Basal part of rhinencephalon

The basal part of the rhinencephalon occupies the base of the cerebral hemisphere. Its lateral boundary is marked by the lateral rhinal sulcus, and its medial boundary by the medial rhinal sulcus. Its rostral part, which is the only truly olfactory region, includes the olfactory bulb, olfactory peduncle, lateral and medial olfactory tracts and olfactory tubercle. Its caudal part contains the piriform lobe.

The basal part of the rhinencephalon shows considerable variation across different mammalian orders, which can be classified into three groups. Carnivores, ungulates, glires, and rodents are considered macrosmatic species, with a highly developed olfactory region. Primates (including humans), pinnipeds, and most cetaceans are microsmatic, while delphinids are anosmatic, as these structures are completely regressed and have lost their function.