ANATOMY OF THE MEDIAL (MIDSAGITTAL) SURFACE OF THE BRAIN IN SITU
The entire extent of the neuraxis, from the spinomedullary junction through the brain stem, diencephalon, and telencephalon, is visible in a midsagittal section. The corpus callosum, a major commissural fiber bundle interconnecting the two hemispheres, is a landmark separating the cerebral cortex above from the thalamus, fornix, and subcortical forebrain below.
The ventricular system, including the interventricular foramen (of Munro); the third ventricle (diencephalon); the cerebral aqueduct (midbrain); and the fourth ventricle (pons and medulla), is visible in a midsagittal view. This subarachnoid fluid system provides internal (the ventricular system) and external (cerebrospinal fluid in the subarachnoid space) protection to the brain and also may serve as a fluid transport system for important regulatory molecules. The thalamus serves as a gateway to the cortex. The hypothalamic proximity to the median eminence (tuber cinereum) and the pituitary gland reflects the important role of the hypothalamus in regulating neuroendocrine function. A midsagittal view also reveals the midbrain colliculi, sometimes called the visual (superior) and auditory (inferior) tecta. See Video 3-1.
The right and left hemispheres are interconnected by commissural fiber bundles. The largest is the corpus callosum, which interconnects all lobes with their counterparts. The anterior commissure interconnects regions of the temporal lobes. When these commissural fiber bundles are disconnected (split brain), the hemispheres do not know what their counterparts are doing, and inputs to one hemisphere cannot produce an appropriate response from the opposite hemi- sphere. With a split brain, only a more generalized recognition of mood states occurs between the two hemispheres, presumably communicated through interconnections between lower structures, such as the diencephalon and brain stem.