The splenium is part of the corpus callosum, a structure of the brain that enables communication between the brain's two hemispheres. Its underside connects with a brain structure called the fornix, which a fibrous and arch-shaped formation of the brain. Overall, the splenium is a folded structure with a thick rounded shape, and is the thickest part of the corpus callosum.
The corpus callosum is band of nerve fibers dividing the cerebrum's two hemispheres. The cerebrum is the largest part of the brain, and it covers most of the brain's inner structures. The corpus callosum allows the two hemispheres of the cerebrum to communicate.
The splenium is the rear, or posterior, part of the corpus callosum. It is thick and convex in shape, and overlaps the part of the third ventricle called the tela choroidea, and the mid-brain, which is the part of the central nervous system associated with hearing, motor control, vision, body temperature regulation, and the sleeping and waking cycle. The tela choroidea is also a part of the pia mater, which is a layer of meningeal tissue that covers the brain.
The brain's ventricles, or the ventricular system, are a system of fluid-filled spaces in the brain. The fluid in them is called cerebrospinal fluid and also cerebral spinal fluid. There are four ventricles in total: the two lateral ventricles, which are found on either side of the brain, and the third and fourth ventricles.
The third ventricle, which is located in the center of the brain, connects with both lateral ventricles. The fourth ventricle is in the back of the brain and connects to the spinal cord. The third and fourth ventricles are connected by a passageway called the aqueduct of Silvius, or the Sylvian aqueduct, and also the cerebral aqueduct.
The underside of the splenium connects to the fornix, which is an arch-shaped bundle of white fiber that communicates with the hippocampus. In front of the splenium is the middle portion of the corpus callosum. In the front, or the anterior, portion of the corpus callosum is the section called the genu.
The top part of the splenium is covered by the cerebrum. The bottom of the splenium, behind and below the part that connects to the fornix, sits the cerebellum. These two structures are free of each other. The cerebellum is not covered by the cerebrum, and is known to play a part in emotional and cognitive functions as well motor control.