But a large part of the tissues which make up the bodies of adult animals have, in a great measure, lost their resemblance to the protoplasm from which they were derived, through an extreme development which has resulted in tissues quite unlike each other—some of them, as the bones, for example, evidently serving a purely mechanical purpose in the body; and it may be said that a comparatively small proportion of the tissues of the body are, strictly speaking, living. Professor Beale classifies all the material of the tissues, and even of the ultimate cells from which the tissues are derived, under two heads, as formative and formed—that is, matter which has the power of producing new matter like itself out of pabulum or food, and that which has no such power, but which has been produced by the former. He considers that a muscle-fiber is not, like the protoplasm which produced it, living; and that the nerve-fiber also consists of formed material of which protoplasm is the builder.
In accordance with this view, only the lowest—that is, the nutritive—processes can be regarded as truly vital. The higher functions performed by the perfected tissues—the bones, the muscles, and the entire system of nerve-fibers and nerve-centers, including the brain are mechanical rather than vital; the character of the function in each case depending on the character of the mechanism, i. e., on the particular relations of the parts concerned.
Muscle-tissue, for example, has but a single and simple physiological property, the power of change of form called contraction, and even this is a function of formed material rather than of living matter; but, through the mechanical relations of bones and tendons, of joints and ligaments, of associated and opposing individual muscular bundles, all the complicated and varied movements of the body are brought about.
The action of muscle is one and the same, whether it be expended in the grosser movements of locomotion, in the finer manipulations of the skilled artisan and the musician, or in the still more delicate adjustments of the vocal cords, by means of which the exquisite modulations, almost infinite in variety, of the voice of a Patti or a Campanini are produced.
These various adjustments are evidently mechanical in their nature. The so-called vital processes—processes identical with those taking place in the simplest animal that lives, and in the very grass beneath our feet, perform a comparatively humble part in the production of the vast results. The vital processes are concerned in the building up of the tissues and organs produced by and from protoplasm out of the food supplied to it; and the chemical changes involved in the breaking down of the tissues furnish the initial force in each case; but the actual forces manifested are due to transformations of this initial force brought about by the complicated mechanism which this force serves to set in operation.