Three surrounding layers of brain and spinal cord are known as meninges which mainly serve a protective function. The delicate inner layer is the pia mater. The middle layer is the arachnoid, a web-like structure filled with fluid that cushions the brain. The tough outer layer is called the dura mater. Image via: https://www.instagram.com/p/CMbzbKvBP45/?igshid=121nbmx3yo6a