Sight

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 world-but 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 a dif

ferent property. Thus, for example, the first pair of cranial nerves—the 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-the 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 determin

ing 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 severally 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 subdivisions, viz., the conscio-voluntary, the reflex, and the ganglionic, each with its center and its afferent and efferent fibers. Neglecting, again, the two others, and selecting only the conscio-voluntary, 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, and it is now believed that heat, cold, pressure, and pain are each perceived by distinct fibers of the nerves of feeling. Odors and tastes are almost infinite in number, but whether they are perceived each by special fibers, or by different affections of the same fibers, is not known. The following diagram (Fig. 1) illustrates these successive differentiations.

Gradation among the Senses.-1. As to Response to Vibrations.-Now all these higher special senses may

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Sight: An Exposition of the Principles of Monocular and Binocular Vision