Mid-line thalamic nuclei in the dog.

Within the massa intermedia, which links the two halves of the thalamus dorsalis, lie several nuclei which have been given the name of mid-line thalamic nuclei. The area occupied by these nuclei is fairly large, and stretches from the parataenial nucleus level orally, as far as the plane which vertically cuts the n. interventralis caudally. Nuclei of this group are linked to nuclei of the same name of the anterior thalamic group: n. interanterodorsalis, n. interanteromedialis, n. para-taenialis; both n. dorso-mediales, and beside this both laminae medullares interne. Dorsally and latero-dorsally this group of nuclei lie next to the third ventricle, the stria medullaris thalami and the n. parataenialis, and in the caudal section to the habenular nuclei. Laterally lie n. antero-medialis, n. reticularis and n. ventralis thalami, as well as peduncullis thalami inferior. Next, caudally, n. dorsalis medialis, n. paracentl-alis, n. submedius, n. ventralis thalami, and lamina medullaris externa. On the ventral side this group of nuclei is bordered by the hypothalamus and the third ventricle. The nuclei under discussion are arranged in a definite order which is found among the lower and higher mammals, and also in the dog (Fig. 1A). N. paraventricularis, lying directly under the ependymal layer of the third ventricle, is the most dorsally situated nucleus of this group. Moving downwards into the massa intermedia ventrally, we come successively to n. interparataenialis, n. interantero-dorsalis, and n. interanteromedialis, which lies next to the ventrally and caudally situated n. centralis medialis and n. rhomboidalis. The most ventrally situated nucleus of this group is n. reuniens, which for the whole of its length borders on the dorsally lying n. centralis medialis, and on the ventral side with the hypothalamus.