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CIIAPTER VI.

to binocular vision, 297; less perfect as we go down, and

disappears in fishes, 298. 2. PositioN OF OPTIC Axes, 298;

parallel in man and apes, 299; more and more divergent as

we go down, until 180° in fishes, 299; relation of position

of axes to binocular vision, 299; even in mammals easy con-

Extreme interest of the subject. 303.

1. INVERTEBRATE EYE: General sensibility to light, 303; coes-

tensive with lite, 303; this not special sense, 303; first be-

ginnings of special sense determined by light itself, 303;

define eye-spot, 304; several stages in development of eye-

spots and diagrams illustrating, 304; may perceive light,

not objects, 305; earliest appearance of eye-(1) pin-hole

image, 305; (2) lens-image, 305; (3) compound lens-image,

305 ; same order in embryonic development of a squid's eye,

306; thus far no chiasm, 306; no binocular vision and can

not be any blind-spot, 306.

2. VERTEBRATE EYE: Invertebrate versus vertebrate eye, 306 ;

(1) as to mode of termination of the fibers in the rods, 306 ;

(2) as to mode of formation of the retina, 307; diagrams illus-

trating formation of optic vesicle and the retina in vertebrates,

307; formation of lens, 307; origin of hacillary layer, 308 :

of choroid, 303; explanation of turning back of nerve-fibers

to terminate in rods and cones, and thus of the blind spot,

308; retina epidermal in invertebrates and cerebral in ver-

tebrates, 309; bacillary layer in both epithelial, 309; transi-

tion from invertebrate to vertebrate eye, 309; the difficulty,

309; Beranek's view, 309; the gradual perfecting of the

vertebrate eye, 310; decreasing divergence of optic ases,

310; formation of the chiasm, 310; the use of the two eyes

as one instrument, 311; the addition of fovea is a condition

of thoughtful attention and mental development, 312.

SI G H T.

INTRODUCTORY.

THE RELATION OF GENERAL SENSIBILITY TO SPECIAL

SENSE.

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 extremnity 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 wo 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 in response to external impression. It gives knowledge of 9 externality-of the existence of the external world—but si 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

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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

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 eliminationi. 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 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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