Because only a small portion of the retina, the fovea, is actually employed for distinct vision, it is vitally important that the motor apparatus governing the direction of gaze be extremely precise in its operation, and rapid. Thus, the gaze must shift swiftly and accurately during the process of reading. Again, if the gaze must remain fixed on a single small object—e.g., a golf ball—the eyes must keep adjusting their gaze to compensate for the continuous small movements of the head and to maintain the image exactly on the fovea. The extraocular muscles that carry out these movements are under voluntary control; thus, the direction of regard can be changed deliberately. Most of the actual movements of the eyes are carried out without awareness, however, in response to movements of the objects in the environment, or in response to movements of the head or the rest of the body, and so on. In examining the mechanisms of the eye movements, then, one must resolve them into a number of reflex responses to changes in the environment or the individual, remembering, of course, that there is an overriding voluntary control.
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The axes of the eye
It is worthwhile at this point to define certain axes of the eyes employed during different types of study. The optic axis of the eye is a line drawn through the centre of the cornea and the nodal (central) point of the eye; it actually does not intersect with the retina at the centre of the fovea as might be expected, but toward the nose from this, so that there is an angle of about five degrees between (1) the visual axis—the line joining the point fixated (the point toward which the gaze is directed) and the nodal point—and (2) the optic axis.
Actions of muscles
The general modes of action of the six extraocular muscles have been described in connection with their anatomy: rotation of the eye toward the nose is carried out by the medial rectus; outward movement is by the lateral rectus. Upward movements are carried out by the combined actions of the superior rectus and the inferior oblique muscles, and downward movements by the inferior rectus and the superior oblique. Intermediate directions of gaze are achieved by combined actions of several muscles. When the two eyes act together, as they normally do, and change their direction of gaze to the left, for example, the left eye rotates away from the nose by means of its lateral rectus, while the right eye turns toward the nose by means of its medial rectus. These muscles may be considered as a linked pair; that is, when they are activated by the central nervous system this occurs conjointly and virtually automatically. This linking of the muscles of the two eyes is an important physiological feature and has still more important pathological interest in the analysis of squint, when the two eyes fail to be directed at the same point.
The binocular movements (the movements of the two eyes) fall into two classes, the conjugate movements, when both eyes move in the same direction, as in a change in the direction of gaze, and disjunctive movements, when the eyes move in opposite directions. Thus, during convergence onto a near object both eyes move toward the nose; the movement is horizontal, but disjunctive, by contrast with the conjugate movement when both eyes move, say, to the right. The disjunctive movement of convergence can be carried out voluntarily, but the act is usually brought about reflexly in response to the changed optical situation—i.e., the nearness of the object of gaze. A seesaw movement of the eyes, whereby one eye looks upward and the other downward, is possible, but not voluntarily; to achieve this a prism is placed in front on one eye so that the object seen through it appears displaced upward or downward; the other eye sees the object where it is. The result of such an arrangement is that, unless the eye with the prism in front makes an upward or downward movement, independent of the other, the images will not fall on corresponding parts of the retinas in the two eyes. Such a noncorrespondence of the retinal images causes double vision; to avoid this, there is an adjustment in the alignment of the eyes so that a seesaw movement is actually executed. In a similar way, the eyes may be made to undergo torsion, or rolling. A conjugate torsion, in which both eyes rotate about their anteroposterior (fore-and-aft) axes in the same sense, occurs naturally; for example, when the head tips toward one shoulder, the eyes tend to roll in the opposite direction, with the result that the image of the visual field on the retina tends to remain vertical in spite of the rotation of the head.
The nerves controlling the actions of the muscles are the third, fourth, and sixth cranial nerves, with their bodies (nuclei) in the brainstem; the third, or oculomotor nerve, controls the superior and inferior recti, the medial rectus, and inferior oblique; the fourth cranial nerve, the trochlear nerve, controls the superior oblique; and the sixth, the abducens nerve, controls the lateral rectus. The nuclei of these nerves are closely associated; especially, there are connections between the nuclei of the sixth cranial nerve, controlling the lateral rectus, and the nucleus of the third, controlling the medial rectus; it is through this close relationship that the linking of the lateral rectus of one eye and the medial rectus of the other, indicated above, is achieved. Another type of linking is concerned with reciprocal inhibition; that is, when there are two antagonistic muscles, such as the medial and the lateral rectus, contraction of one is accompanied by a simultaneous inhibition of the other. Muscles show a continuous slight activity even when at rest; this keeps them taut; this action, called tonic activity, is brought about by discharges in the motor nerve to the muscle. Hence, when the agonist muscle contracts its antagonist must be inhibited.
In examining any reflex movement, one must look for the sensory input—i.e., the way in which messages in sensory nerves bring about discharges in the motor nerves to the muscles. This study involves the connections of the motor nerves or nuclei with other centres of the brain.
When a subject is looking straight ahead and a bright light appears in the periphery of his or her field of vision, the subject’s eyes automatically turn to fix on the light; this is called the fixation reflex. The sensory pathway in the reflex arc leads as far as the cerebral cortex, because removal of the occipital cortex (the outer brain substance at the back of the head) abolishes reflex eye movements in response to light stimuli. If the occipital cortex is stimulated electrically, movements of the eyes may be induced, and in fact one may draw a pattern of the visual field on the occipital cortex corresponding with the directions in which the gaze is turned when given points on the cortex are stimulated. This pattern corresponds with the pattern obtained by recording the visual responses to light stimuli from different parts of the visual field.
The remainder of the pathway—i.e., from the occipital cortex to the motor neurons in the brainstem—has long been considered to involve the superior colliculi as relay stations, and they certainly have such a role in lower animals; but in humans a pathway from the cortex to the eye-muscle nuclei independent of the superior colliculi of the midbrain is now generally assumed.
Continual movements of the eyes occur even when an effort is made to maintain steady fixation of an object. Some of these movements may be regarded as manifestations of the fixation reflex; thus, the eyes tend to drift off their target, and, because of this, the fixation reflex comes into play, bringing the eyes back on target.
Experimentally, the fixation reflex can be studied by observation of the regular to-and-fro movements of the eyes as they follow a rotating drum striped in black and white. (Such movements of the eyes directed at a moving object are called optokinetic nystagmus; nystagmus itself is the involuntary movement of the eye back and forth, up and down, or in a rotatory or a mixed fashion.) While the eyes watch the moving drum, they involuntarily make a slow movement as a result of fixing their gaze on a particular stripe. At a certain point, fixation is broken off, and the eyes spring back to fix on a new stripe. Thus, the nystagmus consists of a slow movement with angular velocity equal to that of the rotation of the drum, then a fast saccade, or jump from one point of fixation to another, in the opposite direction; the process is repeated indefinitely.
Another type of nystagmus reveals the play of another set of reflexes. These are mediated by the semicircular canals—i.e., the organs of balance or the vestibular apparatus. Such a reflex may be evoked by rotating the subject in a chair at a steady speed; the eyes move slowly in the opposite direction to that of rotation and, at the end of their excursion, jump back with a fast saccade in the direction of rotation. If rotation suddenly ceases, the eyes go into a nystagmus in the opposite direction, the postrotatory nystagmus.
During rotation, certain semicircular canals are being stimulated, and the important point is that any acceleration of the head that stimulates these canals will cause reflex movements of the eyes. Thus, acceleration of the head to the right causes a movement of the eyes to the left, the function of the reflex being to enable the eyes to maintain steady fixation of an object despite movements of the head. The reflex occurs even when the eyes are shut, and, when the eyes are open, it obviously cooperates with the fixation reflex in maintaining steady fixation. In many lower animals, this connection between organs of balance and eyes is very rigid; thus, one may move the tail of a fish, and its eyes will move reflexly. In humans, not only do the semicircular canals function in close relation to the eye muscles but so also do the gravity organ—the utricle—and the stretch receptors in the muscles of the neck. Thus, when the head is turned upward, there is a reflex tendency for the eyes to move downward even if the eyes are shut. The actual movement is probably initiated by the reflex from the semicircular canals, which respond to acceleration, but the maintenance of the position is brought about by a reflex through the stretch of the neck muscles and also through the pull of gravity on the utricle, or otolith organ, in the inner ear.
The eyes are under voluntary control, and it is thought that the cortical area subserving voluntary eye movements is in the frontal cortex. Stimulation of this in primates causes movements of the eyes that are well coordinated, and a movement induced by this region prevails over one induced by stimulation of the occipital cortex. The existence of a separate centre in humans is revealed by certain neurological disorders in which the subject is unable to fixate voluntarily but can do so reflexly; e.g., he or she can follow a moving light.