The diencephalon consists of the third ventricle and the structures that define its rostral, caudal, superior, and inferior boundaries. It is situated in the midline of the brain, and most of its components are bilateral and symmetrically arranged, with free communication between the two sides of a given diencephalic structure.
The telencephalon consists of the cerebral hemispheres. These are two bilaterally and symmetrically arranged structures separated by a sagittal midline fissure, and are connected across their midline by the commissural fibers of the corpus callosum.
The structures of the diencephalon are dealt with in more detail in later sections; the components of the diencephalon can be summarized as consisting of the third ventricle, and the major structures surrounding it, namely the thalamus, subthalamus, epithalamus, and the hypothalamus. Within each of these structures are nuclei, pathways and subsidiary structures which are considered in more detail later. The thalamus is a complex, highly organized and compartmentalized relay station for ascending tracts, situated centrally in the cerebrum, and plays an important part in the integration of somatic and visceral function. The hypothalamus forms the floor and part of the lateral walls of the third ventricle, and plays a critical role in endocrine, metabolic, autonomic, and emotional function. The subthalamus, which lies immediately below the thalamus, is concerned with the modulation of involuntary movement, and is considered to be one of the extrapyramidal motor nuclei. The epithalamus consists of the pineal gland and thehabenular nuclei, which play a part in the integration of somatic and olfactory information.
Each cerebral hemispherce of the telencephalon has a highly convoluted and folded surface covering of gray matter, the cerebral cortex, and inner core of white matter consisting of fiber tracts. Deep with the hemispheres are masses of gray matter, the basal nuclei (also called basal ganglia) and the lateral ventricles. The infoldings of the surface greatly increase the surface area of the cortex; these folds are termed gyri (singular, gyrus), separated from each other by fissures called sulci.
The basal nuclei occur bilaterally and symmetrically in the hemispheres, and consist of the amygdaloid nucleus, situated in the temporal lobe, the claustrum and corpus striatum, which lies lateral to the thalamus. The corpus striatum is split by the internal capsule, a band of nerve fibers, into the caudate nucleus and lentiform nucleus. These nuclei are further subdivided by nerve fiber sheets into other nuclei.