Chapter 6: after Galen Late Antiquity and the Islamic world.
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It is usually assumed that after Galen there was nothing new until the Renaissance. Contrary to this view, there were significant modifications of the inherited legacy in Late Antiquity, followed by fundamental changes within the Arabic/Islamic world. Their formative influence extends from the medieval period of transmission to the Renaissance and the 17th century. The increasing emphasis on the primacy of the brain initiated the beginnings of ventricular localization of function in Late Antiquity, which was subsequently developed into a theory and transmitted to the West via Arabic. Following the unprecedented translation movement in 9th-century Baghdad, the cumulative Greek and Hellenistic knowledge of the brain, nerves, and the senses from diverse sources were brought together in the systematic, logically unified Arabic medical compendia of encyclopedic proportions, which embody divergence from accepted views and new diagnostic observations. Their Latin versions became standard texts in medical schools. The oldest extant schematic diagrams relevant to neurology (the eye, the ventricles, the visual system, and the nerves) date from this period, and served as models for the medieval Latin West. The development of coherent descriptions of the motor and sensory systems, and related clinical disorders, by analogy with the mechanisms of hydraulic automata, foreshadows some of the explanatory methods associated with the 17th century. Furthermore, an entirely new approach resulted in a paradigm shift in theory and methodology through the experimental studies on the physics of light and vision of Ibn al-Haytham (d. 1040), who showed that what is sensed is not the object itself, but a punctate optical "image" due to light reflected from its surface to the eye. This revolutionary approach to vision destroyed the viability of the Greek tradition of holistic forms and tactile sensory impressions. Ibn al-Haytham's theory of point-to-point correspondence formed the basis of subsequent attempts to find a topological representation of external objects in the eye (the retinal surface with Kepler) and in the brain (the pineal gland with Descartes and ultimately in the cortex with Munk). His serial re-projection of the punctate image in the eye, chiasma, and the brain can be regarded as dramatic precursors of the principles in neurology of hierarchical anatomical organization of sensory information from simple to complex. Due to Ibn al-Haytham, an understanding of vision increasingly required a synthesis of anatomy with the physics of light. Subsequently the visual enquiry shifted from the global question of "how do we perceive the external world by the sense of sight?" to specific concerns arising from the implications of Ibn al-Haytham's "optical" image in the eye: (a) the preservation of a point-to-point correspondence between object and image; (b) image inversion and the veridical (upright) perception of the object; (c) the unity of perception, or the binocular fusion of the two separate images, one from each eye; and (d) the anatomical projection of the retinal maps to various brain centers. The fact that these became central issues extending to Descartes and beyond underscores the unique legacy in neurology of Ibn al-Haytham's conception of a punctate sensory map, which was re-projected to the brain for complex sensory functions.
