[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anatomy](https://durgical.com/en/anatomy/) › [Head and neck](https://durgical.com/en/anatomy/region/head-and-neck/) › Commissures and septum [Nervous system](https://durgical.com/en/anatomy/system/nervous-system/) · [Head and neck](https://durgical.com/en/anatomy/region/head-and-neck/) # Commissures and septum The commissures are small fibre bridges linking the two sides of the brain; the septum is the thin wall between the front of the two lateral ventricles. Official name: Anterior commissure [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=head_neck&select=commissures_septum&side=r) Opens in the Durgical app, no sign-up needed. ## What it is Small fibre bridges across the midline and the thin wall between the lateral ventricles. ## What it does Link the two sides; the septum separates the front of the lateral ventricles. ## Key points - Anterior commissure: joins the temporal lobes (middle and inferior temporal gyri) and olfactory regions; it lies in the lamina terminalis, in front of the fornix columns - Posterior commissure: crosses above the opening of the cerebral aqueduct, carrying pretectal fibres for the consensual light reflex; it is compressed in Parinaud syndrome - The AC–PC line joining the two commissures is the standard reference for axial MRI slices and for stereotactic surgery - Septum pellucidum: two thin leaves between the corpus callosum and the fornix forming the medial wall of each frontal horn; midline shift on CT is measured at it ## Key facts **Where** Anterior commissure in front of the fornix columns; posterior commissure above the aqueduct; septum pellucidum between the corpus callosum and fornix **Function** Link the two sides (temporal lobes, smell; pupillary light reflex); the septum separates the frontal horns of the lateral ventricles ## Relations **Above** [Corpus callosum](https://durgical.com/en/anatomy/corpus-callosum/), from whose underside the septum pellucidum hangs **Below** Body and columns of the [fornix](https://durgical.com/en/anatomy/fornix/) along the lower edge of the septum **Lateral** Frontal horns and bodies of the [lateral ventricles](https://durgical.com/en/anatomy/lateral-ventricle/) on each side of the septum **In front** Lamina terminalis, the front wall of the [third ventricle](https://durgical.com/en/anatomy/third-ventricle/), which holds the anterior commissure **Behind** [Pineal gland](https://durgical.com/en/anatomy/pineal-gland/) just above and behind the posterior commissure; the [cerebral aqueduct](https://durgical.com/en/anatomy/aqueduct-of-midbrain/) opens beneath it ## How to find it On midsagittal MRI the anterior commissure is a small round white dot in front of the fornix columns, and the posterior commissure a smaller dot above the opening of the aqueduct, below the pineal gland. The septum pellucidum is the thin midline membrane between the frontal horns on coronal and axial images. ## On imaging A fluid space between the two leaves (cavum septi pellucidi) is normal in fetuses and newborns; its backward extension is the cavum vergae. Absence of the septum suggests septo-optic dysplasia or holoprosencephaly. ## Clinical notes - Parinaud (dorsal [midbrain](https://durgical.com/en/anatomy/midbrain/)) syndrome: upgaze palsy, light–near dissociation of the pupils and convergence–retraction nystagmus, from a pineal tumour or hydrocephalus pressing on the region - When only one lateral ventricle is obstructed, an endoscopic opening in the septum (septostomy) lets fluid cross to the other side - Absence of the septum pellucidum is a feature of septo-optic dysplasia, together with small optic nerves and, in many children, pituitary hormone deficiency ## Anatomical variants - A cavum septi pellucidi persists in some adults, usually as an incidental finding; it is not part of the ventricular system (no ependymal lining, no CSF connection), so the old name "fifth ventricle" is a misnomer; a large one is more frequent after repeated head trauma, as in boxers ## Development The commissures and the septum form in the commissural plate of the lamina terminalis; the cavum between the septal leaves normally closes in the months after birth. ## Test yourself Think of the answer, then tap the question to see it. ## Which fibres does the posterior commissure carry, and where does it cross? Pretectal fibres for the consensual light reflex, crossing above the opening of the cerebral aqueduct; it is compressed in Parinaud syndrome. ## What is the AC–PC line used for? It joins the anterior and posterior commissures and is the standard reference for axial MRI slices and stereotactic surgery. ## Where is the septum pellucidum, and what is measured at it on CT? Between the corpus callosum and the fornix, forming the medial wall of each frontal horn; midline shift is measured at it. [Add to my cards (3)](https://durgical.app/estudiar/?add=/en/anatomy/anterior-commissure/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** commissura anterior TA2 5613 **Official Spanish (DeCS)** Comisuras y septo pelúcido DeCS D012688 **English (TA2)** Anterior commissure Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=13070); [Neuroanatomy, Corpus Callosum](https://www.ncbi.nlm.nih.gov/books/NBK448209/); Tratado de anatomía humana (9.ª ed.), tomo II, Libro V, cap. VI: Cerebro (configuración exterior e interior, ventrículos, formaciones coroideas, epífisis). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/anatomy/anterior-commissure/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/anterior-commissure/