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mixtures of different strengths of these primary colors, producing different degrees of stimulation of each set of nerve terminals.

The view that such special nerve apparatus exists for red, green and violet is supported by the fact that the most anterior or marginal part of the retina is incapable of being stimulated by red objects, which look black when only seen by this part of the retina. This inability to see red may extend over the whole retina, as is found in some persons who may be said to be “ red blind." If we investigate our negative after images, after looking for a long time at a red object, we find them to be greenish blue. That is to say, the nervous mechanism for receiving red impressions is fatigued, and that of its complementary color is easily stimulated.

MENTAL OPERATIONS IN VISION. Our visual sensations enable us to perceive the existence, position and correct form of the various objects around us. For visual perception much more is necessary than the mere perfection of the dioptric media of the eye, and of the retinal nerve mechanisms. Besides the changes produced in the retina by light and the excitations in the nerve cells of the visual centre, there must be psychical action in other cells of the cortex of the brain. This psychical action of the brain consists of a series of conclusions drawn from the experiences gained by our visual and other sensations.

Our ideas of external objects are not in exact accord with the image produced on the retina and transmitted to the brain, but are the result of a kind of argument carried on unconsciously in our minds. Thus, when no light reaches the retina, we say (without what we call thought) that it is dark; our retina being unstimulated, no impulse is communicated, and the sensation of blackness arises in our sensorium. When luminous rays are reflected to the retina from various objects around us, the physiological impulse starts from the eye, but in the brain, by unconscious psychical activity, it is referred in our minds to the objects around us, so that mentally we project into the outer world what really occurs in the eye. So also, from habit, we re-invert in our minds the image which is thrown on the retina upside down, by the lens, and so unconscious are we of the psychical act that we find it hard to believe that our eyes really receive the image of everything inverted, and our minds have to reinstate it to the upright position.

One of the most important means employed to enable us to form accurate visual perceptions is the varied motion which the eyeballs are capable of performing.

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MOVEMENTS OF THE EYEBALLS. The eyeballs may be regarded as spherical bodies, lying in loosely fitted sockets of connective tissue padded with fat, in which they can move or revolve freely in all directions, in a limited degree. The muscles which

Fig. 236. act directly on the eyeball are

061.sup. six in number.

Four recti passing from the back of the orbit are

obl.int attached to the eyeball, one at

F.SUA each side and one above and below, not far from the cornea. These move the front of the

r.unt eye to the right or left, up or down respectively. Two oblique passing nearly horizontally out

obl.int ward, and a little backward, are attached to the upper and under surface of the eyeball respectively. These muscles can slightly rotate the eye on its antero-posterior axis, the upper one draw

riert. T.sup. r.inc ing the upper part of the eyeball

Diagram of the direction of action of the inward, and its antagonist, the muscles of the eyeball, which is shown by

the dark lines. The axes of the rotation lower, drawing the lower part caused by the oblique and upper and lower

recti are shown by the dotted lines. The inward, so as to rotate the eye- inner and outer recti rotate the ball on its

vertical axis, which is cut across. ball in an opposite direction abbreviated names of the muscles are

affixed to the lines. round the same axis.

The internal and external recti draw the centre of the cornea

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to or from the median line respectively, directly opposing one another.

As the direction of the superior and inferior recti is different from that of the axis of the eyeball, they draw the outer edge of the cornea, not its centre, up and down respectively, and at the same time tend to give the eyeball a slight rotation in the same direction as the corresponding oblique muscles. The tendency to rotation is counteracted by the antagonistic oblique muscle when simple elevation or depression is formed.

Thus, pure abduction or adduction only requires the unaided action of the internal or external recti, while direct depression of the eye requires the combined action of the inferior rectus and superior oblique, and direct elevation requires the superior rectus and inferior oblique to act together. The oblique movements are accomplished by various combined coördinations of movement of the different muscles,

From the foregoing it is obvious that the simplest movements of the eye require the coöperation of different muscles.

The diagram shows the directions toward which the different muscles tend to draw the eyeball.

In the ordinary movements of both eyes more than this is necessary. Byth eyes must move in the same direction at the same time, now to the right, now to the left, so that while the external rectus moves the right eye to the right side, the internal rectus moves the other eye in the same direction. The coördination of the movements of the eyeball is so arranged that the contractions of the external and internal recti of opposite sides must occur together, and are called “ associated movements." This associated movement has been acquired by the habit of voluntarily directing both eyes at the same object, and has gradually become involuntary, for few persons have the power of exerting control over the muscles of one eye alone.

BINOCULAR VISION. When we look at an object with both eyes we have a separate image thrown upon each retina, and therefore two sets of impulses are sent to the sensorium, one from the right and one from

the left eye.

Yet we are only conscious of the occurrence of one stimulation. The reason of this is, that experience has taught us that similar images thrown upon certain parts of the two retinæ correspond to the same object, and in our minds we fuse the sensations caused by the two images so that they produce but one idea.

These points of the retina which are thus habitually stimulated by the same objects are called “ corresponding points."

Besides being of great use in making up for such deficiencies as the blind spots (which are not corresponding points), binocular vision is useful for the following purposes :

To judge of distance. When using one eye only, some knowledge of distance may be gathered by the force employed to accommodate, but a much more accurate judgment can be formed when both eyes are used and the muscular sense of the ocular muscles, employed in converging the eyeballs for near objects, gives further evidence of their distance.

In judging of size, in the same way, with one eye, we can only have an idea of the apparent size of an object, which will vary with its distance. With a knowledge of apparent size and distance such as is gained by binocular vision, we can come to a fairly accurate conclusion as to the size of an object.

To judge of the relative distances of objects so as to see depth in the picture before our eyes, binocular vision is necessary. If one eye alone is used we see a flat picture, without having an accurate idea of the relative distances of the different objects. With each eye, however, we get a slightly different view of each object, and thus we are helped to a conclusion as to their exact distances and shapes, and arrive at fairly correct judgments as to their form, etc.

CHAPTER XXXIII.

HEARING.

Just as impulses traveling along the optic nerves can only give rise, in the sensorium, to impressions of light, so impulses passing to the sensorium via the auditory part of the portio mollis of the seventh pair of cranial nerves can only excite impressions of sound, and any stimulation of that nerve gives rise to sound sensations.

The peripheral end of the special nerve of hearing is distributed to an organ of very peculiar construction situated in the internal ear, which, from its complexity, has been called the labyrinth. The nerve endings are spread out between layers of fluid, so that they must be stimulated by very gentle forms of movement; and when we consider their delicacy, we cannot be surprised that even sound vibrations suffice to stimulate these terminals and transmit nerve impulses to the brain. The organs of hearing of mammalia are so deeply placed in the petrous part of the temporal bone, that special mechanisms have to be adopted to convey the sound with sufficient intensity from the air to the fine nerve terminals. These make up complex piece of anatomy which will be briefly referred to presently.

SOUND. Before attempting to describe the complex mechanisms by which sound is conveyed from the air to the nerve endings, some notion must be formed of what sound is from a merely physical standpoint. By means of the sense of hearing we form an idea of sound, and here the knowledge of sound ends with many people, since they only think of it as something they can hear. A physicist, however, regards sound in a different way. He knows that it is produced by the vibrations of elastic bodies, such as a tense string, a metal rod, or an elastic membrane. These vibrations, being communicated to the air, are conveyed

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