Domestic Encyclopædia (1802)/Animal Life
ANIMAL LIFE is that organized principle which distinguishes animals from vegetables, and is susceptible of sensation and reflection.
Various conjectures have, at different periods, and by eminent philosophers, been held respecting the nature and origin of this important principle, but it still remains involved in obscurity. In a late Dissertation, addressed to the President and Fellows of the College of Physicians, Dr. Beattie resolves it into that inherent tendency to approximation and cohesion, in some parts of matter, and that resilition and elasticity in others, the source of which is yet undiscovered, and which is not deducible from any material, secondary cause. As it was found that no animal could exist when suddenly deprived of large quantities of blood, it was inferred that this fluid was the vital principle; an opinion, indeed, which was much strengthened by the injunction of the Mosaic Law, not to eat meat in which there was blood, "that being the life." A late celebrated anatomist adopted this opinion, and boldly declared that the blood was alive. By some physiologists it has been conjectured, that the electric fluid is the source and principle of animal life: on the contrary, modern chemists maintain that it is conveyed by that elastic elementary gas, termed oxygen, or vital air, which, according to their notions, is the true principle of vitality.
Without entering into a minute investigation of these theories, it will be sufficient to state a few of the leading circumstances which accompany the progress towards animation.
Heat is a material agent in the production and continuation of life, as is beautifully illustrated in the incubation, or hatching of an egg; the progress of which towards maturity, is nearly as follows:—On the first day, no perceptible alteration takes place; on the second, the treadle changes to a pale yellow colour; and every following day it becomes yellower, till at length it grows red, and afterwards of a deep blood-colour, which soon thickens to a firmer substance; this speedily assumes a form, which, when it thickens into life, is nourished by the yolk, and laid in the white as in a bed provided for its accommodation; thus it continues increasing, till it grows too large for its narrow bounds, when it bursts tire walls of its prison, and comes forth a perfect animal.