Falx Cerebri: Anatomy, Function, and Dural Reflections

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

The falx cerebri (also called the cerebral falx) is a sickle-shaped fold of dura mater that descends into the longitudinal fissure between the two cerebral hemispheres, attaching rostrally to the crista galli and caudally to the tentorium cerebelli, and enclosing the dorsal sagittal sinus along its convex dorsal margin. It is one of three major dural reflections inside the skull, alongside the tentorium cerebelli and the falx cerebelli, and it helps separate, stabilize, and protect the cerebral hemispheres while channeling venous blood from the dorsal cerebrum.

Understanding the falx cerebri matters for more than exam recall. Dural reflections define the compartments of the cranial cavity, they carry the venous sinuses that drain the brain, and they appear on every cross-sectional imaging study of the head. A veterinary student who can trace the falx cerebri from the crista galli to the tentorium, and who knows which sinus runs in which fold, can read a magnetic resonance image, a computed tomography scan, or a cadaver dissection with far more confidence. The same knowledge explains why dural disease, such as the hypertrophic pachymeningitis reported in a dog by Daudén and colleagues [1], produces signs tied to the region of dura involved.

What the Falx Cerebri Is

The dura mater is the outermost and toughest of the three meninges. In the skull, the dura forms a periosteal layer against bone and a meningeal layer against the brain. Where the meningeal layer folds inward, it creates dural reflections, sometimes called dural folds or dural partitions. These folds are the major gateway for accessing extra-axial and intra-axial lesions of the central nervous system, and they provide a protective barrier against external trauma, infection, and spread of malignant cells [2].

The falx cerebri is the most rostral and most dorsal of these folds. It is a vertical sheet of dura that occupies the longitudinal fissure, the deep groove that separates the left and right cerebral hemispheres. Its shape is crescentic or sickle-like, which is where the name comes from: falx is Latin for sickle.

Three features define the falx cerebri:

1. Location. It lies in the midline, in the longitudinal fissure between the cerebral hemispheres.
2. Attachments. Rostrally it attaches to the crista galli, a bony ridge on the ethmoid bone. Caudally it attaches to the tentorium cerebelli, the tent-like dural fold that separates the cerebrum from the cerebellum.
3. Contents. Its convex dorsal (upper) margin splits into two layers that enclose the dorsal sagittal sinus, a major venous channel of the dorsal cranial cavity.

The falx cerebri is a dural reflection, not a nerve, not a muscle, and not part of the brain parenchyma. It is mesodermal connective tissue, and it is continuous with the rest of the dura at every attachment.

Why the Falx Cerebri Matters

Dural reflections organize the intracranial space. They create compartments, they carry venous sinuses, and they give surgeons and imagers fixed landmarks. The falx cerebri specifically separates the cerebral hemispheres, resists excessive side-to-side movement of the brain, and houses the dorsal sagittal sinus, which receives blood from the dorsal cerebral veins and channels it toward the transverse sinuses.

In dogs, magnetic resonance venography has shown that the dorsal dural venous sinus system is highly variable. Fenn, Lam, and Kenny found that 70.6% of 51 dogs with structurally normal brains had at least one anatomic variation in the dorsal sagittal sinus, including deviation from the midline in 33.3% and collateral branches from the dorsal sagittal sinus or dorsal cerebral veins in 54.9% [3]. Transverse sinus asymmetry was present in 58.8% of dogs, with hypoplasia in 39.2% and aplasia in 23.5% [3]. This matters because a normal variant can look like a pathologic finding if the clinician does not know the range of normal anatomy.

Anatomy of the Falx Cerebri

Shape and Position

The falx cerebri is a thin, roughly triangular or crescentic sheet. Its broad base is dorsal, where it meets the inner surface of the skull along the midline. Its narrow free edge is ventral, dipping into the longitudinal fissure between the hemispheres. The rostral tip is anchored at the crista galli. The caudal end blends into the tentorium cerebelli.

Because the cerebral hemispheres are separated by the longitudinal fissure, the falx cerebri fills that fissure. In species with large, well-developed cerebral hemispheres, the falx is correspondingly more extensive. In species with smaller or less convoluted brains, the falx is shorter and shallower.

Attachments

The falx cerebri has three principal attachment zones:

• Rostral attachment. The crista galli of the ethmoid bone. This is a small midline bony projection that gives the falx a firm anterior anchor.
• Dorsal attachment. The inner surface of the skull along the midline, where the two layers of the falx separate to enclose the dorsal sagittal sinus.
• Caudal attachment. The tentorium cerebelli. The falx cerebri and tentorium cerebelli meet at the level of the transverse sinuses and the straight sinus region. This junction is a key landmark in the caudal cranial cavity.

The Dorsal Sagittal Sinus

The convex dorsal margin of the falx cerebri contains the dorsal sagittal sinus. A venous sinus is a channel lined by endothelium and enclosed within the dura, not a vessel with a muscular wall like an artery or vein. The dorsal sagittal sinus runs caudally along the top of the falx and typically continues toward the confluence of sinuses near the tentorium.

The dorsal sagittal sinus receives tributaries from the dorsal cerebral veins. In dogs, collateral branches from the dorsal sagittal sinus or dorsal cerebral veins were identified in 54.9% of the 51 dogs studied by Fenn and colleagues [3]. The sinus can also deviate from the midline, which was seen in 33.3% of those dogs [3]. These variations are relevant when interpreting venographic images and when planning any procedure near the midline.

Relationship to the Tentorium Cerebelli

The tentorium cerebelli is a transverse dural fold that forms a roof over the cerebellum and separates the cerebrum above from the cerebellum below. The falx cerebri meets the tentorium at the caudal end of the longitudinal fissure. At that junction, the dorsal sagittal sinus and the straight sinus converge. The tentorium also contains the transverse sinuses along its attached margins.

The tentorium is not part of the falx cerebri, but the two are continuous. A student who understands that continuity can trace the dura from the crista galli, along the dorsal midline, to the tentorium, and then laterally to the transverse sinuses.

Function of the Falx Cerebri

The falx cerebri serves several functions:

1. Compartmentalization. It separates the left and right cerebral hemispheres and helps define the supratentorial space.
2. Mechanical support. It resists excessive lateral movement of the brain within the skull.
3. Venous drainage. It houses the dorsal sagittal sinus, which drains dorsal cerebral blood toward the transverse sinuses.
4. Barrier function. As part of the dura, it contributes to the protective barrier against trauma, infection, and malignant spread [2].
5. Anatomical landmark. It provides a midline reference for imaging and dissection.

These functions are structural and mechanical. The falx does not generate electrical activity, does not secrete cerebrospinal fluid, and does not directly participate in neuronal signaling.

How the Falx Cerebri Is Studied and Observed

Gross Dissection

In a cadaver dissection, the falx cerebri is exposed after removing the calvaria (the skull cap) and reflecting the dura. The dorsal sagittal sinus is visible along the dorsal midline, and the falx is seen descending into the longitudinal fissure. The rostral attachment at the crista galli and the caudal attachment at the tentorium can be traced with a probe.

Magnetic Resonance Imaging

On MRI, the falx cerebri appears as a thin midline structure. The dorsal sagittal sinus is visible on venographic sequences. Fenn and colleagues used phase-contrast intracranial MR venography to describe the dorsal dural venous sinus system in dogs, documenting midline deviation and collateral branches as common findings [3]. This study established that anatomic variation in the dorsal sagittal sinus is common in dogs with structurally normal brains.

Computed Tomography

On CT, the falx and the dural sinuses can be identified by their density. In a forensic study of children, increased density of the tentorium on postmortem CT reflected dural hemorrhage rather than subarachnoid hemorrhage [4]. That finding is human, but it illustrates a general principle: dural folds and their contents can mimic other lesions on imaging, and the interpreter must know the normal anatomy.

Clinical and Pathological Observation

Dural disease can involve the falx and adjacent dura. Daudén and colleagues reported an 8-year-old Whippet crossbreed with hypertrophic pachymeningitis and generalized eosinophilic disease. MRI showed thickening of the dura mater around the left parietal and temporal cerebral cortex, the left piriform lobe, and the cerebellar hemispheres [1]. This case shows that dural thickening can be regional and can involve multiple dural surfaces, not just the falx. The falx itself is part of the same dural sheet, so dural pathology can affect it.

Comparative Anatomy: Species Differences

The falx cerebri varies across domestic species. The general plan is the same, but the size, completeness, and degree of ossification differ.

Dogs and Cats

In dogs and cats, the falx cerebri is relatively small. It may be incomplete in some individuals, and in some breeds it can contain ossified areas. The dorsal sagittal sinus is present but variable. Fenn and colleagues documented that 70.6% of 51 dogs had at least one anatomic variation in the dorsal sagittal sinus, including midline deviation and collateral branches [3]. This variability is normal and should not be mistaken for disease.

Horses and Cattle

In horses and cattle, the falx cerebri is more extensive. The larger cerebral hemispheres and the greater depth of the longitudinal fissure are matched by a longer, deeper falx. The dorsal sagittal sinus is correspondingly larger and more consistent in its course.

Other Species

In pigs, sheep, and other domestic mammals, the falx cerebri follows the same general pattern but with species-specific proportions. The key comparative principle is that the falx cerebri scales with the size and complexity of the cerebral hemispheres.

The Three Major Dural Folds Compared

The falx cerebri is one of three major dural reflections in the cranial cavity. Students often confuse them, so the following table compares them directly.

The falx cerebelli is a retrocerebellar dural reflection [5]. In a fetal MRI study, the falx cerebelli was found to be linear in 23% of cases, Y-shaped in 15%, V-shaped in 22%, and U-shaped in 21%, and it was unusually multiplicated in 17% and absent in less than 2% [5]. That study was in human fetuses, but it demonstrates that the falx cerebelli is a variable structure even in normal development.

Variations of the dural folds and dural venous sinuses are infrequently reported. Nayak and Shetty described a rare concurrent variation: partial duplication of the tentorium cerebelli and complete duplication of the falx cerebelli, with a single occipital venous sinus within the attached border of the duplicated falx cerebelli [6]. This case highlights that dural fold anatomy is not always textbook-perfect, and that variations can be clinically relevant.

Clinical Relevance, Limitations and Common Mistakes

Clinical Relevance

Dural reflections are clinically relevant because they carry venous sinuses, because they define compartments, and because they can be involved in disease. Dural arteriovenous fistulas, for example, are classified partly by their location relative to the tentorium. Davies and colleagues analyzed 102 intracranial dural arteriovenous fistulas and found that 69% of hemorrhages occurred in anterior cranial fossa or tentorial lesions [7]. That study is human, but it illustrates that the tentorium and adjacent dura are clinically important locations.

In veterinary medicine, dural disease is less commonly reported but does occur. The case of hypertrophic pachymeningitis in a dog showed that dural thickening can be regional and can involve multiple areas, including the dura around the cerebral cortex and cerebellar hemispheres [1]. The falx cerebri is part of the same dural sheet, so it can be involved in such processes.

Dural mobilization exercises in dogs have been shown to produce measurable movement of nerve roots and dura at the thoracolumbar and lumbosacral levels [8]. That study was about the lumbar spine, not the falx, but it confirms that dura is a mobile, mechanically responsive tissue, not a rigid fixed structure.

Limitations

The falx cerebri is small and variable in dogs and cats. Imaging findings that would be abnormal in one species may be normal in another. The dorsal sagittal sinus can deviate from the midline in normal dogs [3], so midline deviation alone is not proof of disease. The falx cerebelli can be absent or multiplicated in normal development [5], so absence of the falx cerebelli is not automatically pathologic.

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

Common Mistakes

1. Confusing the falx cerebri with the falx cerebelli. The falx cerebri is between the cerebral hemispheres. The falx cerebelli is between the cerebellar hemispheres. They are in different compartments and contain different sinuses.
2. Assuming the falx cerebri always contains a sinus. The dorsal sagittal sinus runs along the dorsal margin of the falx cerebri, but the sinus is a separate channel. The falx itself is dural connective tissue.
3. Treating midline deviation of the dorsal sagittal sinus as always abnormal. In dogs, midline deviation was found in 33.3% of structurally normal brains [3].
4. Forgetting that the falx cerebri attaches caudally to the tentorium. The two folds are continuous, and the junction is a key landmark.
5. Assuming the falx cerebri is the same size in all species. It is small and sometimes incomplete in dogs and cats, and more extensive in horses and cattle.
6. Mistaking dural thickening for a brain lesion. Dural disease can thicken the dura and mimic other findings on imaging [1].

Quick Review

1. The falx cerebri is a sickle-shaped dural fold in the longitudinal fissure between the cerebral hemispheres.
2. It attaches rostrally to the crista galli and caudally to the tentorium cerebelli.
3. Its convex dorsal margin encloses the dorsal sagittal sinus.
4. The falx cerebelli is a separate fold between the cerebellar hemispheres, and the tentorium cerebelli is a transverse fold between cerebrum and cerebellum.
5. In dogs and cats the falx cerebri is relatively small and may be incomplete or ossified. In horses and cattle it is more extensive.
6. Dorsal sagittal sinus anatomy is variable in normal dogs, with midline deviation and collateral branches being common [3].
7. Dural folds are part of the protective barrier of the central nervous system and are clinically relevant in dural disease [2][1].

Frequently Asked Questions

What is the falx cerebri?

The falx cerebri is a sickle-shaped fold of dura mater that sits in the longitudinal fissure between the two cerebral hemispheres. It attaches to the crista galli rostrally and the tentorium cerebelli caudally, and it encloses the dorsal sagittal sinus along its dorsal margin.

What is the difference between the falx cerebri and the falx cerebelli?

The falx cerebri separates the cerebral hemispheres and contains the dorsal sagittal sinus. The falx cerebelli separates the cerebellar hemispheres, sits below the tentorium, and may contain an occipital sinus.

What sinus runs in the falx cerebri?

The dorsal sagittal sinus runs along the convex dorsal margin of the falx cerebri. It receives blood from dorsal cerebral veins and channels it caudally toward the transverse sinuses.

Is the falx cerebri the same in dogs and horses?

No. In dogs and cats the falx cerebri is relatively small and may be incomplete or ossified in some breeds. In horses and cattle it is more extensive, matching the larger cerebral hemispheres.

Can the falx cerebri be seen on MRI?

Yes. The falx cerebri is visible on MRI as a thin midline structure, and the dorsal sagittal sinus can be evaluated with MR venography. Anatomic variations in the dorsal sagittal sinus are common in normal dogs [3].

What happens if the dura around the falx cerebri becomes diseased?

Dural disease can cause thickening of the dura mater, which may be visible on MRI. A case of hypertrophic pachymeningitis in a dog showed regional dural thickening around the cerebral cortex and cerebellar hemispheres [1]. Any suspected dural disease requires veterinary evaluation.

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Sources

1. Eosinophilic Cerebrospinal Fluid Pleocytosis in A Dog With Hypertrophic Pachymeningitis and Generalized Eosinophilic Disease.
2. Arachnoid and dural reflections.
3. Variations in magnetic resonance venographic anatomy of the dorsal dural venous sinus system in 51 dogs.
4. Dural hemorrhage of the tentorium on postmortem cranial computed tomographic scans in children.
5. The fetal falx cerebelli.
6. Partial duplication of tentorium cerebelli and complete duplication of falx cerebelli.
7. The validity of classification for the clinical presentation of intracranial dural arteriovenous fistulas.
8. Evaluation of the anatomic effect of physical therapy exercises for mobilization of lumbar spinal nerves and the dura mater in dogs.