Chapter 05: The Reaction to Near Vision

CHAPTER 5 The Reaction to Near Vision CONTENTS A. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . B. Historical Background . . . . . . . . . . . . . . . . . C. Appearance . . . . . . . . . . . . . . . . . . . . . . . . 1. Reactions in Animals . . . . . . . . . . . . . . . 2. Convergence, Accommodation, and Pupillary Contraction in Man . . . . . . . . . 3. Other Factors . . . . . . . . . . . . . . . . . . . . . (a) Fusional Convergence . . . . . . . . . . . . (b) Awareness of Nearness . . . . . . . . . . . . D. Neural Pathways . . . . . . . . . . . . . . . . . . . . . 1. The Afferent Path . . . . . . . . . . . . . . . . . . 2. Central Connections . . . . . . . . . . . . . . . . 3. The Motor Center and Preganglionic 295 296 296 296 302 303 303 304 305 305 306 Efferent Path . . . . . . . . . . . . . . . . . . . .. (a) The Oculomotor Nucleus and Third Nerve . . . . . . . . . . . . . . . . . . . . . . . . (b) The Ciliary Ganglion and ... Postganglionic Ciliary Nerves (c) "Light-near Dissociation" by Postganglionic Damage . . . . . . . . . . . (d) The Ciliary Muscle . . . . . . . . . . . . . E. Influence of Sympathetic Nerves . . . . . . . . . . F. The Near-Vision Reaction as Clinical ... Indicator: What Is Normal? . . . . . . . . ... G. Misconceptions . . . . . . . . . . . . . . . . . 307 307 309 311 311 312 315 317 A. Summary When the eyes are moved from a far to a near point, three events occur together: convergence of the visual axes keeps the image placed upon corresponding retinal areas of the two eyes; accommodation of the lens corrects the focus of the eye; and contraction of the pupils increases the depth of field. Together, these adjustments allow the individual to maintain a sharp, stereoscopic single image of the object viewed. The near-vision reaction is the most highly developed of all eye movements and requires exquisite coordination of the striated extraocular and mooth intraocular muscles. Correspondingly, it developed late phylogenetically, being complete only in primate and man. Ontogenetically, also, the near-vision triad i the last of the eye movements to be perfected. The pupillary near-vision re ponse has often been said to depend upon accommodation or convergence, but in actual fact it is not brought about by either of the two. It is a co-movement, elicited by the forebrain together with accommodation and convergence during the voluntary act of near vi ion. It can occur when either accommodation or convergence is abolished by len es or by prisms, respectively, or when either of the e functions is lost due to disease. The near-vision reaction can be produced by electric stimulation of certain cortical areas. Its motor center is the third nerve nucleu . For convergence, the largecelled subnuclei for the internal recti are activated. For accommodation, the anteromedial portion, and, for the pupils, the Edinger-We tphal portion of the mall-celled oculomotor group are brought into play. The efferent impulses for the pupillary near-vi ion reaction thus are conveyed by the same neuron that erve the light reflex. Their axons run with the oculomotor nerve and synapse with the cells of the ciliary ganglion. The postganglionic short ciliary nerves then transmit the me age to the pupillary sphincter. It has been said many times that the light reflex and the pupillary contraction to near-vision each have their own pre- and postganglionic neurons, and that the fibers for the pupillary near response fail to synapse in the ciliary ganglion. They are said either to reach the sphincter muscle directly or to synapse with ganglion cells situated more peripherally in the orbit or within the eye. These assumptions are not based on fact, and they offend against fundamental physiologic laws. There is only one common motor path for the pupillary sphincter, just as for all other muscles of the body. However, the parasympathetic pre- and postganglionic neurons for accommodation of the lens are separate from those for the pupil; and probably they differ from ordinary autonomic neurons to some extent. Besides this cholinergic nerve supply, the ciliary muscle receives a small number of sympathetic nerves. These vary among species and in different parts of the muscle. They serve to inhibit the ciliary muscle, which-in the human and the monkey eye-contains beta-adrenergic receptors almost exclusively. Activation of these sympathetic fibers causes a slight change of refraction toward "far." In normal young adults the pupillary reaction to a vigorous near-vision effort is about as extensive as the contractions to bright light. The amplitude of the response depends upon the effort expended, not only upon the distance of the target from the eye. The reaction is the same in the two eyes even when one eye does not converge because of asymmetric placement of the target or when one eye is covered. In many people the near-vision contraction of the pupil is less extensive than the light reflex. This is especially true in older persons who have lost most of their accommodation, which has been replaced by reading glasses, for some years; or in patients who cannot or do not want to cooperate properly. Because of these 295 296 I l. Anatomy and Physiology subjective features, a reduced pupillary near-vision response is not clinically significant unless it can be proven that an adequate near-vision effort was made. However, when the near-vision contraction is more extensive than the patient's best light reflex, a "light-near dissociation" exists. This sign is always part of a pathologic pupillary syndrome. B. Historical Background After Scheiner (1616) had described accommodation and the pupillary constriction that occur during near vision, others commented on this interesting phenomenon. In these reactions the iris appeared to obey an effort of will, brought about by the wish to see nearby objects clearly. This fitted weJJ with the idea current at the time that pupillary movements in general were voluntary in animals and voluntary at birth in human infants also, to become unconscious and involuntary only later, by habit. Descartes (1664), however, pointed out that the pupil was not under direct voluntary control: it was impossible to constrict it by mental effort alone. The contraction merely accompanied a different voluntary act, that is, the shifting of gaze from a far to a near point (see Chapter 13). During the next three centuries, a great deal of discussion revolved around the question of whether the pupillary contraction to near vision depended upon convergence or upon acommodation; and many ingenious experiments were devised to prove one or the other view (Table 5-1). It is curious that these speculations have continued to the present, since it was shown over and over again that each component of the near-vision reaction can be divorced from the other two: pupillary constriction alone, accommodation alone, and convergence alone can be abolished; and each of them can be evoked selectively in different ways. The controversy continued because various authors supposed that the factor excluded in their experiments (that is, accommodation or convergence) was not essential to produce the pupillary response, leaving the remaining one as cause. But, as pointed out by Behr (1924) in an excellent discussion, the pupillary reaction is not a reflex in the usual sense. It is a movement elicited by a central coordinating mechanism that serves to maintain clear binocular vision. In the clinical literature the pupillary near-vision contraction played an important role because it remains active while the light reflexes are reduced or lost in two important pupillary syndromes, the Argyll Robertson syndrome and "Adie's" tonic pupil. Because of this "light-near dissociation" a number of theories were proposed regarding the mechanism and the reflex path for the near-vision contraction. C. Appearance 1. Reactions in Animals The ocular near-vision apparatus varies greatly among species; this is related to differences in the construction of animals' eyes, the environment and feeding habits, the stage of development of the central nervous system, and many other factors. For example, for accommodation, Gillum (1976) wrote that "some vertebrate eyes are so small as to possess a large depth of field, while others with larger eyes have structures that circumvent the need for an active lenticular focussing mechanism. Cyclostomes and teleosts are myopic and move the lens backward to accommodate for distance. Selachians, amphibians, and snakes are hypermetropic and move the lens forward to accommodate for near. Birds and reptiles have powerful mechanisms that compress the lens to accommodate for near. Amphibious vertebrates have the greatest accommodative amplitude of all. The mammalian mode of accommodation is relatively new in the phylogenetic sense. It is poorly developed in most forms except primates, the most advanced of which is man's." For convergence, an effective motor apparatus appears quite late phylogenetically, together with the shift from panoramic vision, with the eyes placed on the side of the head, to binocular stereoscopic vision, with the eyes looking forward and the visual fields of the right and the left eye largely overlapping. Correspondingly, convergence occurs late ontogenetically: in human infants it begins to develop only some months after birth. 1 A small pupil during near vision enhances the depth of focus of the eye, which contributes to clear vision. In human vision this effect is quite marked (Campbell, 1957; Alpern, 1969; Hennesy et al., 1976). Among the commonly used laboratory animals, monkeys have the most pronounced near-vision reactions. The good convergence, bulging of the lens, and pupillary constriction can be seen well when the animals look at a raisin or other small bit of favorite food nearby. In cats and dogs, the accommodative apparatus is much less well developed, and in rabbits it is only rudimentary. Convergence, similarly, is poor in cats and dogs and virtually absent in rabbits. This agrees with the fact that binocular vision with superimposition of the two eyes' visual fields is quite imperfect in these animals (see Chapter 3). 1. See also the interesting and detailed descriptions of the accommodative apparatus in various species given by G.L. Walls in his book on the vertebrate eye [Bloomfield Hills, Mich.: The Cranbrook Institute, 1942). 5. The Reaction to Near Vision Table 5-1. YEAR Pupillary movements related to near vi ion: Literature considered AUTHOR 1619 1659 1664 1751 1780 1797 1 811 Scheiner Plempius Descartes Whytt Olbers Monro Wells 1821} 1851 1834 1 835 1836 1843 1853 1853 1 854 1855 1856 1863 1863 1864} 1 865 1866 1 868 Weber Faria Plateau Kohlrausch Ruete Cramer De Ruiter von Graefe Helmholtz von Graefe Robertson Wells Danders Trautvetter Hering 1869 1869 von Arlt Le Conte 1 869 1870 Wells AdamUk 1871 1881 1 888 1892 Ada.milk & Woinow Coccius Krenchel (& Mulder} Drouin Hensen & Voelckers Angelucci & Aubert Leeser Danders Barthen 1893 Du Bois -Reymond 1895 1895 &Greeff Seggel Heine 18 7 3 1874 1876 1 878 1880 CHIEF SUBJECT OF I TEREST experiments on accommodation (first description) similar to Scheiner's description voluntary vs. involuntary pupillary movements near vision reaction aescribea pupillary contraction with accommodation similar to Sclieiner's exoeriments . effect of atropine was different for pupil and accommodation; convergence also was independent co movement pupillary contraction relatea to accommodation and convergence ("'1) similar t.o Descartes's discussion pupil contraction with accommodation alon~ ( * 1) effect of atropine on pupil and accommodat~on effect of atropine on pupil and accommodation pupil contraction with accommodation alone ( *l) pupil contraction with accommodation alone ( * 1) . . aEEarentl;}:'.: paradox reaction in dogs (PS,Y:Cho-sensor,Y: dilation) mechanism of accommodation case with EOM palsl and good pupillar;y: near response effect of calabar bean on p u_eil and accommodation effect of calabar bean on EUJ2il and accommodation pupils can constrict with accommodation alone and with convergence alone ( *1 , *2) apparently paraaox near response in aog:s (see von Graefe) pupil constriction with fusion of entoptic images (with accommodation z but not with convergence) timing of pupil contraction and of accommodation relations between pupillary constriction, accommodation, and convergence (*l) effect of atropine on pupil and accommodation pupillary contraction, accommodation, and eye movements elicited by electric stimulation in the midbrain (dogs) effect of electric st. of efferent nerves on the pupil,. lens curvature 1 and depth of anterior chamber (dog) pupillary contraction with accommodation & convergence (*l) effects of muscarine on the pupil ana on accommoaation relations between pupil, accommodation, and convergence isolated pupil constriction, accommodation , and contraction ot internal rectus and other ele muscles to electric st. in dogs speed of pupillary contraction and of change in Purkinje images, showing accommodation (pupil was slower) relations between pupil, accommodation, and convergence (*1 * 2) puEil and accommodation versus convergence ( * 1, *~) relations between pupillary constriction, accommodation, and _ convergence J clinical, * l, *2 ) flash photos of pupils during near -vision efforts in darkness: pupils contracted ( *3 ) pupil versus convergence ( *2 ) speed of pupillary contraction & of accommodation in birds The symbols mean the following: *1 = used lenses; *2 = used prisms; *3 = used more complex optical methods; the underlined was deemed the essential factor, supposedly eliciting pupi_llary contraction; (A) = cited after Alpern, 1969. E OM= extraocular muscle reactions; C G = ciliary ganglion. / 297 298 / I. Anatomy and Physiology Table 5-1 (continued) YEAR AUTHOR 1898 1898 Hess & He ine Ovi o 1900 1900 1903 1903 1903 Hes s Vervoort Fri ber ger Hess Marina 1904 Mach 1905 1905 190 7 1908 1908 1909 1909 19 12 1912 1912 1913 19 12 Mo de r ow Wlotzka Weidlich Kanngies s e r Lohmann Magitot SU!rc ke Hesse Isak.owitz Karplus & Kreidl Terrien & Hillian Pick Sattler Caspary & Goeritz Behr 1920 1920 1922 1924 1925 1926 1928 1928 1929 1930-} 1931 1930 1930 1931 1931 1931 1931 1933 1936 1936 1937 193 7 1937 CHI EF SUBJECT OF INT EREST extent of accommodation in dog , cat , rabbit, and« man r e l ations between pupil, accommodation, and convergence { pupilla r ,y: reactions dependen on distance to target) wor k on ac commodation (n . r . ) pupil, accommodation, convergence, and fusion { *3) speed o fpupil movementwith accommodation &with convergence accommodation ancl pupil ( n . r . ) EOM transpl antations in monkey : exchanged lat. & medial recti and then tested convergence and lateral gaze clinical cas es : accommodative or convergence palsy, or blind ness in a single eye {tested pupil reactions) r eactions of the pueil 2 accommodation & convergence (literature) relations of pu:eil , accommodation and convergence rel ations of pupil and accommodation ,eupil vs. accommodation: effects of atro,eine { *I) p u pil , accommodation , and convergence (clinical) embryologic development of :12upillary reflexes & of accommod . "convergence" reflex of the :eupils {clinical) pupils and accommodation in cases with EOM pals,y: _ convergence im,eutse vs • convergence movement dissociation of light reflexes and near-vision reactions in monkey Paton & Mann IIoIIada,Y Kesten baum & Eidelberg Rakonitz Goer itz child with good accommodation, absent pupillary reflexes to near 2 and absent convergence pupil, accommodation , convergence 2 and fusion pupillary contraction and convergence in high myo_eia relations of pupillary constriction , accommodation, fusion, and convergence excellent discuss ion of pupillary near vision reactions vs • accommodation and convergence { literature) development of 3rd nerve nucleus and of iris sphincter ,eu_ei llary effects of forced conver gence ( * ) pupillary contraction , accommod ati on , and convergence {clinical , *l, *2) the pupils can contract with ac c ommodation or with convergence pupil &accommodation ; (s ynergi stic function? (dissertation, nr) Gual di pupil , accommodation , and c o nvergence ( *1, *2) Ikezawa Lampis Dupuys - Dutemps Kyriel eis Ronchi Sucfa Gualdi Rapis a r da Schubert &Burian Fr,Y Haessle r S c huber t near vision vs. :eu:eils { related to meter angles ) pupil , accommodation and conve r gence {clinical) pupil , ac c ommodation , and convergence pupils in convergence ,eals y after ence:ehalitis pupils , acco mmodati on 2 and conver gence r elation of pu pils to convergence ,eu:eils 2 accommo d ation 2 and convergenc e effect of fatigue on pu pils 2 accommodation and IOP ,eu:eils wit h fu s ional movements 2 withou t accommod or converg. relations betwee n acco mmodation and convergence ( *3) pu pill ary cont racti on to near in {almos t ) darkness "fusion reflex" - see Schubert & Burian , 1936 5. The R eaction to Near Vision Table 5-1 YEAR (continued) AUTHOR 1938 1943 Labadze Bender & Weinstein 1945 Fry 1947 Renard 1949 Knoll 1949} Marg& 1950 Morgan 1950 1951 1953 1954 1954 1955 1955 1955 1956 Marg & Morgan Renard & Massonet-Nau.x Schober Marg & al. Warwick Burian & Allen Schubert {A) Warwick Lowenstein 1956} Sigg & Schneider 1956 Moore, Sigg & Schneider 1956-} Shakhnovich 1961 1957 Bornschein & Schubert {A) 1957 Ha;y:es 1957 Rohen 1957 Stegemann 1958 Akimoto & al . 1958 A112ern & al. {A} 1958 Jacobson & al. {A} 1958 Sabin & Ogle 1959 Cam2bell & al . 1959} 1960 Jampel Mawas 1959 1960 Campbell & Westheimer 1960 Otsuka Alpern, Mason & 1961 Jardinico 1961 RiEES & al. Samoj loff & al . 1961 1962 Hagihara & _Ishikawa 1962 Ripps 8i al . 1962 RiJ2ES &al. Stark & al. 1962 BonvaUet & 1963 Zbrozyna CHIEF SUBJECT OF I TEREST pupil, accommodation, and convergence (central influenc~ ) isolated pupll constriction, accommodation, and contraction of the internal rectus by el. stim. of monke;y' s 3rd n. nucleus pupil size vs. convergence and accommodation ( *3) relation of eueil z accommodation, & converP"ence { *_ l, *2} pupil, accommodation, accomm. vergence , and ~s1.on ( *3) pupillary diameter, accommodation, accommodative v~rgence, pupils, accommodation, accommodative vergence, fus1onal con vergence and awareness of nearness {*3} EUEil size related to fusion , indeeendent of accomm & converg. relations between pupil, accommodation , and convergence {*l, *2} relations between pupil, accommodation , and convergence ( *3) accommodation vs. 12arameters of eI. stim . to c1Iiary g. (cat , ;i:13) detailed anatom;y of ocular parasympathetic innervation mechanical changes in iris during accommodation { gonioscopy) action potentials of ciliar;y: muscle in man anatomy of the so -called nucleus of convergence { Per lia} isolated pupil constriction, accommodation , & adduction of the globe upon el. stimulation of the 3rd nerve ( monkey) transmission of electric stimuli of various frequencies through the ciliary ganglion and the superior cervical ganglion of cats (and effects of drugs) simultaneous recording of pupil constriction and of convergence in man; analysis of tem:eoral relation 0 action potentials of the ciliary muscle in man relations of 12upil , accommodation , and forced lidclos ure detailed anatomic study of ciliar;y bod,Y cybernetics on the near response pupilloconstrictor path not related to light (rabbit) action potentials of the ciliary muscle in man ( • 3) action potentials of the ciliary muscle in man effects of miotics &mydriatics on accommodation-convergence {*3} unrest of eueil and of accommodation { *3 ) cortical representation of the near response in the macaque , and and connections to the midbrain (see Figure 5 - 6) innervation ot the ciliary muscle , and speed of contraction (man) latent periods and speed of contraction for accommod ation and for the pupil (man; *3) rabbit's pupillary reaction to postural change relation between changes in pupil size , accommodation , and accommodative vergence {*3; see Figure 5 - 3 } accommodation to electric stim . at various frequencies {cat} pupillary contraction vs. convergence in man ( drugs ) action ootentials of the ciliary muscle in man ( *3) effect of pupil size on accommodation , convergence, & AC/A r atio functional organization of cat ciliary muscle {electric stim . 1 cybernetic study of dynamics of the lens system midbrain integration of pupillary & of accommodative discharges to the short ciliary nerves of cats ( stim. and recor ding) See als o th e rel a tions bet Vveen pupil constr1ct10n and accommodation described 1 n Chapter 3 ( especially Tables 3-4 5 through 2 -50) , and the clinical Chapters of this book ( Chapters 19 and 24_) • / 299 300 / I. Anatomy and Physiology Table 5-1 YEAR (continued) AUTHOR CIIlEF SUBJECT OF INTEREST 1963 1964 1965 1964 Isaki Baacl<er & Ogle Burian Elul & Marchiafava 1964 1965 1967 Pitts Fenton & Hunter Voloshin & Bonvallet T5rnqvist Jampel & Mindel Pitts 1967 1968 Wilcek Armaly 1968} 1969 1968 1968 Kayamvra. accommodation, depth of anterior chamber, and convergence Morgan O'Neill & Stark accommodative vergence and pu:eil simultaneous records of pupillary changes, accommodation, and convergence {see Figure 5 -9} separate groups of discharges for accommodation and for pupillary constriction via the short ciliary nerves of cats effect of dim light on the pupillary near response ( accommodation and :eu:eils are se:earate I inde2endent co-movements} influence of amygdaloid stimulation (and central connections) u:eon cortical and accommodative discharges influence of age and illumination on pupillary near reactions anatomy of the ciliary ganf?ilion after evisceration of the globe iris bulging during accommodation in cats (photos) ; and relation of iris to anterior lens surface pupillary near response (n.ro) relation between pupillary movements and accommodation in normal human subjects and in patients {*2 simultaneous recording of pupillary changes and of accommodation by electronic means ( *3) miotic, fixed pupils and/or defective convergence or divergence due to stereotactic midbrain lesions {pretectal region} dynamics of :eu:eil & of accommodation, recorded simultaneously vestibular influence on accommodation in cats intraocular effects of accommodation (radiographic study, *3) transmission of stimuli of various frequencies through the ciliary ganglion, and brain stimulation in monkeys (see Cho 3) timing of accommodation and accommodative vergence {*3} :ehylogenetic development of accommodation in vertebrates effect of 2u2il size on accommodative amelitude {*3 2 relation of refraction and 2u2il size in rats pupil, accommodation, and vision {normal & clinical; *3) pupillary near reaction 1966 1966 1967 1969 1969 1970 Bonvallet & Voloshin Roth Gary Bobo & Bonvallet Sch11fer & Weale Tobari 1970 1971 1972} Hughes 1973 1972 Krueger 1972 Narasaki & Shimizu 1972 Shimizu & Narasaki 1972 Nashold & Seaber 1972 van der Wildt 1973 Markham & al. 1973 Suzuki 1973 Westheimer & Blair 1973 Wilson 1976 Gillum 1976 Hennessy & al. 1977 Hughes 1977 Yoshimura 1978 Imai & Ishida rabbit's Eu:eillary contraction to Eostural change pupillary reactions to fusional convergence { *3; see Figure 5 -5} letter to the Editor in reference to Backer & Ogle's Qa~r recording of accommodative changes in cats to visual targets that elicited attention, and reactions to near and far vision (*3) changes in refraction elicited in cats by stimulation of the CG congenital or long-standing 3rd nerve palsy: degeneration in the :eu:eillary s:ehincter and ciliary muscle of man supranuclear control of pupillary and of accommodative discharges travelling via the short ciliary nerves of cats effect of carbachol on the :eu:eil and refraction { monkey} accommodative changes (forward bulging of the lens) elicited by electric stimulation of the 3rd no nucleus of the macaque influence of narcotics and of stimulus voltage on effects of 3rd nerve stimulation measurement of pupillary reactions, accommodo, & convergence degeneration of iris sphincter and of ciliary muscle atter removal of the ciliary ganglion of monkeys ADDITIONS 1984 1984 Banda Banda 1985 Clarke & al. 1985 Lovasik & Szymkiw Ward & Charman 1985 neuronal control of accommodation and pupillarv constriction (cat) overlapping or closely placed cortical areas yielding accommodation and :eupillary constriction in cats anatomic & electrophysiolog1c work on oculomotor subnuclei related to pupillary constriction and accommodation in the marmoset effect of pupil s ize, accommodation, and other factors on stereops is in adult human subiects effect of pupil size on steady-state accommodation 5. The Reaction to Near Vision In conscious cats an accommodative shift can be een in oblique light by movements of a slight kink vi ible on the anterior iris surface (Figure 5-1). The central portion of the iris that rides on the anterior len surface i bent forward; and thi tilted portion become larger when the animal looks near because the anterior len surface pushes forward and become more convex (Hughes, 1973; see Figure 5-2). The cat's pupils do not always become smaller when accommodation increa es because cats tend to innervate their ciliary mu cle when exposed to stimuli that cause visual attention and cortical arousal. Since these animals are often u ed in experiments, it is important to keep this in mind. In a fascinating eries of experiments EluJ and Marchiafava (1964) recorded cortical di charge and accommodation simultaneously in con ciou cats. In some experiments, sustained near-vision shifts of accommodation coincided with pupillary constriction. Such responses always were elicited by a nearby visual target: as the cats followed an approaching object with their eyes, accommodation and miosis increased; and a the object receded, they decreased. The e reaction appeared related to the animals' attention to the visual stimuli: they were more extensive for some objects than for others. The most effective one-a dusting brush with feathers-also evoked the mo t marked cortical arousal. In other experiments, "general orienting reactions" developed when an object was unexpectedly presented or removed from the cats' visual field: increased accommodation was combined with cortical arou al and pupillary dilation. These reactions were non pecific. They occurred regardless of whether the visual target was nearby or far away; and they were elicited also by nonvisual stimuli uch a touch, smells, or sounds. They were hart-lasting, and when the same object was repeatedly introduced and then removed, they faded away. These facts fit well with tho e demonstrated by Bonvallet and her group (Bonvallet, 1967; Bonvallet and Voloshin, 1969; Gary Bobo and Bonvallet, 1970; Hugelin, 1970), who recorded increased accommodative di charges together with cortical arousal and decreased pupilloconstrictor impulse in cat under the influence of sensory stimulation or electric stimulation of certain brain structures (see Chapter 9). 2 Pupillary constriction therefore is not nece arily Jinked to increa ed accommodation. The two function are independent and may even work in oppo ite directions. They work together during normal near vi ion when they are set in motion by a common cortical influence ( ee below), but this proce s may be overridden by other factors, a just mentioned. Further, pupil- / 301 lary contractions to near vision can ?e re?uced or blocked by interfering sensory or emotional mpu_t, e~actly a the light reflex may be blocked by these stu::~mh. The pupils may therefore dilate even tho~gh _an arnmal doe follow an approaching visual target with its eyes. For dogs, similarly, " paradoxical" pupillary dilation during near vision was descnbed _by von Graefe, who reported a finding of Muller's m 1854. The experiment was repeated b_y Trautvette:r (1866); and it has since been ~ent10ned sporadically by others that in dogs-m contrast_ t? other mammals-the pupil dilates upon near v1s1on and contracts upon far vision. But the "visual targets" presented to the dogs were pieces of meat or sau age. Trautvetter described graphically how a Figure 5-1. Diagram of a cat's eyes, looking far (A) a nd near (B) . The arrow show the direction of the illuminating light. Note the shadow produced by the central part of the iris that is tilted Coward by the large lens behind it. As the lens moves forward and becomes more convex during near vision, the tilted iris portion becomes larger (B). -=- stroma sp ,;,11,. .. ..··•· lens ......-··' posterior pigment epithelium clllary muscles 2. As de cribed below, such firing of parasympathetic impulses to the ciliary mu cle, causing it contraction, bring on relaxation of the zonular fiber and thu a pha ic hift toward near vision. Figure 5-2. . Cro s- ection through the iris a nd lens of a dog with a lar~e pupil. ote the forward kink of the iris due to the forwardpu hmg len · (Modified from E. Purtscher Z. mikrosk.-anat Forch., 65 [1959):477) ' .