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General Physiology Of Vision

General Physiology Of Vision

Introduction. The visual apparatus in its entirety constitutes a mechanism the excitation of which gives rise (1) to the sensation of light and its different qualities colors; (2) to the perception of light and color under the form of pictures of external objects; (3) to the production of muscular sensations by which we judge of the size, distance, and direction of objects. The specific physiological stimulus to the terminal apparatus of the optic nerve is the impact of the undulations of a perfectly elastic medium, the ether. The transfer of the energy of the ether vibrations into that form of energy known as a nerve impulse takes place in the pigment of the neuroepithelial laver of the retina. The nerve impulses so generated are transmitted by the fibers of the optic nerve to the cells of the cerebral cortex, in which some molecular process takes place out of which the mind forms the sensations of light and color. In general, it may be said that, at least for the same color, the intensity of the objective vibrations determines the intensity of the sensations.

The optic nerve, obeying the same general laws of nerve stimulation, reacts also to the electric current and to mechanical agencies, as shown by flashes of light with varying shades of color.

The formation of images on the percipient elements of the retina, which by their forms and associated colors give rise to the perception of objects, is made possible by the introduction of a complex refracting apparatus consisting of the cornea, aqueous humor, crystalline lens, and vitreous humor. Without these agencies ether vibrations would only give rise to a sensation of diffused luminosity. The movements of the eyeball occasioned by the contractions of the ocular muscles are attended by muscular sensations, out of which the mind draws its conclusions as to the size, distance, and direction of objects.

The Eye a Living Camera. In its construction, in the arrangement of its various parts, and in their mode of action the eye may be compared to a camera obscura Though the comparison may not be absolutely exact, yet in a general way it is true that there are many striking points of similarity between them e.g. the sclera and choroid may be compared to the wall; of the camera; the combined refractive media to the single lens, the action of which results in the focusing of the light rays ; the retina to the sensitive plate receiving the image formed at the focal point; the iris to the diaphragm for the regulation of the amount of light to be admitted, and for the partial exclusion of those marginal rays which give rise to spherical aberration; the ciliary muscle to the adjusting screw, by means of which the image is brought to a focus on the sensitive plate, notwithstanding the varying distances of the object from the lens. The presence of the visual purple in the rods of the retina capable of being altered by light makes the comparison Still more striking.

The Retinal Image. The existence of an image on the retina can be readily seen in the excised eye of an albino rabbit, the coats of which are quite transparent from the absence of pigment. Its presence in the human eye can be demonstrated with the ophthalmoscope. It is this image, composed of focal points of luminous rays, which is the basis of our sightperceptions, and which stimulates the rods and cones, and out of which the mind constructs space relations of external objects. In only two essential respects does the image on the retina differ from the object, aside from the fact that the object has usually three, the image only two, dimensions viz. in size and relative arrangement of its parts. Whatever the distance, the image is generally smaller than the object: it is also reversed, the upper part of the object becoming the lower part of the image, and the right side of the object the left of the image, and the reverse.

The Dioptric Apparatus. The media by which rays of light entering the eye are refracted and brought to a focus with the production of an image consist of the cornea, aqueous humor, lens, and vitreous body. As the two surfaces of the cornea are practically parallel, and as the index of refraction of the aqueous humor is the same as that of the cornea, they may be regarded as but one medium. The refracting surfaces may therefore be reduced to the anterior surface of the cornea, the anterior surface of the lens, and the posterior surface of the lens.

Rays of light emanating from one point that is, homocentric rays entering the eye must traverse successively the different refractive media. In their passage from one to the other they undergo at their surfaces changes in direction before they are converged to a focal point. In order to mathematically follow the rays in all their deviations through the media, to determine their focal point, and to construct the image, knowledge of the form of the refracting surfaces, the refractive index of the different media, and the distance of the surfaces from each other, must be obtained.

The following constants are now accepted: The radii of curvature of that portion of each refracting surface used for distinct vision are for the cornea 7.829 mm., for the anterior and posterior surfaces of the lens 10 and 6 mm., respectively. The indices of refraction of the different media are as follows: cornea and aqueous humor, 1.3365; lens, 1.4371 ; vitreous body, 1.3365. The distance from the vertex of the cornea to the lens is 3.6 turn. ; the thickness of the lens, 3.6 mm. ; the distance from the posterior surface of the lens to the retina, 15 mm.

Homocentric rays of light entering the eye pass from air with a refractive index of 1.00025 into the cornea with an index of 1.3365. In passing from the rarer into the denser medium they undergo refraction and are rendered somewhat convergent. The extent of this first refraction and convergence is sufficiently great to bring parallel rays, if continued, to a focus about 10 mm. behind the situation of the retina. On entering the lens they are for the same reason again refracted and converged, and if continued would come to a focus about 6.5 mm. behind the retina. On passing into the vitreous body they are again converged to an extent sufficient to focalize them on the retina (Fig. 48).

While it is possible thus to geometrically follow the rays through these media by means of the above mentioned factors, the procedure is attended with many difficulties. Moreover, as the relations all change when rays enter the eye from objects situated progressively nearer the eye, a separate calculation is necessitated for each distance for the determination of the size of the image.

A method by which these difficulties are much reduced was suggested by Gauss and developed by Listing. It was demonstrated by Gauss that in every complicated system of refracting media separated by spherical centered surfaces there may be assumed certain ideal or cardinal points, to which the system may be reduced, and which, if their relative position and properties be known, permit of the determination, either by calculation or geometrical construction, of the path of the refracted ray, and the position and size of the image in the last medium of the object in the first.

Every dioptric system can be replaced, as Gauss showed, by a single system composed of six cardinal points and six planes perpendicular to the common axis e. g. two focal points, two principal points, two nodal points, two focal planes, two principal planes, and two nodal planes.

Properties of the Cardinal Points. The first focal point, F, in Fig. 49, have the property that every ray which before refraction passes through it after refraction is parallel to the axis.

The second focal point, _F, has the property that every ray which before refraction is parallel to the axis passes after refraction through it.

The second principal point, H2, is the image of the first, H1 ; that is, rays in the first medium which go through the first principal point pass after the last refraction through the second. Planes at right angles to the axis at these points are principal planes. The second principal plane is the image of the first. Every point in the first principal plane has its image after refraction at a corresponding point in the second principal plane at the same distance from. the axis and on the same side.

The second nodal point, A,, is the image of the first, N1 : a ray which in the first medium is directed to the first nodal point passes after refraction through the second nodal point, and the directions of the rays before and after refraction are parallel to each other. In Fig. 49 let A B represent the axis. The distance of the first focal point, F1, from the first principal plane, Hi, is the anterior focal distance. The distance of the posterior focal point, F2, from the second principal plane, H2, is the posterior focal distance. The distance of the first nodal point, A',, from the first focal point is equal to the second focal distance. The distance of the second nodal point, N2, from the posterior focal point is equal to the anterior focal distance. It is evident, therefore, that the distance of the corresponding principal and nodal points from each other is equal to the differences between the two focal distances. Also the distance of the two principal points from each other is equal to the distance of the two nodal points from each other. Finally, the focal distances are proportional to the refractive indices of the first and last media Planes passing through the focal points vertically to the axis are known as focal planes.

From these properties of the cardinal points the position of an image in the last medium of a luminous point in the first may be determined, and the course of a refracted, ray in the last medium be constructed d if its direction in the first be given according to the following rules:

The Schematic Eye Accepting the system of cardinal points, Listing, Donders, and v. Helmholtz have constructed "schematic” eyes to be substituted for the refracting system of the natural eye.

For this purpose it is necessary to deduce from the various estimates of the indices of refraction of the different media, of the radii of curvatures of the different refractive surfaces, and of the distances separating them an average eye as a basis for calculation. The most recent attempt is that of v. Helmholtz. The data he assumed are as follows: The refractive index of air = 1 ; of the cornea and aqueous humor, 1.3365 ; of the lens, 1.4371 ; of the vitreous humor, 1.3365 ; the radius of curvature of the cornea, 7.829 mm. ; of the anterior surface of the lens, 10 mm. ; of the posterior surface, 6 mm. ; the distance from the apex of the cornea to the anterior surface of the lens, 3.6 mm. ; thickness of lens, 3.6 mm. From these data v. Helmholtz calculated the position of the cardinal points for the eye as follows (see Fig. 52): The first focal point is situated 13.745 mm. before the anterior surface of the cornea; the posterior focal point is situated 15.619 rum. behind the posterior surface of the lens; the first principal point, 1.753 mm. behind the cornea; the second principal point, 2.106 mm. behind the cornea; the first and second nodal points, 6.968 and 7.321 mm. behind the apex of the cornea, respectively. The anterior focal distance of this schematic eye therefore amounts to 1~.498 mm. and the posterior focal distance to 20,713 rum.

When the eye, however, is accommodated for near vision, the relations of the cardinal points are changed as follows, if the point accommodated for lies 152 mm. from the cornea: Anterior focal distance, 13.990 mm. ; posterior focal distance, 18.689 mm. ; distance from cornea of the first and second principal points, 1.858 and 2.257 mm. respectively ; distance of the posterior focus, 20.955 mm. from cornea. Given this schematic eye in the accommodated state, the course of the rays and the determination of the position of an image in the last medium of a luminous point in the first can easily be determined by the rules above given.

The Reduced Eye. As suggested by Listing, this schematic eye may be vet further simplified or reduced to a single refracting surface bounded anteriorly by air and posteriorly by aqueous or vitreous humor. Without introducing any noticeable error in the determination of the size of the retinal image, the anterior principal and the anterior nodal points may be disregarded, Owing to the minuteness of the distances (0.39 mm.) separating the two systems of points. There is thus obtained one principal point and one nodal point, which latter becomes the center of curvature of the single refracting surface. The dimensions of this "' reduced" eye are as follows (see Fig. 53): from the anterior surface of the cornea to the principal point, 2,106 rum. ; To the nodal point, 7.321 mm. The anterior focal distance is 15.498 mm. the posterior focal distance, 20.713. There is thus substituted for the natwal eye a single refracting surface having a radius of curvature of 5.215 mm. The index of refraction of this eye is 1.3365, which is that of the us humor. In such an eye luminous rays emanating from the anterior point are parallel to the axis after refraction in the interior of the eye. rays parallel to the axis before refraction unite at the posterior focal point. By means of this reduced eye the construction of the refracted ray, various calculations as to the size of the image, the size of diffusion cir are much facilitated.

Accommodation.1 In a normal or emmetropic eye homocentric parallel ray of light after passing through the optic media are converged and brought to a focus on the retina. Rays, however, which come from a luminous point situated near the eye, and which are therefore divergent and passing through the optic media at the same time, are intercepted by the retina before they are focused, and give rise to the formation of diffusion circles and indistinctness of vision. The reverse is also true. When the eye is adjusted for the refraction and focusing of divergent rays, parallel rays will be brought to a focus before reaching the retina, and, again diverging, will form diffusion circles. It is evident, therefore, that it is impossible to simultaneously focus both parallel and divergent rays, and to see two objects distinctly at the same time which are situated at different distances. The eve must be alternately adjusted first to one object and then to another, the capability which the eye possesses of adjusting itself to vision at different distance; is termed accommodation.

The following table of Listing shows the size of the diffusion circles formed of objects situated at different distances when the accommodative power is suspended:

The normal eye when adjusted for distant vision is in a passive condition and unattended with fatigue. In the act of adjustment, however, for near vision the eve passes into an active state, the result of a muscular effort, the energy of which is proportional to the nearness of the object toward which the eye is directed. From the above table it is evident that rays of light coming from infinity or front any object even but 6 in. distant are so nearly parallel and t lie diffusion circles so very small that the indistinctness of the image is scarcely perceived, and hence no perceptible accommodative effort is required. Rays coming from objects situated progressively nearer the eye require for their focalization a constantly increasing effort of accommodation. During accommodation the lens undergoes a change of' shape, becoming more convex, especially on its anterior surface. The greater the degrees of divergence of the rays the greater must be the increase in lens convexity, in order that they may be sufficiently converged and focalized on the retinal surface. Changes in the curvatures of the lens, either of increase or decrease, are attended with corresponding changes in the distinctness of the image.

Mechanism of Accommodation. Though it is generally admitted that the increase in the convexity of the lens is caused by the contraction of the ciliarv muscle and the subsequent relaxation of the suspensory ligament, the exact manner in which this is brought about is not well understood. When the eve is in repose and adjusted for distant vision the lens is somewhat flat from the traction of the suspensory ligament. When the eve requires adjustment for near vision the ciliarv muscle contracts, the suspensory ligament relaxes, and the lens, in consequence of its inherent elasticity, bulges forward and becomes more convex. Its antero posterior diameter is thus increased and its refractive power is proportionally greater.

It is generally admitted that during accommodation the meridional fibers of the ciliarv muscle draw forward the ciliary processes and relax the ligament. At the same time the outer border of the iris is drawn somewhat backward. In extreme efforts of accommodation it is also believed by some observers that the circular fibers, the so called 11 annular muscle," contract exert a pressure on the periphery of the lens, and thus aid other mechan increasing the convexity. 'this view appears to be supported by the that in hyperopia, where there is a constant effort required for distinct L,h even of distant objects, the annular muscle becomes very much hyperhied, thus serving to reinforce the action of the meridional fibers. In pia, on the contrary, where the accommodative effort is at a minimum, mire muscle possesses less than its average size and development (compare with page 135).

Optical Defects.' From a purely physical point of view the eye is not t instrument. It is not quite achromatic, is not free from spherical ion, and is not exactly centered. Moreover, its area of distinct vision is quite limited, and does not correspond with the field of projection, In first class optical instruments the lenses are centered that is, their exact centers are situated on the same axis. In viewing an object through such a system the visual line corresponds with the axis of the lens system. This is not the case with the lens system of the eye.

A line passing through the center of the cornea and the center of the eye, the optic axis 0 A in Fig. 55, does not pass exactly through the center of the lens, and does not fall into the point of most distinct vision, the fovea. This has led to the recognition of other lines, the relations of which must be borne in mind in all optical discussions.

1. The visual axis, or line of vision VL, is the line connecting the point viewed, the nodal point, and the fovea centralis.

2. The line of fixation, or line of regard V Q, is the line connecting the point viewed with the center of rotation, the latter being situated 6 mm. behind the nodal point of the eye and 9 before the retina. The relations of these lines and certain angles in connection with them are shown in the following figure:

The angle included between the line D D (the major axis of the corneal ellipse) and the visual line is the angle alpha, amounting, on the average, to about 5'. The angle included between the optic axis and the line of regard is known as the angle gamma, while the angle between the optic axis and the line of vision is known as the angle beta (see also page 129).

Functions of the Iris. The iris, in virtue of the capability it possesses of alternately enlarging and diminishing the size of its central opening, the pupil, forms in several respects an important corrective apparatus of the eye. It serves as a diaphragm by which the rays of light which would otherwise pass through the margin of the lens are cut off, so that spherical aberration is in a great measure overcome. It also serves, through the contraction of its muscular fibers, to form a fixed point of support for the ciliary muscle during the period of active accommodation. Owing to the fact that the circular fibers of the iris alternately contract and relax with increasing and decreasing intensities of light, it serves to regulate the amount of light entering the eye necessary for distinct vision. In the absence of light the sphincter pupille relaxes and the pupil enlarges. As the light increases in intensity the muscle contracts and the pupil becomes smaller. The contraction of the sphincter muscle is with a given intensity of light greater when the light falls directly into the fovea. Contraction of' this muscle also occurs as an associated movement in the act of convergence of the optic axes in accommodative efforts and in consensus with tile other eye.

The movements of the iris by which the size of the pupil is determined from moment to moment are caused by the contractions of the sphincter pupille and dilatator pupille muscles. The contraction of the sphincter is entirely reflex and involves for its action the parts necessary to the performance of any reflex act viz. a sentient surface, the retina; an afferent nerve, the optic; a central center situated in the gray matter of the aqueduct of Sylvius and an efferent nerve, the motor oculi. The stimulus requisite for the calling forth of a contraction is the impact of ether vibrations on the ends of the rods and cones. According to the intensity of the light or ether vibrations will be the energy of the contraction. The contraction of the dilatator pupille is determined by the activity of a continuously active nerve center situated in the medulla oblongata, which transmits its regulative nerveimpulses to the iris through fibers in the sympathetic.

The exact course of these fibers, however, in man is not satisfactorily determined. From their origin they pass successively through the cervical cord, the anterior roots of the first and second dorsal nerves, the upper thoracic ganglion, the cervical sympathetic, the upper cervical ganglion through fibers to the ophthalmic division of the fifth nerve, the nasal nerve, and long ciliary nerve to the iris.

As to the action of the two sets of muscles, they appear to bear an antago relation to each other, for section of the motor oculi is followed by relaxation of the circular fibers and dilatation of the pupil. Stimulation of the sympathetic in the neck is followed by a much larger dilatation of the pupil. The normal physiological stimulus to the dilator center is probably dyspneic blood, though it is excited by muscular activity and stimulation of various sensory nerves.

Functions of the Retina. Of all the layers of the retina, the rods and cones appear to be the most essential to vision. It is only this layer which is capable of receiving the light stimulus and of transforming it into some specific form of energy, which in turn arouses in the fibers of the optic nerve the characteristic nerve impulses. The nerve fibers themselves are insensible to the impact of the ether vibrations, and require for their excitation some intermediate form of energy. That this is the case was shown by Donders, who reflected a beam of light on the optic nerve at its entrance without the individual experiencing any sensation of light. This region, occupied only by the optic nerve fibers and devoid of any special retinal elements, is therefore an insensitive or blind spot. The diameter of this spot is about 1.5 mm., and occupies in the field of vision a space of about 6'. It is situated about 3.5 mm. to the nasal side of the visual axis. Its existence can be demonstrated by the familiar experiment of Mariotte e. g. if the right eye be directed to the cross in the following figure (56) and the left eye closed, and the paper be held at a distance of 10 inches, the circle will entirely disappear. This occurs when the image falls on the optic nerve at its entrance. (See also page 470.) The experiment of Purkinje demonstrates the same fact.

It is well known that the blood vessels of the retina are situated in its innermost layers a short distance behind the optic nerve fibers. Owing to this anatomical arrangement, a portion of the light coming through the pupil will be intercepted by the vessels and a shadow projected on the layer of rods and cones. Ordinarily, these shadows are not perceived, for the reason that the shaded parts are more sensitive and their excitability less readily exhausted, and perhaps because the mind has learned to disregard them. But if light be made to enter the eye obliquely, the position of the shadows will be changed, when at once they become apparent. This can be shown in the following way:

If in a darkened room a lighted candle be held several inches to the side and to the front of the eve, and then moved up and down, there will be perceived, apparently in the field of vision, an arborescent figure corresponding to the retinal blood vessels. This is due to the falling of the shadows on unusual portions of the layer of rods and cones (see also page 141).

Excitability of the Retina. The retina is not equally excitable in all parts of its extent. The maximum degree of sensibility is found in the macula lutea, and especially in its central portion, the fovea. In this region the layers of the retina almost entirely disappear, the layer of rods and cones only remaining, and in the fovea only the latter are present. That this area is the point of most distinct vision is shown by the observation that when the eve is directed to any given point of light, its image always falls in the fovea. Any pathological change in the fovea is attended by marked indistinctness of vision. The sensibility of the retina gradually but irregularly diminishes from the macula toward the periphery. This diminution in sensibility holds true for monochromatic as well as white light.

As stated above, the nature of the molecular processes which take Place in the retinal tissue, and which are caused on one hand by the light vibrations, and on the other hand develop nerve impulses, is entirely unknown. The discovery of the visual purple. in the outer segment of the rods gave promise of some explanation of the process, especially when it was shown to undergo changes when exposed to the action of light. Kuhne even succeeded in obtaining an optogram, or a fixed image, of an external object in a manner similar to that by which an image is fixed on the sensitive plate of a camera. But as the pigment is wanting in the (, ones, and especially in the fovea, it cannot be considered essential to distinct vision, although that it plays some important role in the visual process is highly probable. The visual purple disappears when the eye is exposed to light, but is restored when light is excluded. It has also been observed that under the influence of light stimulation the cones become shorter, and in the darkness again become longer (see page 69).

Color perception. A beam of sunlight passed through a glass prism is decomposed into a series of colors red, orange, yellow gre en, blue, indigo, and violet the so called spectral colors, so well exemplified in the rainbow. The spectral color,. Are termed simple colors, because they cannot be an further decomposed by a prism. Objectively, the spectral colors consist of very rapid transverse vibrations of the ether, from about 400 millions of millions per second for red to about 760 millions of millions for violet, but subjectively they are sensations caused by the impact of the ether waves the percipient layer of the retina.

is possible to mix or blend these spectral color sensations in the eye stimulating the same area of the retina by different spectral colors, either the same time or in rapid succession. The following table shows the result of such experiments as performed by v. Helmholtz (DR. == dark; Wh. whitish).

These are. the mixed colors. But it is to be observed that only two new sensations can be produced, white and purple, the remaining mixed color already binding their equivalent in the spectrum. White and purple, re, are color sensations, which have no objective equivalent in a simple r of ether vibrations like the spectral colors.

spectral colors which by their mixture produce the sensation of are called complementary colors. Such are red and green blue, golden w and blue, green and purple. The mixture of all the spectral colors uces white again. This is the result of adding two or more color sensation Different results are obtained, however, by adding colored pigments. Yellow and blue, for example, produce in the eye white, but on the painter's e green. For the explanation of such facts reference must be made to r treatises. The colors of nature are usually mixtures of simple colors, be shown by spectroscopic analysis or by a synthesis of spectral colors.

In all color sensations we must distinguish three primary qualities: (1) hue; (2) purity or tint; (3) brightness or luminosity. The first quality gives main name to the color e. g. red or blue this depending on the spectral or the mixture of two spectral colors with which it can be matched. The second quality, the tint, depends on the admixture of white to the d color; and the third quality, brightness, depends on the objective intensity of the light and the subjective sensitiveness of the retina. Color thus far refers only to the most sensitive part of the retina. At more peripheral parts of the retina the colors are seen somewhat differ as is shown by the following table giving the limits up to which the color are recognized.

of Color pereeption. The theory of v. Helmholtz, originated Thomas Young (1807), assumes in its latest form the existence in the retina of three different kinds of end organs, each of which is loaded its own photo chemical substance capable of being decomposed by a color and thus exciting the fiber of the optic nerve.

the first group these end organs are loaded with a red sensitive sub, which is affected mainly by the red part of the spectrum ; the second p has its end organs provided with a green sensitive substance, which is By excited by the green color; while the third group is provided with a e sensitive substance, this latter being mainly affected and decomposed by blue violet portion of the spectrum. All these three different end organs present in every part of the most sensitive area of the retina, and are ected by separate nerve fibers with special parts of the brain, in the cells which each calls up its separate sensation of red or green or blue.

of these three primary color sensations all other color sensation arise. If a light mainly excites the red or green or blue sensitive substance of a retinal area, we term it red, green, or blue, respectively. But if two these photo chemical substances are stimulated simultaneously, quite different sensations arise. Thus simultaneous stimulation of the red and blue, g e stances gives rise to tile sensation of yellow, that of red and blue to t sensation of purple, and that of blue and green to the sensation of blue green Simultaneous stimulation of all three substances of a certain area produce the sensation of white. According to this theory, complementary colors a all those which together excite all the three substances. Color blindness explained by this theory, on the assumption that two of the photo chemical substances have become similar or equal in composition to each other.

The theory of Herring, brought forward in 1874, has the underlying sumption that the process of restitution in a nerve element is capable of exciting a sensation. This theory asserts that there are three visual substances in the retina a white black, a red green, and a yellow blue vis substance. A destructive process in the white black substance, such as induced not only by white light, but also by any other simple or mixed color produces the sensation of white, while the process of restitution or assimilation in this substance produces the sensation of black. Similarly, red light produces dissimilation or decomposition in the red green substance, and this, again, the sensation of red. Green light, however, favors the process of restitution or assimilation in the red green substances, and thus gives rise to the sensation of green. In the same way the sensation of yellow has its cause in the decomposition of the yellow blue substance induced by yellow light, while the sensation of blue is produced by an assimilative process in the same substance. Simultaneous processes of disassimilation and assimilation in the same visual substance antagonize each other, and consequently produce no color sensation by means of this substance, but only the sensation of white, by reason of decomposition, by both colors, in the white black substalim Thus, yellow and blue ' impinging on the same retinal area, have no effect on the yellow blue substance, because they are antagonistic in their action on this substance, but only produce the sensation of white, as both yellow and blue decompose the white black material. Color blindness is explained by the assumption of the absence of either the red green or the yellow blue visual substance in the retina.

Movements of the Eyeball. The almost spherical eyeball lies in a correspondingly shaped cavity of the orbit, like a hall placed in a socket, and is capable of being moved to a considerable extent by the six ocular muscles which are attached to it. The movements of each eye are referred to three fixed lines or axes which have their origin at the point of rotation of the eyeball, this point lying about 1.7 mm. behind the center of the globe. If tile eye looks straight forward in the horizontal plane (the bead being erect), the line joining the center of rotation with the object looked at is the visual line or visual axis. Around this antero posterior axis the eye may be regarded as performing its circular rotation or torsion. At right angles to this line, and joining the center of rotation of both eyes, is the horizontal or transverse axis around which the movements of elevation (up to 34') and depression (down to 57') take place. At right angles to both of these lines there is the vertical axis, around which the movements of adduction (toward the nose up to 45') and abduction (toward the temple up to 420) occur. The six muscles may be divided into three pairs, each of which has a common axis around which it tends to move the eyeball. But only the common axis of the internal and external recti coincides with one of three axes before mentioned namely, with the vertical axis thus moving the ball only inwardly or outwardly, respectively. The other two pairs, however, have their own axes of action, their movements of the ball must be therefore analyzed with regard to the three axes, each of' these four muscles producing rotation, elevation, depression, and abduction or adduction. The superior and inferior recti muscles, forming one pair, move the eye around a horizontal axis which intersects the median plane of the body in front of' the eyes at an angle of , and the superior and inferior oblique muscles forming the third pair rotate the globe around a horizontal axis which cuts the median plane of the body behind the eyes at an angle of 390. Thus it is that each muscle moves the eye as follows, the movement for practical purposes being referred to the cornea: The rectus externus draws the cornea simply to the temporal side, rectus internus simply to the nose; the superior rectus displaces the cornea upward, slightly inward, and turns the upper part toward the nose (medial torsion); the inferior rectus moves the cornea downward, slightly inward, and twists the upper part away from the nose (lateral torsion) ; the superior oblique displaces the cornea downward, slightly outward, and 'produces medial torsion; while the inferior oblique moves the cornea upward, slightly out and produces lateral torsion. These facts show that for certain move of the eye at least three muscles are necessary (see following table)

If both eyes have their line of vision in the horizontal plane parallel with each other and with the median plane of the body, they are said to be in the primary position. All other positions are called secondary. Both eyes always move simultaneously, which is called the associated movement of the eyes. There are three forms of associated movements: (1) movement of both eyes in same direction; (2) movements of convergence by which the visual lines converged on a point in the middle line of the body; (3) movements of divergence, by which the eyes are brought back from convergence to parallelism, or even to divergence, as in certain stereoscopic exercises. A combination of (1) and (2) or of (1) and (3) takes place for certain positions of the object looked at.

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