MOLECULAR AND MICROSCOPIC SCIENCE.
PART III.
ANIMAL ORGANISMS.
SECTION I.
FUNCTIONS OF THE ANIMAL FRAME.
Although animal life is only known to us as a manifestation of divine power not to be explained, yet the various phases of life, growth, and structure in animals, from the microscopic Monad to Man, are legitimate subjects of physical inquiry, being totally independent of those high moral and religious sentiments which are peculiar to Man alone.
The same simple elements chemically combined in definite but different proportions form the base of animal as well as of vegetable life. But besides the elementary gases and carbon, many substances, simple and compound, are found in the animal frame; the phosphate and carbonate of lime, iron which colours the blood, and common salt which, with the exception of water, is the only article of food we use in a mineral state. Animals derive their nourishment, both directly and indirectly, from vegetables. Their incapacity to change inert into living matter is one of the most characteristic distinctions between the animal and vegetable kingdoms.
Protoplasm was shown to be rudimentary formative vegetable matter: so Sarcode, or rudimentary flesh, forms the whole or part of every animal structure. It is a semi-fluid substance, consisting of an albuminous base, mixed with particles of oil in a state of very fine division. It is tenacious, extensile, contractile, and diaphanous, reflecting light more than water, but less than oil. It is rendered perfectly transparent by citric acid, and is dyed brown by iodine. This substance, in a homogeneous state, constitutes the whole frame of the lowest grade of animal life; but when gradually differentiated into cell-wall and cell-contents, it becomes the origin of animal structure from that which has little more than mere existence to man himself; in fact, cellular origin and cellular structure prevail throughout every class of animal life. Unicellular plants and animals live for themselves independently and alone; but the cells which form part of the higher and compound individuals of both kingdoms, may be said to have two lives, one peculiarly their own, and another depending on that of the organized beings of which they form a part.
Flesh or muscle, which is organized sarcode, consists of two parts, namely, bundles of muscular fibre imbedded in areolar tissue. Nervous matter also consists of two parts, differing much in appearance and structure, the one being cellular, the other fibrous. The vital activity of the nerves far surpasses that of every other tissue; but there is an inherent irritability in muscular fibre altogether independent of nervous action: both the nervous and muscular tissues are subject to decay and waste.
The blood, which is the ultimate result of the assimilation of the food and respiration, conveys nourishment to all the tissues during its circulation; for with every breath, with every effort, muscular or mental, with every motion, voluntary or involuntary, at every instant of life, asleep or awake, part of the muscular and nervous substances becomes dead, separates from the living part, is returned to the circulation, combines with the oxygen of the blood, and is removed from the system, the waste being ordinarily in exact proportion to the exertion, mental and physical. Hence food, assimilated into blood, is necessary to supply nourishment to the muscles, and to restore strength to the nervous system, on which all our vital motions depend; for, by the nerves, volition acts upon living matter. Waste and repair is a law of nature, but when nature begins to decay, the waste exceeds the supply.
However, something more than food is necessary, for the oxygen in the blood would soon be exhausted were it not constantly restored by inspiration of atmospheric air. The perpetual combination of the oxygen of the air with the carbon of the blood derived from the food is a real combustion, and the cause of animal heat; but if the carbonic acid gas produced by that chemical union were not continually given out by the respiratory organs, it would become injurious to the animal system. Thus respiration and the circulation of the blood are mutually dependent; the activity of the one is exactly proportional to that of the other: both are increased by exercise and nervous excitement.
External heat is no less essential to animals than to vegetables; the development of a germ or egg is as dependent on heat as that of a seed. The amount of heat generated by respiration and that carried off by the air is a more or less constant quantity; hence, in hot countries, rice and other vegetable diet is sufficient, but as the cold increases with the latitude, more and more animal food or hydrocarbon is requisite for the production of heat.