Physiology: Living Beings as Machines. Machines in Action. Classification of Functions.
Thus far living beings have been regarded merely as definite forms of matter, and Biology has presented no considerations of a different order from those which meet the student of Mineralogy. But living things are not only natural bodies, having a definite form and mode of, structure, growth, and development. They are machines in action; and under this aspect, the phenomena which they present have no parallel in the mineral world.
The actions of living matter are termed its functions; and these functions, varied as they are, may be reduced to three categories. They are either -- (1), functions which affect the material composition of the body, and determine its mass, which is the balance of the processes of waste on the one hand and those of assimilation on the other. Or (2), they are functions which subserve the process of reproduction, which is essentially the detachment of a part endowed with the power of developing into an independent whole. Or (3), they are functions in virtue of which one part of the body is able to exert a direct influence on another, and the body, by its parts or as a whole, becomes a source of molar motion. The first may be termed sustentative, the second generative, and the third correlative functions.
Of these three classes of functions the first two only can be said to be invariably present in living beings, all of which are nourished, grow, and multiply. But these are some forms of life, such as many Fungi, which are not known to posses any powers of changing their form; in which the protoplasm exhibits no movements, and reacts upon no stimulus, and in which any influence which the different parts of the body exert upon one another must be transmitted indirectly from molecule to molecule of common mass. In most of the lowest plants, however, and in all animals yet known, the body either constantly or temporarily changes its form, either with or without the application of a special stimulus, and thereby modifies the relations of its parts to one another, and of the whole to surrounding bodies; while, in all the higher animals, the different parts of the body are able to affect, and be affected by, one another, by means of a special tissue, termed nerve. Molar motion is effected on a large scale by means of another special tissue, muscle; and the organism is brought into relation with surrounding bodies my means of a third kind of special tissue -- that of the sensory organs -- by means of which the forces exerted by surrounding bodies are transmitted into affections of nerve.
In the lowest forms of life, the functions which have been enumerated are seen in their simplest forms, and they are exerted indifferently, or nearly so, by all parts of the protoplasmic body; and the like is true of the function of the body of even the highest organisms, so long as they are in the condition of the nucleated cell, which constitutes the starting point of their development.
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