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EVOLUTION AND DISEASE OF THE BRAIN PERCIVAL BAILEY, Ph.D., M.D.* The evolution ofthe brain in vertebrates has been reconstructed in considerable detail. There is no direct evidence because the brain does not become fossilized, but some evidence can be obtained by making casts of the interior ofthe cranium. In this way it has beenpossible for T. Edinger (i) to follow the evolution ofthe brain ofthe horse. Most ofthe reconstruction , however, has been inferred from laborious study of the brains ofsurviving animals believed, for sufficient reasons, to be closely related to ancestral forms. The details have been gathered into such compilations as that ofKappers, Huber, and Crosby (2). We learn from such studies that the brain has retained a fundamental pattern within which increasing complication has taken place, new parts evolving out ofold ones—typically from their central regions—but never entirely replacing them. These alterations in form andrelationship were accompanied by an increasing internal complication of structure. The final result was a bewildering complexity which seemed to defy any attempt to understand the human brain. The hope that the simpler brains of so-called lower vertebrates would help to clarify this puzzling organ provided the principal incentive for all the laborious studies ofcomparative neurology. Many students of the brain were clinicians anxious to understand the symptoms from which their patients suffered. They carefully described the symptoms and then tried to correlate themwith the lesions which were found in the brains ofthe patients after death. When thecomparative anatomists had progressed to a certain extent with their studies, it became apparent that some ofthelesions that thepathologists observedwere related * Director, Illinois State Psychopathic Institute. This essay is the body ofa paper that the author will present at the Darwin Centennial Celebration to be held at and sponsored by the University of Chicago, November 18-24, 1959. 62 Percival Bailey · Evolution and Disease ofthe Brain Perspectives in Biology and Medicine · Autumn 1938 to the evolutionary story which the anatomists were unfolding. We may take as an example the cerebellum. This part ofthe brain has a very complicated structure to which a bewildering host ofnames had been applied, but these subdivisions, having no physiological significance, served no useful purpose, as Flourens long ago pointed out. Its structure did not begin to make sense until the comparative anatomists had analyzed it. They found (3, 4) that it began as two outpocketings, one on each side ofthe fourth ventricle. These are clearly seen in amphibians but are better developed in certain fishes and persist throughout the vertebrate phylum; in the human cerebellum, they are known as the "flocculi." Together with related tracts and nuclei they came to be known as the "archicerebellum ." Later a mid-line structure developed which is called in the human cerebellum the "vermis." This part is gready developed in birds (Brouwer); together with related parts, it came to be called the "paleocerebellum ." In the older literature, the archicerebellum was not distinguished but was included in the paleocerebellum (5). Still later large masses developed on either side of the vermis which are called in the human brain the "hemispheres." With related parts, they are now spoken of in general as the "neocerebellum." Van Valkenburg (6) has shown that, in the development of the human fetus, the paleocerebellum completes its development before the neocerebellum. The pathologists noted that these fundamental evolutionary parts of the cerebellum might be involved separately in disease processes. One part might fail to develop. For example, Jelgersma (7) found a family of cats in which there was marked atrophy of the hemispheres and pons. The vermis, except for its dorsal part, and the flocculi were less affected. The resemblance ofthe cerebellum in these cats to that ofbirds was striking . In 1913 Brouwer (8, 9) studied the cerebellum of a man whose left cerebellar hemisphere was only about one-third the size ofthe right. The atrophywas found to affectnot only thelefthemispherebutalso thehomolateral middle peduncle, the homolateral transverse pontine fibers, the crossed pontine gray cells, the heterolateral lower olive, the homolateral pontine striae, and the homolateral striae arciformes externae—nuclei and tracts which Ingvar (10) had shown to belong to the phylogenetically newer parts ofthe cerebellar complex. Many similar cases ofneocerebellar...


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