CorpusHuman Anatomy

Brain — Midsagittal Section

Medial surface of the right half, anterior to the left

A cut through the midline between the two hemispheres, viewed from the left so that the front of the brain faces left. Around the medial cortex and the cingulate gyrus arches the corpus callosum; beneath it lie the septum pellucidum, the fornix and the diencephalon (thalamus, hypothalamus, pineal gland), which hangs the pituitary gland from its floor. The brainstem (midbrain, pons, medulla oblongata) continues into the spinal cord, with the cerebellum behind it, its white matter branching as the arbor vitae.

The ventricular system is shown in blue: the lateral ventricle (projected through the septum), the slit-like third ventricle around the thalamus, the cerebral aqueduct and the tent-shaped fourth ventricle. Outside the brain, the subarachnoid space (blue band) lies between the pia and the arachnoid, beneath the dura mater and the superior sagittal sinus. The moving dots trace the circulation of cerebrospinal fluid from the choroid plexus to its absorption through the arachnoid granulations into the venous blood.

Step-by-step

  1. Midline structures

    The corpus callosum arches over the diencephalon (thalamus, hypothalamus). Below it the brainstem — midbrain, pons and medulla oblongata — continues as the spinal cord, with the cerebellum behind. The CSF spaces are shown in blue.

  2. 1 · Secretion by the choroid plexus

    CSF is secreted mainly (≈70–80%) by the choroid plexus — tufts of fenestrated capillaries covered by a cuboidal choroid epithelium whose tight junctions form the blood–CSF barrier. About 500 mL is produced per day (≈0.35 mL/min), so the total volume of ≈150 mL is replaced 3–4 times a day.

  3. 2 · Lateral ventricles

    Most CSF is made in the two lateral ventricles, C-shaped cavities in each hemisphere with a frontal horn, body, atrium, occipital horn and temporal horn. They are shown here projected through the thin septum pellucidum.

  4. 3 · Interventricular foramen

    CSF leaves each lateral ventricle through the interventricular foramen (of Monro), between the column of the fornix and the front of the thalamus, and enters the third ventricle.

  5. 4 · Third ventricle

    The third ventricle is a midline slit between the two thalami, bridged by the interthalamic adhesion. Its floor is the hypothalamus, and recesses extend toward the optic chiasm, the infundibulum and the pineal gland. Its roof also carries choroid plexus.

  6. 5 · Cerebral aqueduct

    The narrow cerebral aqueduct (of Sylvius, ≈1–2 mm across) passes through the midbrain, between the tectum and the tegmentum. As the narrowest point of the system, it is the commonest site of obstruction: aqueductal stenosis causes non-communicating hydrocephalus with enlarged lateral and third ventricles but a normal fourth ventricle.

  7. 6 · Fourth ventricle

    The tent-shaped fourth ventricle lies between the pons and upper medulla in front (the rhomboid fossa) and the cerebellum behind. CSF leaves it through the single median aperture (Magendie) and two lateral apertures (Luschka); a little enters the central canal of the spinal cord.

  8. 7 · Subarachnoid space

    From the cisterna magna, CSF spreads through the subarachnoid space — between the arachnoid mater and the pia mater — around the brain, through the basal cisterns and down around the spinal cord. Here it cushions the brain, which effectively weighs only ≈50 g instead of ≈1400 g. CSF is sampled from the lumbar cistern, below the end of the cord, by lumbar puncture at L3/L4 or L4/L5.

  9. 8 · Absorption into venous blood

    CSF returns to the blood mainly through the arachnoid granulations (villi), which project through the dura into the superior sagittal sinus. They work as pressure-dependent, one-way valves: absorption increases as CSF pressure rises above venous pressure. Perineural and lymphatic routes also drain CSF. When absorption fails (for example after meningitis or subarachnoid haemorrhage), the result is communicating hydrocephalus.

  10. Endocrine links of the diencephalon

    The hypothalamus controls the pituitary gland through the infundibulum (by axons to the posterior lobe, and by the hypophyseal portal system to the anterior lobe), and the pineal gland secretes melatonin. Note the optic chiasm just in front of the pituitary stalk.

Labelled parts