While it is true that the younger animals suffer least from cold, it is, at the same time, to be considered that they cool down more rapidly. On this principle depends the mortality of our domestic fowls and other animals, the management of which requires so much observation and experience in order to rear them. In regions where the temperature is liable to great alterations in the course of the year, man and other vertebrated animals of warm blood are liable to suffer in their health; for, though cold should produce the resistance derived from the necessary constitutional developement of heat, this increase of caloric, having its limits, often exposes the constitution to the effects of too great reduction of temperature, as is exemplified in the frozen regions of the North Pole, in Siberia, and in Russia.
The young of mammiferæ, in general, were found by Dr. Edwards to differ very materially in the duration of their lives, in a state of asphyxia, often being limited to from five to eleven minutes, according to their developement at birth, the most advanced in organization living the longest period. The author proved these facts by placing animals in a state of asphyxia under water; and it is remarkable that, in all his experiments, the voluntary motions were always first destroyed, the involuntary outliving them. With dogs, cats, and rabbits, sensibility existed only three or four minutes. A puppy showed automatic signs of life nearly half an hour. The best divers appear to be able to remain under water from three to four minutes.
When animals are entirely deprived of aërial contact, it may be inquired, what are the principal functions exercised? When the air circulates through the lungs, it imparts to the blood a peculiar quality, by which its colour becomes changed. Deprived of this influence from the air, the blood acquires a dark colour, and the nervous function is [p306] simultaneously affected. Among reptiles, Dr. Edwards found that life could be maintained by this dark blood; but it is questionable whether the circulation of venous or dark blood can promote life in animals of the warm-blooded kind. Temperature certainly modifies their capability of existence. Under 20°, they live longest; at 0°, their existence is shortest. The vitality of the nervous system seems to be thus directly influenced by temperature.
Of all the phenomena of animal life, those relative to the blood’s state in asphyxia are, perhaps, the most interesting and curious, from loss of consciousness, sensation, and voluntary motion attending its disoxygenated state. If, however, animals differ so materially under the influence of a deprivation of air, as to the duration of such existence, we may imagine a corresponding difference relative to their respirations modified by species, age, &c. Air, the pabulum vitæ, is not equally consumed by all, but in different proportions; at least, such is the presumption from the experiments upon animals of warm blood. The relative proportions of this difference are sought to be ascertained. Warm-blooded animals of equal size and age, at their liveliest period of age, were the objects of comparative inquiry. We must refer the reader to the table at the end of the work for the results. A marked difference is observable between the quantity of air consumed by the cold-blooded animals and that required for the support of the warm-blooded; and each has an organization appropriated to the individual distinctions. Thus the structure of the reptile and fish entails the lesser consumption of air, compared with that of the mammiferæ and birds. Fish consume least air, reptiles stand next, then the mammiferæ, and, lastly, birds consume most. The two last, however, very nearly approach each other; so do also the two first; and the distinction between the organization and the consumption of air is most strongly marked between the fish and reptiles on the one hand, and the mammiferæ and birds on the other, which, indeed, has caused their separation into two distinct groups, by the appellation of cold and warm blooded animals,—a distinction which clearly separates the whole of the vertebrated animals into two groups, bearing different physiological characters in their relations to animal heat and respiration.
The mere temperature of the blood in each group is insufficient for our knowledge of their distinctive characters. We [p307] further find them characterised by a consumption of air in union with their heat, so as to unite these two functions, and thus render them dependent upon the same organs. Dr. Edwards has further shown, that from birth to maturity the production of heat goes on increasing with the consumption of air. And thus age (as well as the seasons) has been shown to be a modifier of animal heat; for, as the hot season advances, the consumption of air becomes diminished, and when the cold sets in, it increases; and this decrease and increase are accompanied by corresponding developements of heat.
In cases of fainting, of hysteric and asthmatic fits, the principle here laid down, as to the balance between the air consumed and heat, is instinctively acted upon by the most ignorant persons, who open all the doors and windows to admit cold air, and dash cold water in the patient’s face. The addition or continuance of heat increases the affection. The application of cold produces instant relief. The state of asphyxia is relieved, the senses return, the pulse beats at the wrist, and the respiration goes on naturally. The cooling renders the air, unfit before, fitted for the purposes of life.
The effects of temperature upon the respiratory movements are indicated also in those constitutional changes which diminish the production of heat and the consumption of air. Organic affection of the heart or lungs may produce this change, which entails the necessity of a change of climate, or an alteration of temperature artificially, to restore the balance between the air and the animal heat.
A very elaborate and complete argument, and series of experiments, are devoted to the subject of transpiration, and the effect upon it of the influence derived from repose of the body and sleep, by the air’s motion or stillness, and by the pressure of the atmosphere.
We have, however, pursued the interesting points touched upon so far as to render it impossible to enter at present upon this portion of the work. The importance of the subject demands a fuller investigation and report than we have now room for; and we must, therefore, defer it to another opportunity. [p308]