Which structure is described as resembling a spiderweb in the brain?

The structure that resembles a spiderweb in the brain is the arachnoid. The arachnoid membrane is one of the three layers of the meninges, which are protective coverings that envelop the brain and spinal cord. It is situated between the outer dura mater and the inner pia mater. The term "arachnoid" is derived from the Greek word for spider, reflecting its web-like appearance. This membrane has fine, spiderweb-like trabeculations that connect it to the pia mater, creating a space filled with cerebrospinal fluid that acts as a cushion for the brain.

The other options represent different parts of the brain or its structure: the cerebellum is involved in coordination and balance; the cortex is responsible for higher brain functions such as thought and voluntary movement; and the medulla regulates vital life functions like heart rate and breathing. None of these structures exhibit the characteristic web-like form that defines the arachnoid membrane.