not equally injurious at all ages. The younger the animal, the less is the injury sustained by cold, because the faculty of producing heat is less powerful with the young than with the matured animal, the power increasing as the animal grows, and also with the increase of cold.
Still, however, this subject is open to inquiry, for the great variety of species, and other circumstances belonging to the animal creation, so modify the phenomena as to create an almost endless field of investigation. When warm-blooded animals are exposed by their parents to the atmospheric influence at an early age, they are better provided against the perils of cold, being born with an abundant source of heat. But, if the cold exceeds their powers of generating heat, the mortality is so much readier. Hence arises the danger of animals being born in the winter season. [p305] Two circumstances are distinguishable, the refrigeration of the body, and the temperature it is capable of sustaining. The cooling is so much less injurious with the young. If two young animals of the same species be cooled down equally, the youngest suffers the least. But, in order to lower to the same number of degrees the temperature of bodies of different ages, the external heat should be lowered in proportion to the advancement of the animal towards maturity, in order to compensate for the difference which the modification of age produces.