Collateral sulcus

The collateral sulcus can be seen on the lower surface of the cerebrum, near the back of the brain. It stretches forward from near the occipital pole, but doesn’t reach the frontmost part (i.e. the temporal pole). The collateral sulcus turns into the rhinal sulcus as it continues forward.

At the back, the collateral sulcus lies below and to the lateral side of the calcarine sulcus, with the lingual gyrus separating them. Towards the front, it is situated between the parahippocampal gyrus on the inside and the medial occipitotemporal gyrus on the outside. As we move further forward, the collateral sulcus becomes the rhinal sulcus, which further separates the front part of the parahippocampal gyrus (i.e. uncus) from the medial occipitotemporal gyrus.

There might be some confusion due to different terms used in the literature. For example, the occipitotemporal gyrus is sometimes referred to as the fusiform gyrus. Similarly, the terms occipitotemporal sulcus and mid-fusiform sulcus are used interchangeably. The mid-fusiform sulcus divides the fusiform gyrus into medial and lateral parts. Therefore, the medial and lateral fusiform gyri are alternative names for the medial and lateral occipitotemporal gyri. In other words, we can also say that the collateral and rhinal sulci separate the front part of the parahippocampal gyrus from the medial fusiform gyrus.