Lateral Sulcus (Sylvian Fissure): Anatomy and Borders

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The lateral sulcus, also called the Sylvian fissure, is the deep cleft on the lateral surface of the cerebral hemisphere that separates the temporal lobe below from the frontal and parietal opercula above. Its floor is formed by the insula, and its three named rami (anterior, ascending, and posterior) define the landmarks that neurosurgeons, anatomists, and imaging specialists use to orient themselves on the lateral hemisphere.

This article covers the borders of the lateral sulcus, the parts of the inferior frontal gyrus its rami divide, the opercular relationships above and below the cleft, and how the structure differs across domestic species. The lateral sulcus is one of the most consistent landmarks on the mammalian cerebrum, and it is the single most useful reference point for lateral-surface neuroanatomy in both human and veterinary medicine.

What Is the Lateral Sulcus?

The lateral sulcus is a deep, operculated fissure on the lateral surface of each cerebral hemisphere. It runs roughly horizontally from the anterior temporal region posteriorly and slightly upward, and it is the boundary between the temporal lobe and the overlying frontal and parietal lobes. The term "Sylvian fissure" honors Franciscus Sylvius, and the Latin term sulcus lateralis is the standard anatomical name used in the Nomina Anatomica Veterinaria and human anatomical terminology [1][2].

A key structural feature distinguishes the lateral sulcus from ordinary cortical sulci: its walls are not simple opposing banks of cortex. Instead, the frontal, parietal, and temporal cortices grow over and cover a deeper region called the insula during development. The lateral sulcus is therefore the surface opening of a buried cavity, and the cortex that forms its upper and lower lips is called the operculum (Latin for "lid"). The insula sits at the bottom of this cavity as the floor of the lateral sulcus [3][4][5].

This buried architecture explains why the lateral sulcus appears as a wide, gaping cleft on the brain surface rather than a narrow groove. The gap is the space between the opercular lips, and the insula is visible only when the opercula are retracted or removed.

Borders of the Lateral Sulcus

The borders of the lateral sulcus are defined by the gyri that form its upper and lower lips and by the rami that extend from its anterior end.

Upper Lip (Frontal and Parietal Opercula)

The upper lip of the lateral sulcus is formed by the frontal operculum anteriorly and the parietal operculum posteriorly. The frontal operculum consists of the inferior frontal gyrus and the lower part of the precentral gyrus. The parietal operculum consists of the lower part of the postcentral gyrus and the anterior part of the supramarginal gyrus.

The inferior frontal, precentral, and postcentral gyri cover the anterior, middle, and posterior thirds of the lateral surface of the insula respectively [6]. This three-to-one relationship is a useful memory aid: three upper opercular gyri cover three insular thirds.

Lower Lip (Temporal Operculum)

The lower lip of the lateral sulcus is the temporal operculum, formed by the superior temporal gyrus and its deeper extensions. The temporal operculum lies in direct opposition to the frontal and parietal opercula. The planum polare of the temporal lobe faces the inferior frontal and precentral gyri, Heschl's gyrus (the primary auditory cortex) faces the postcentral gyrus, and the planum temporale faces the supramarginal gyrus [6]. These appositions are not arbitrary. They reflect the developmental folding that brings the temporal lobe up against the frontal and parietal lobes.

Anterior Sylvian Point

The anterior Sylvian point is the most anterior extent of the lateral sulcus. It is located in three-dimensional space between the pars triangularis of the frontal lobe and the pars opercularis of the temporal lobe [7]. This point is where the anterior and ascending rami of the lateral sulcus originate. In a morphometric study of 40 cerebral hemispheres, the anterior Sylvian point was classified into three types based on its distance: type A (less than 5 mm), type B (5.1 to 10 mm), and type C (greater than 10 mm). Type B was the most common, representing 45% of right and 40% of left hemispheres [7].

The Three Rami of the Lateral Sulcus

The lateral sulcus has three named rami at its anterior end: the anterior ramus (also called the anterior horizontal ramus), the ascending ramus (also called the anterior ascending ramus), and the posterior ramus (the main trunk of the fissure). These rami are the key landmarks for identifying the subdivisions of the inferior frontal gyrus.

Anterior Ramus

The anterior ramus extends anteriorly and slightly downward from the anterior Sylvian point. It separates the orbital part of the inferior frontal gyrus below from the triangular part above. The orbital part wraps around the inferior surface of the frontal lobe, and the triangular part is the triangular-shaped cortical region between the anterior and ascending rami.

Ascending Ramus

The ascending ramus extends upward and slightly forward from the anterior Sylvian point. It separates the triangular part of the inferior frontal gyrus from the opercular part. The opercular part is the cortical region between the ascending ramus and the precentral sulcus, and it forms the anterior part of the frontal operculum that covers the insula.

In a cadaveric study of 62 adult hemispheres, the anterior and ascending rami arose from the anterior Sylvian point and either divided at that point (43 hemispheres, 69.4%) or shared a common short stem before separating distally, producing a Y-shaped configuration [8]. This variation matters for surgical planning because the position of the anterior Sylvian point determines the approach corridor.

Posterior Ramus

The posterior ramus is the main trunk of the lateral sulcus. It extends posteriorly and slightly upward from the anterior Sylvian point, running between the temporal operculum below and the frontal and parietal opercula above. The posterior ramus is the longest segment of the fissure. The sylvian fissure extends from the basal to the lateral surface of the brain and presents two compartments on each surface: a superficial compartment (the temporal stem and its rami) and a deep compartment (the anterior and lateral operculoinsular compartments) [6].

The Insula as the Floor of the Lateral Sulcus

The insula is a buried region of cortex that forms the floor of the lateral sulcus. It is divided into a smaller anterior portion (the anterior insula, with short gyri) and a larger posterior portion (the posterior insula, with long gyri). The limen insulae is the threshold region at the anteroinferior margin of the insula, where the insular cortex transitions to the temporal lobe.

The insula is delineated by the peri-insular sulci: the superior limiting sulcus, the inferior limiting sulcus, and the anterior limiting sulcus. The superior limiting sulcus is the only sulcus on the lateral surface of the brain that does not have an artery running along its axis [9]. This anatomical constant is surgically useful because it provides a relatively avascular plane for dissection.

The relationship between the insula and the opercula is not uniform. The pars triangularis of the inferior frontal gyrus covers the apex of the insula, which is located immediately distal to the genu of the middle cerebral artery [6]. The anterior, middle, and posterior thirds of the insular surface are covered by the inferior frontal, precentral, and postcentral gyri respectively [6].

Table: Rami of the Lateral Sulcus, Adjacent Gyri, and Functional Relevance

Development of the Lateral Sulcus

The lateral sulcus forms through a process called opercularization. After the prosencephalon divides sagittally at about 4.5 weeks of gestation, the early fetal cerebral hemisphere bends or rotates posteroventrally from about seven weeks of gestation. This rotation, called the telencephalic flexure, forms the operculum and eventually the lateral cerebral or Sylvian fissure. The ventral part of the hemisphere is infolded to become the insula [3].

The frontal and temporal lips of the lateral sulcus, along with the insula, all derive from the ventral margin of the primitive telencephalon. This shared origin means they may be influenced by genetic mutations with a ventrodorsal gradient of expression [3]. The telencephalic flexure also shifts the hippocampus from a dorsal to a ventral position, with the early rostral pole of the hippocampus becoming caudal and the dorsal becoming ventral [3].

A digital fetal brain atlas study using MRI imaging of 81 healthy fetuses between 21 and 38 weeks of gestation showed that the highest rate of organized growth occurred in regions bordering the Sylvian fissure of the frontal, parietal, and temporal lobes. Volumetric changes were first observed in the posterior aspect of the fissure, moving anteriorly to the frontal lobe and laterally in the direction of the temporal pole. The insular region expanded to a much lesser degree [5]. The gene expression profile before folding begins was significantly different in the developing opercular cortex compared to the insula, supporting the idea that the Sylvian fissure forms by relative overgrowth of the frontal and temporal lobes over the insula [5].

Many major malformations include lack of telencephalic flexure (holoprosencephaly, extreme micrencephaly) or a dysplastic Sylvian fissure (lissencephalies, hemimegalencephaly, schizencephaly). Although fissures and sulci are genetically programmed, mechanical forces of growth and volume expansion are proposed to be mainly extrinsic for fissures and intrinsic for sulci [3].

Comparative Anatomy Across Species

The lateral sulcus is present in all primates and is well developed in humans, great apes, and Old World monkeys. In these species, the opercularization process is extensive, and the insula is deeply buried beneath well-formed frontal, parietal, and temporal opercula.

In carnivores such as dogs and cats, the lateral sulcus (called the Sylvian fissure or sulcus lateralis in veterinary anatomy) is present but less distinct. The opercularization is less complete, and the insula is not buried as deeply as in primates. The surrounding gyri and sulci are also less elaborate. The carnivore brain is gyrencephalic but with a simpler folding pattern than the primate brain.

In ungulates, the picture is different again. The horse has a highly gyrencephalic cerebrum, and the lateral surface is dominated by the Sylvian fissure with an accessory Sylvian fissure at its origin. A standardized color-coded atlas of the equine brain based on the Nomina Anatomica Veterinaria documented a consistent lateral network dominated by the Sylvian fissure, with a three-limbed ectosylvian sulcus that subdivides the ectosylvian gyrus, a continuous suprasylvian sulcus, and a distinct oblique sulcus [1]. The accessory Sylvian fissure is a feature not typically described in the human brain.

In the horse, the Sylvian fissure is a major landmark for identifying the temporal lobe and the ectosylvian gyrus. The ectosylvian gyrus is the equine equivalent of the superior temporal gyrus in primates, and it contains auditory cortex. The three-limbed ectosylvian sulcus divides this gyrus into distinct subregions [1].

The comparative differences in lateral sulcus anatomy reflect differences in cortical folding and opercularization. Species with more complex cortical folding (primates, horses) have more elaborate perisylvian anatomy, while species with simpler folding (carnivores) have a less distinct lateral sulcus.

Surgical and Clinical Relevance of the Lateral Sulcus

The lateral sulcus is a critical landmark in neurosurgery. The transsylvian approach, which involves dissecting the fissure to access the insula and the middle cerebral artery, is used for aneurysm clipping, arteriovenous malformation resection, and insular tumor removal [9][6][10].

In a study of insular arteriovenous malformations, microsurgical resection was performed via the ascending ramus of the Sylvian fissure to achieve safer resection with less retraction of the frontal operculum [11]. The ascending ramus provides a corridor to the superior limiting sulcus of the insula, which is the site of some insular AVMs.

The transsylvian approach has anatomical constraints. The anteroinferior part of the insula (including the limen insulae) is the most technically easy-to-reach area, while the superior parts of the insula are the most difficult to reach. For tumors in the superior insula, a transcortical approach may be recommended because it does not require significant retraction of brain matter and provides a larger surgical corridor [10].

The lateral sulcus also has functional significance beyond surgery. The posterior Sylvian fissure region, close to the posterior insula, is involved in vestibular processing. Functional MRI studies have identified a parieto-insular vestibular cortex area (PIVC) and a posterior insular cortex (PIC) in this region. Both respond to caloric vestibular stimulation, but only the right PIVC reliably represents the direction of the caloric stimulus [12]. The PIC is located at a more posterior site in the Sylvian fissure than the PIVC [12].

The lateral sulcus shows significant leftward asymmetry in humans. A morphometric study of 50 formalin-fixed adult human brains found significant leftward asymmetry of the lateral sulcus, with a mean left length of 8.94 mm (after correction for fixation shrinkage) [13]. This asymmetry is thought to relate to leftward functional lateralization for language processing. A longitudinal study of 71 infants born extremely preterm found a more pronounced opercularization in the left hemisphere, contrasting with a previously reported right hemisphere advance in sulcation around birth [4]. These findings suggest that lateral sulcus asymmetry is encoded early in development.

Clinical Relevance, Limitations and Common Mistakes

The lateral sulcus is a high-value landmark for anyone studying or working with the brain. In veterinary medicine, it is used to orient the lateral surface of the cerebrum during gross anatomy, to interpret MRI and CT images of the brain, and to plan surgical approaches to the temporal lobe, insula, and middle cerebral artery territory.

One common mistake is to treat the lateral sulcus as a simple sulcus. It is not. It is a deep, operculated fissure with a buried floor (the insula) and a complex three-dimensional anatomy that includes superficial and deep compartments [6]. A second common mistake is to confuse the anterior and ascending rami. The anterior ramus runs forward and separates the orbital and triangular parts of the inferior frontal gyrus. The ascending ramus runs upward and separates the triangular and opercular parts. In the dominant hemisphere, the opercular and triangular parts together constitute Broca area, which is critical for language production.

A third mistake is to assume that the lateral sulcus is identical across species. In dogs and cats, the fissure is present but less distinct, with less opercularization and a less deeply buried insula. In horses, the Sylvian fissure is a dominant landmark with an accessory Sylvian fissure at its origin, a feature not seen in humans [1]. These differences matter for comparative anatomy and for interpreting veterinary neuroimaging.

A fourth mistake is to overlook the morphometric variability of the anterior Sylvian point. The distance from the anterior Sylvian point to the pars triangularis and pars opercularis varies considerably, with type B (5.1 to 10 mm) being the most common in one study of 40 hemispheres [7]. This variability affects surgical planning and the interpretation of imaging studies.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Frequently Asked Questions

What is the lateral sulcus?

The lateral sulcus, also called the Sylvian fissure, is the deep cleft on the lateral surface of the cerebral hemisphere that separates the temporal lobe from the frontal and parietal opercula. Its floor is formed by the insula.

What are the three rami of the lateral sulcus?

The three rami are the anterior ramus, the ascending ramus, and the posterior ramus. The anterior and ascending rami separate the orbital, triangular, and opercular parts of the inferior frontal gyrus. The posterior ramus is the main trunk of the fissure.

Where is the insula located?

The insula is the buried cortex that forms the floor of the lateral sulcus. It lies deep to the frontal, parietal, and temporal opercula and is divided into anterior and posterior portions by the central insular sulcus.

What is the function of the lateral sulcus?

The lateral sulcus is a structural landmark rather than a functional region itself. The cortex along its banks includes Broca area (inferior frontal gyrus), primary auditory cortex (Heschl's gyrus on the temporal operculum), and somatosensory cortex (postcentral gyrus on the parietal operculum).

Is the lateral sulcus the same in dogs and cats?

No. In carnivores, the lateral sulcus is present but less distinct than in primates. The opercularization is less complete, and the insula is not buried as deeply. The surrounding gyri and sulci are also simpler.

Why is the lateral sulcus important in surgery?

The lateral sulcus is the corridor for the transsylvian approach, which is used to access the insula, the middle cerebral artery, and insular lesions. Dissecting the fissure allows surgeons to reach deep structures with minimal brain retraction.

What is the anterior Sylvian point?

The anterior Sylvian point is the most anterior extent of the lateral sulcus, located between the pars triangularis and the pars opercularis. It is where the anterior and ascending rami originate.

Does the lateral sulcus differ between the left and right hemispheres?

Yes. The lateral sulcus is longer on the left side in most people, and this asymmetry is thought to relate to leftward language lateralization. The asymmetry appears early in development, including in preterm infants.

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