What structure separates the two hemispheres of the brain?

The structure that separates the two hemispheres of the brain is the falx cerebri. This is a sickle-shaped fold of the dura mater, the tough protective layer that envelopes the brain. The falx cerebri extends from the crest of the brain down into the longitudinal fissure, providing a partition between the left and right cerebral hemispheres. Its primary function is to help stabilize the position of the brain within the cranial cavity, preventing excessive movement that could lead to injury.

The other structures mentioned, such as the corpus callosum, thalamus, and brainstem, serve different roles within the brain. The corpus callosum, for instance, is a large bundle of nerve fibers that connects the two hemispheres and facilitates communication between them, but it does not separate them. The thalamus acts as a relay station for sensory and motor signals, and the brainstem is responsible for basic life functions such as breathing and heart rate. Therefore, while these structures are vital to brain function, they do not perform the role of separating the hemispheres like the falx cerebri does.