The perineurium is a protective sheath that surrounds bundles of nerve fibers, known as fascicles, within a nerve. It is composed of several layers of flattened cells that create a barrier, helping to maintain the internal environment of the nerve and protect it from external factors. This structure plays a crucial role in the overall function of the nervous system. In addition to providing protection, the perineurium also contributes to the nerve's ability to transmit signals efficiently. It helps to regulate the exchange of substances between the nerve fibers and surrounding tissues, ensuring that the nerve can function properly. This makes the perineurium an essential component of the peripheral nervous system.