The projection of the midline and intralaminar nuclei of the thalamus of the rabbit.

Electrophysiological studies have indicated that the nuclei of the midline and internal medullary lamina of the thalamus together constitute a functionally distinct system which may be of considerable importance in the maintenance and control of normal cerebral activity. Since most of the physiological observations on this system have recently been collated in the publication of the symposium on "Brain Mechanisms and Consciousness" (Council for International Organizations of Medical Sciences, 1954) a detailed review of the literature will not be given here. Suffice it to say that whereas each of the principal thalamic nuclei is related to a localized cortical area, stimulation studies have shown that the midline and intralaminar nuclei are capable of exerting widespread effects on the activity of the cerebral cortex. The normal pathways by which these effects are mediated are still obscure since it has long been known that these nuclei have no direct connexion with the neopallial cortex as they do niot undergo retrograde cell degeneration even after decortication. That their projection is extrathalamic is, however, well established (Rose and Woolsey, 1943, 1949) and there is some evidence to suggest that they may be related to the corpus striatum (Stefens and Fortuyn, 1953 ; Powell and Cowan, 1954). In view of the important functional significance attributed to these nuclei more precise information concerning their efferent connexions is desirable. In the present work the projection of the individual elements of this group has been studied in the thalamus of the rabbit using the technique of retrograde cell degeneration after lesions in the rostral part of the cerebral hemisphere.