Plates 4, 5] As the result of detailed anatomical study, the ventral nucleus of the mammalian thalamus has been shown to be a composite structure, con sisting of several nuclear elements which are to be distinguished by their cell and fibre architecture, and also by their fibre connexions. It has been demonstrated that medial fillet fibres (as well as ascending fibres from the spinal nucleus of the trigeminal nerve) terminate in the pars externa of the ventral nucleus (Vogt 1909; Clark 19366;

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... lker 1937), and that this element projects on to the gyrus postcentralis of the cortex. Brachium conjunctivum fibres, on the other hand, terminate in the rostral part of the ventral nucleus (extending forwards from approximately the plane of the anterior margin of the centre median nucleus) which, in turn, is connected by thalamo-cortical fibres with the motor cortex of the precentral gyrus (Clark and Boggon 1935). It appears, therefore, that, although the motor cortex certainly receives afferent fibres from the thalamus, these have not the same functional implications as the afferent fibres which reach the postcentral gyrus. Of all the elements of the ventral nucleus of the thalamus, the best de fined in higher mammals is the pars arcuata or arcuate nucleus (semilunar nucleus of Fleschig, nucleus vb of von Monakow and Friedemann, nucleus ventralis postero-medialis of other authors). The contour of this element can readily be determined by a naked-eye inspection of the freshly cut human or macaque brain. It is sharply circumscribed by a thin medullary capsule, it is darker in colour than the adjacent elements of the ventral nucleus, and it occupies a position in the postero-medial part of the thalamus immediately ventral to the centre median nucleus. The cells of the arcuate nucleus stain rather more deeply with methylene blue than those of the pars externa of the ventral nucleus, and they tend to be arranged in 1 166 ]