The term collateral eminence refers to a surface feature of the occipital horn of the lateral ventricle in the human. Defined on the basis of dissection it is a swelling in the lateral wall of the ventricle next to the hippocampal complex. It is continuous with the flatter collateral trigone. It is caused by the invagination of the collateral sulcus. It's size depends on the depth of that sulcus, so it is not always present ( Wikipedia ).