EYE, HUMAN

specialized sense organ capable of receiving visual images, which are then carried to the brain. Additional reading Hugh Davson (ed.), The Eye, 4 vol. (1962; 2nd ed., vol. 1, 1969), covers the whole field of eye physiology, written by a group of experts; Stewart Duke-Elder et al. (eds.), System of Ophthalmology, 15 vol. in 19 (195876), authoritative accounts of the anatomy and physiology of the eye; E. Wolff, Anatomy of the Eye and Orbit, 5th ed. (1961), the classic work on this aspect; Hugh Davson, Physiology of the Eye, 3rd ed. (1971), an account of eye physiology covering all aspects; M.H. Pirenne, Vision and the Eye, 2nd ed. (1967), a simple account of certain features of eye physiology; R.A. Weale, The Eye and Its Function (1960), a short and elementary account; H. von Helmholtz, Handbuch der physiologischen Optik, 3rd ed. (188696; Eng. trans., Physiological Optics, 3 vol., 192425; reprinted in 2 vol., 1962), a classic account of the psychological aspects of visionnot at all out of date, although written over 100 years ago; H.H. Emsley, Visual Optics, 5th ed., 2 vol. (195253), a technical account of the detailed optics of the eye. Physiological aspects of vision are discussed in Hitoshi Shichi, Biochemistry of Vision (1983). Psychology of vision, including motion, depth, binocular vision, visual effects, and colour, is discussed in Mark Fineman, The Inquisitive Eye (1981). Edward S. Perkins The higher visual centres The visual pathway The axons of the ganglion cells converge on the region of the retina called the papilla or optic disk. They leave the globe as the optic nerve, in which they maintain an orderly arrangement in the sense that fibres from the macular zone of the retina occupy the central portion, the fibres from the temporal half of the retina take up a concentric position, and so on; when outside the orbit, there is a partial decussation (crossover). The fibres from the nasal halves of each retina cross to the opposite side of the brain, while those from the temporal halves remain uncrossed. This partial decussation is called the chiasma. The optic nerves after this point are called the optic tracts, containing nerve fibres from both retinas. The result of the partial decussation is that an object in, say, the right-hand visual field produces effects in the two eyes that are transmitted to the left-hand side of the brain only. With cutaneous (skin) sensation there is a complete crossing-over of the sensory pathway; thus, information from the right half of the body, and the right visual field, is all conveyed to the left-hand part of the brain by the time that it has reached the diencephalon (the posterior part of the forebrain). Fusion of retinal images Partial decussation is an arrangement that serves the needs of frontally directed eyes and permits binocular vision, which consists in the fusion of the responses of both eyes to a single objectmore loosely, one speaks of the fusion of the retinal images. In many lower mammals, with laterally directed eyes and therefore limited binocular vision, the degree of decussation is much greater, so that in the rat, for example, practically all of the optic nerve fibres pass to the opposite side of the brain. The fibres of the optic tracts relay their messages to nerve cells in those parts of the diencephalon called the lateral geniculate bodies, and from the lateral geniculate bodies the messages are relayed to nerve cells in the occipital cortex of the same side. (The occipital cortex is the outer substance in the posterior portion of the brain.) The visual process The work of the auxiliary structures The protective mechanisms The first line of protection of the eyes is provided by the lids, which prevent access of foreign bodies and assist in the lubrication of the corneal surface. Lid closure and opening are accomplished by the orbicularis oculi and levator palpebri muscles; the orbicularis oculi operates on both lids, bringing their margins into close apposition in the act of lid closure. Opening results from relaxation of the orbicularis muscle and contraction of the levator palpebri of the upper lid; the smooth muscle of the upper lid, Mller's muscle, or the superior palpebral muscle, also assists in widening the lid aperture. The lower lid does not possess a muscle corresponding to the levator of the upper lid, and the only muscle available for causing an active lowering of the lid, required during the depression of the gaze, is the inferior palpebral muscle, which is analogous to the muscle of Mller of the upper lid (called the superior palpebral muscle). This inferior palpebral muscle is so directly fused with the sheaths of the ocular muscles that it provides cooperative action, opening of the lid on downward gaze being mediated, in effect, mainly by the inferior rectus. Innervation The seventh cranial nervethe facial nervesupplies the motor fibres for the orbicularis muscle. The levator is innervated by the third cranial nervethe oculomotor nervethat also innervates some of the extraocular muscles concerned with rotation of the eyeball, including the superior rectus. The smooth muscle of the eyelids and orbit is activated by the sympathetic division of the autonomic system. The secretion of adrenaline during such states of excitement as fear would also presumably cause contraction of the smooth muscle, but it seems unlikely that this would lead to the protrusion of the eyes traditionally associated with extreme fear. It is possible that the widening of the lid aperture occurring in this excited state, and dilation of the pupil, create the illusion of eye protrusion. Blinking is normally an involuntary act, but may be carried out voluntarily. The more vigorous full closure of the lids involves the orbital portion of the orbicularis muscle and may be accompanied by contraction of the facial muscles that have been described as accessory muscles of blinking: namely, the corrugator supercilii, which on contraction pulls the eyebrows toward the bridge of the nose; and the procerus or pyramidalis, which pulls the skin of the forehead into horizontal folds, acting as a protection when the eyes are exposed to bright light. The more vigorous full closure may be evoked as a reflex response. The work of the retina Some basic facts of vision So far, attention has been directed to what are essentially the preliminaries to vision; it is now time to examine some of the elementary facts of vision and to relate them to the structure of the retina and, later, to chemically identifiable events. Measurement of the threshold An important means of measuring a sensation is to determine the threshold stimulusi.e., the minimum energy required to evoke the sensation. In the case of vision, this would be the minimum number of quanta of light entering the eye in unit time. If it is found that the threshold has altered because of a change of some sort, then this change can be said to have altered the subject's sensitivity to light, and a numerical value can be assigned to the sensitivity by use of the reciprocal of the threshold energy. Practically, a subject may be placed in the dark in front of a white screen, and the screen may be illuminated by flashes of light; for any given intensity of illumination of the screen, it is not difficult to calculate the flow of light energy entering the eye. One may begin with a low intensity of flash and increase this successively until the subject reports that he can see the flash. In fact, at this threshold level, he will not see every flash presented, even though the intensity of the light is kept constant; for this reason, a certain frequency of seeinge.g., four times out of sixmust be selected as the arbitrary point at which to fix the threshold. When measurements of this sort are carried out, it is found that the threshold falls progressively as the subject is maintained in the dark room. This is not due to dilation of the pupil because the same phenomenon occurs if the subject is made to look through an artificial pupil of fixed diameter. The eye, after about 30 minutes in the dark, may become about 10,000 times more sensitive to light. Vision under these conditions is, moreover, characteristically different from what it is under ordinary daylight conditions. Thus, in order to obtain best vision, the eye must look away from the screen so that the image of the screen does not fall on the fovea; if the screen is continuously illuminated at around this threshold level it will be found to disappear if its image is brought onto the fovea, and it will become immediately visible on looking away. The same phenomenon may be demonstrated on a moonless night if the gaze is fixed on a dim star; it disappears on fixation and reappears on looking away. This feature of vision under these near-threshold or scotopic conditions suggests that the cones are effectively blind to weak light stimuli, since they are the only receptors in the fovea. This is the basis of the duplicity theory of vision, which postulates that when the light stimulus is weak and the eye has been dark-adapted, it is the rods that are utilized because, under these conditions, their threshold is much lower than that of the cones. When the subject first enters the dark, the rods are the less sensitive type of receptor, and the threshold stimulus is the light energy required to stimulate the cones; during the first five or more minutes the threshold of the cones decreases; i.e., they become more sensitive. The rods then increase their sensitivity to the point that they are the more sensitive, and it is they that now determine the sensitivity of the whole eye, the threshold stimuli obtained after 10 minutes in the dark, for example, being too weak to activate the cones.