The insula is one of the most important parts of the brian and as mentioned previously, it is sometimes considered, as the fifth lobe of the brain.

The insula has the shape of a pyramid with its apex directed inferiorly and presents an anterior surface and a lateral surface. The anterior surface presents a triangular shape and is constituted by transverse and accessory gyri and the insular pole. The medial portion of the insular pole is marked by an arched ridge of variable prominence, the limen insula. It is composed of fibers of uncinate fascilulus covered by a thin layer of gray matter that extends from the anterior end of the long gyrus and pass through the medial part of the insular pole, and ends at the middle of the posterior orbital gyrus.

Limen means threshold and the Limen insula in the threshold between the carotid cistern medially and the sylvian fissure laterally. The insula is encircled and seperated from the opercula by a deep furrow called the circular or limiting sulcus of the insula which presents 3 parts, the superior, anterior and inferior parts. From the limen insula, the gyri and the sulci of the insula are directed superiorly in a radial manner. The deepest sulcus, the central sulcus of the insula is directed superior and backward in the general line of the central sulcus of the cerebrum. It divides the lateral surface of the insula into an anterior and posterior zone. The anterior zone is divided into several short sulci into 3 to 5 short gyri. The posterior zone is formed by anterior and posterior long gyri. From microsurgical and radiologic viewpoints, the insula presents the external covering of the central core, constituted by the extreme, external and internal capsules, the claustrum, basal ganglia and the thalamus. The anterior, inferior and posterior limits of the insula prosent the anterior, inferior and posterior limits of the central core. The upper limit of the central core (the caudate nucleus), however, is higher than the upper limit of the insula.