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experiments illustrating, 164-172; the statement of the laws,
173; contrary statement by Helmholtz explained, 175; rota-
tion on optic axes in parallel motion only apparent, 176.
SECTION II.—LAWS OF CONVERGENT Motion, 177 ; the rotation
in this case real, 178; difficulty in experimenting, 178; ex-
periments proving rotation on optic axes in convergence,
180-187; effect of' elevation and depression of visual plane,
188; experiments illustrating, 188; cause of the rotation,
189; laws of parallel and convergent motion contrasted, 189.
263; when extreme, incompatible with binocular vision, 264;
central spot, 266; how far it exists in lower animals, 267;
importance of this spot, 267; general changes in the cye as
we go up the vertebrate scale, 269.
THE RELATION OF GENERAL SENSIBILITY TO SPECIAL
SENSORY nerve-fibers are cylindrical threads of microscopic fineness, terminating outwardly in the sensitive surfaces and sense-organs, and inwardly in the nerve-centers, especially the brain. Impressions on their outer extremity are transmitted along the fiber with a velocity of about one hundred feet per second, and determine changes in the nerve-centers, which in turn may determine changes in consciousness, which we call sensation. The simplest and most general form of sensation is what is called general sensibility, or common sensation. This is a mere sense of contact, an indefinite response to external impression. It gives knowledge of externality-of the existence of the external worldbut not of the properties of matter. The lowest animals possess this, and nothing more. But, as we go up the scale of animals, in order to give that wider and more accurate knowledge of the various properties of matter necessary for the complex relations of the higher animals, sensory nerve-fibers are differentiated into several kinds, so that each may give clear knowledge of differ
ent properties. Thus, for example, the first pair of cranial nerves—olfactive-is specially organized to take cognizance of certain impressions, called smells, and nothing else. If, therefore, these nerve-fibers are irritated in any way, even mechanically, by scratching or pinching, they do not feel but perceive an odor. The second pair of cranial nerves--the optic–is specially organized in a truly wonderful way to respond to the ethereal vibrations called light, and nothing else. If, therefore, these nerves be mechanically irritated, we do not feel anything, but see a flash of light. In a similar manner, the eighth pair-auditive nerve-is specially organized to respond to sound-vibrations, and nothing else; and therefore mechanical irritation of this nerve produces only the sensation of sound. Similarly, the ninth pair, or gustative nerve, is organized for the appreciation of taste only; and, therefore, a feeble electric current through this nerve produces a peculiar taste.
We have in these facts only an example of a very wide law, viz., the law of differentiation. In the lowest animals all the tissues and organs which are so widely distinct in the higher animals are represented by an unmodified cellular structure, performing all the functions of the animal body, but in an imperfect manner. Each cell in such an organism will feel like a nervous cell, contract like a muscular cell, respire like a lungcell, or digest like a stomach-cell. As we go up the animal scale, this common structure is differentiated first into three main systems, viz., the nutritive or epithelial system, the nerve-system, and the blood-system : the first, presiding over absorption and elimination, i.e., exchange of matter between the exterior world and the organism; the second, over exchange of force between exterior and interior by impressions determining changes in
consciousness, and by will determining changes in external phenomena; the third, presiding over exchanges between different parts of the organism. The first kind of exchange.may be likened to foreign commerce; the second, to exchange of intelligence by telegraphic communication with foreign countries; the third, to the internal carrying trade. These three systems are very early differentiated in the embryo, since they are seyerally produced from the three primitive layers of the germinal disk, viz., the endoderm, the ectoderm, and the mesoderm.
Neglecting now all but the second or nervous system as we still go up, this is again differentiated into three sub-systems, viz., the conscio-voluntary, or sensorimotor, the reflex, and the ganglionic, each with its center and its afferent and efferent fibers. Neglecting, again, the two others, and selecting only the sensorimotor, the sensory fibers of this sub-system are again differentiated into five kinds, each to respond to a different kind of impression, and perceive a different property, viz., the five special sense-fibers for sight, hearing, smell, taste, and touch. Even these are probably again further differentiated; for the perception of different colors and different musical sounds is probably effected by means of special fibers of the optic and auditive nerves. The following diagram (Fig. 1) illustrates these successive differentiations.
Gradation among the Senses.- Now all these higher special senses may be regarded as the result of refinements of common sensation--each a more refined touch. Coarse vibrations are perceived by the nerves of common sensation as a jarring. When the vibrations are so rapid that there are sixteen complete movements back and forth in a second, an entirely different sensa