He who is theoretic as well as practical is therefore doubly armed: able not only to prove the propriety of his design but equally so to carry it into execution.

This volume is the product of the cooperativeefforts of the two authors. Parts I and II were written by Burian, and Parts III and IV by von Noorden; however, both authors take full responsibility for the complete text.

In this work, our aim is to provide the practicing ophthalmologist as well as the budding one with the theoretic knowledge and practical know-how that will enable him to pursue the field of neuromuscular anomalies of the eyes in the manner set forth in the precept of Vitruvius.

The sound physiologic tradition of Hering, Helmholtz, Donders, Tschermak, Hofmann, and their schools forms the solid ground upon which was built the clinical work of Javal, Worth, Bielschowsky, Duane, Lancaster, and, more recently, Harms, Cüppers, Lyle, Bagolini, ourselves, and many others. Our purpose has been to convey this physiologic basis as concisely and simply as possible, always with the practicing ophthalmologist in mind and wherever possible emphasizing its immediate clinical application. But much has happened in our field since the days of the old masters, and due consideration is given to the exciting and significant modern studies in the psychophysical and neurophysiologic areas as well as in the field of clinical management of strabismus. This volume is not a handbook or a system, however, and is not intended to be systematically or historically complete. We, therefore, have omitted many points that are to be found in reference works. Neither does this book supplant the Atlas of Strabismus by von Noorden and Maumenee, which continues to be a useful guide to the diagnostic aspects of strabismus.

The theoretic foundation has served us as a means to make the strictly clinical chapters both “theoretic and practical,” telling the ophthalmologist not only the “what and how” but also the “why.” We hope that the long hours of labor expended on this volume may be of real usefulness in the study of strabismus, particularly to the younger generation of ophthalmologists.

The two human eyes with their adnexa and nervous system connections form an indivisible entity. This fact must always be kept in mind, but for the purpose of study a distinction between the sensory and the motor systems is necessary.

Light stimuli, having gone through the changes imposed on them by the refractive media, reach the peripheral organ of vision, the retina, and produce physical and chemical alterations in the retinal receptors. In turn, these alterations provoke in the retinal neurons physicochemical and electrical changes that are transmitted as impulses to the central nervous system. Eventually, visual sensations of form, spatial relationships, and color appear in our consciousness. This sequence of events may be called the sensory aspect of the visual process. The events in the sensory part of the visual system also precipitate a chain of responses in the motor system of the eyes, in the central and peripheral nervous arrangements, and in the inner and outer muscles of the eyes.

In this unitary sensorimotor system, the sensory system transmits and elaborates the information received about the outside world. The motor system has no independent significance and is entirely in the service of the sensory system, by which it is largely governed. Understanding of this system is essential for the interpretation of the neuromuscular anomalies of the eyes.

The tasks of the motor system are (1) to enlarge the field of view by transforming the field of vision into the field of fixation, (2) to bring the image of the object of attention onto the fovea and keep it there, and (3) to position the two eyes in such a way that they are properly aligned at all times, thereby ensuring the maintenance of single binocular vision.

In agreement with a time-honored classification, a distinction is made between voluntary and involuntary eye movements. Voluntary simply implies that the movements are “willed” by the individual, presumably as a result of a chain of impulses that originate in the cortex. Involuntary eye movements are not willed by the individual and, indeed, occur without awareness. They are elicited mainly by stimuli arising from outside the body, for example, visual or auditory, or those arising from within the body, for example, vestibular. The former are referred to as exteroceptive, the latter as interoceptive stimuli.

When the illuminance of the retina changes, the pupil of the eye constricts or dilates. When we tilt our head to one shoulder, the eyes make a parallel movement around their anteroposterior axes, so the vertical meridians of the retinas turn in the direction opposite that of the head. The eyes attempt to right themselves. Both these motor reactions are highly useful unconditioned reflexes. The central nervous system structures that mediate these reflexes are subcortical. The individual is not aware they are taking place.

When a light stimulus reaches the retinal periphery, the eye turns and causes the stimulus to impinge on the area of highest resolving power, the fovea. If a binocularly fixated object approaches the eyes, the visual axes converge to maintain fixation. If for some reason the proper alignment of the visual axes has been lost, corrective fusional movements occur and restore binocular fixation. All these movements are highly useful, and most of them are also reflexive, but there is a significant difference between them and reflex movements.

If a person is lost in thought or concentrating on an object of regard, another object approaching from one side may not be noticed—at times with regrettable results. One can voluntarily stop convergence or voluntarily overconverge. In inattentive states, one may fail to make fusional movements. All these movements, then, though basically reflexive, require the cooperation of the cerebral cortex, in particular a state of visual attention. Hofmann and Bielschowsky,⁸ who published their classic study on fusional movements in 1900, clearly noted the reflex nature of these movements, but were also aware that they did not come about without the concurrence of attention. They designated the fusional movements as psycho-optical reflexes. At the time Hofmann and Bielschowsky published their paper, Pavlov had just begun his work on conditioned reflexes, and his findings were not yet published. Today, reflex movements that require cooperation of the cerebral cortex are designated as conditioned reflexes.

In summary, all eye movements, insofar as they are not voluntary, are unconditioned or conditioned reflexes performed in the service of the sensory system of the eyes, specifically in the interest of clear, distinct vision and of binocular fixation.