Crossed Connexion of the Cerebral Hemispheres with the Muscles and Sense Organs

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PROF. ROAF'S interesting speculation published under the above title in NATURE of Feb. 8, (p. 203) is based on the assumption that the two eyes possessed by most vertebrate animals have arisen in the course of evolution from a single median eye such as is found in the free-swimming larva of an Ascidian. He argues (if I understand him rightly) that when the image of an object falls on the left half of the retina of an animal of this type, the appropriate response is a contraction of the muscles of the right side of the creature's body and tail, and that the efferent nerve paths from the brain will therefore be simplified if the afferent fibres involved end in the right half of the central nervous system. Such a view may be held to account for the central projection of the retina of each of the two eyes of a mammal in such a way that fibres from its upper half are connected with the superior lip of the calcarine fissure; and that fibres from its right margin are connected with cerebral points situated to the left of those with which areas of retina lying farther to the left are connected. (This may legitimately be inferred from the work of Gordon Holmes and others on cortical projection in man.) But Prof. Roaf goes further and suggests that it may also account for the fact that in most vertebrates the right eye is directly connected only with the left side of the brain, and the paths from the two eyes undergo complete decussation. At this point the argument seems to me to become less convincing.

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