SUMMARY OF THE DIENCEPHALON.
The diencephalon or inter-brain is subdivided into the thalamencephalon and the pars mamillaris hypothalarni.
A. The thalamencephalon includes : The thalamus, The epithalamus, The metathalmus.
To the epithalamus belong : The corpus pineale, The regio habenulae—trigonum habenulae, commissura habenularum, The commissura posterior.
To the metathalamus belong : The corpora geniculata.
B. The pars mamillaris hypothalami includes the corpora mamillaria.
The thalamus consists of three chief nuclei : Nucleus anterior or dorsalis, Nucleus medialis ( + centrum medianum), Nucleus lateralis ( + nucleus semilunaris).
The lateral boundary of the thalamus is formed by the lamina medullaris externa and the stratum reticulare. Medially the thalamus is covered by the central gray substance, which likewise clothes the medial surface of the hypothalamus and forms the massa intermedia.
Within the trigonum habenulae lies the nucleus or ganglion habenulae.
The corpora geniculata contain the nucleus corporis geniculati medialis and lateralis. Within the hypothalamus, as special centres, are found the nuclei of the corpora mamillaria and the nucleus hypothalamicus, or body of Luys.
The capsula intern lies between the nucleus lenticularis, on the one side, and the nucleus caudatus and the thalamus, on the other. It consists of an anterior limb, pars frontalis or pars lenticulo-caudata, a posterior limb, pars occipitalis or pars lenticulo thalamica, with the pars retrolenticularis, and the genu capsulae internae. In horizontal
sections through the hypothalamic region the pars frontalis is wanting.
The diencephalon encloses the third ventricle which communicates with the lateral ventricles by means of the foramen interventriculare, and with the fourth ventricle through the aquaeductus cerebri.
The boundaries of the third ventricle are as follows :— Anterior wall: Lamina terminalis, Commissura anterior, Columnae fornicis.
Posterior wall: Commissura habenularum, Corpus pineale, Commissura cerebri posterior.
Lateral walls : Medial surfaces of the thalami and the hypothalami.
. Floor : Cerebral peduncles, Substantia perforata posterior, Corpora mamillaria, Tuber cinereum, with infundibulum and hypophysis, Chiasma opticum.
Roof : Lamina chorioidea epithelialis ; secondarily, tela chorioidea ventriculi tertii, fornix and corpus callosum.
The diencephalon, together with the telencephalon, constitutes the prosencephalon or the fore-brain. The pars optics hypothalami and the pars mamillaris hypothalami together form the hypothalamus.
The foregoing relations are presented in recapitulation in the following table :