THIRTEEN FIGURESAlthough the di-and mesencephalon have been the subject of numerous investigations dealing with the anatomy as well as the physiology of these parts of the brain, many points have remained obscure, and among these may be mentioned the delimitation and functional significance of the area of transition between thalamus and tectum opticum, the pretectal region and posterior commissure of this study. I n particular for the pretectal region, configuration and homologies vary from report to report. The present paper will consider as pretectal region the area of transition between thalamus and tectum opticum, lying immediately lateral to the nucleus of the posterior commissure (Brown, '43), on the inward side of the large-celled nucleus of the optic tract, medial to the nucleus suprageniculatus of the thalamus, fusing caudally with the superior colliculus, and limited ventrally by pretecto-mesencephalic fibers running parallel with or penetrating the medial elements of the trigeminal and medial fillets. I n the cat a distinct nucleus limitans separates the thalamus and pretectal region. Our delimitation corresponds well with the pretectal group as described by Rose ( '42) for the sheep and cat. We agree with this author that sagittal and horizontal sections show up the area much more strikingly than frontal 139
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