Lunate sulcus

At a glance

English

Lunate sulcus

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the lunate sulcus

The lunate sulcus (sulcus lunatus, from the Latin for moon-shaped) is a curved furrow on the outer surface of the occipital lobe, near the occipital pole. It is concave towards the back, like a crescent, and runs roughly up and down across the lateral occipital surface. In monkeys and apes it is a large, constant fissure, sometimes called the simian sulcus or by its German name, Affenspalte. In human brains it is inconsistent: it may be well marked, small and broken, or absent altogether.

In apes and monkeys the sulcus sits at the edge of the striate cortex (V1), which spreads widely over the lateral surface of their occipital lobe, and the lunate sulcus marks its junction with the next visual area, V2. In humans the primary visual cortex has been pushed back and inwards, so that most of it lies on the medial surface in and around the calcarine sulcus, with only a small part reaching the lateral surface at the pole. Where a lunate sulcus is present in a human brain, it lies further back than in a chimpanzee and close to the pole.

That position has made the sulcus important in the study of human evolution. Early in the twentieth century the anatomist Grafton Elliot Smith proposed that the human furrow was the homologue of the ape fissure and that its backward shift reflected the great growth of the parietal, temporal and occipital association cortex in front of it. Because sulci press faint marks into the inside of the skull, researchers have looked for the lunate sulcus on endocasts of fossil hominins, such as australopithecines, to judge whether their brains had begun this reorganisation; the reading of these casts remains disputed. Studies comparing apes suggest that the position of the sulcus predicts the size of V1, and that the human V1 is smaller than expected for our brain size.

Not everyone accepts that a true lunate sulcus exists in humans; a high-resolution MRI study of a large group of human brains failed to find a consistent homologue. For students it is mostly a reminder that the lateral occipital lobe is variable and that the primary visual cortex lies mainly on the medial surface.

Function and supply

Function

A variable landmark on the lateral occipital lobe; where present it lies near the junction of the primary and secondary visual cortex (V1 and V2).

Blood supply

Branches of the posterior cerebral artery at the occipital pole, overlapping with terminal branches of the middle cerebral artery.