THE CENTRAL NERVOUS SYSTEM
WHITE MATTER OF THE HEMISPHERES
Commissural fibres
There are three types of fibre within the cerebral hemispheres:
The corpus callosum (Figs 2. 4 and 2. 5)
• Commissural fibres, which connect corresponding areas of the two hemispheres; • Association (arcuate) fibres, which connect different parts of the cortex of the same hemisphere; and • Projection fibres, which join the cortex to lower centres.
The corpus callosum is a large midline mass of commissural fibres, each of which connects corresponding areas of both hemispheres. It is approximately 10 cm long and becomes progressively thicker towards its posterior end. Named parts include the: • Rostrum - this is the first part, which extends anteriorly from the anterior commissure (vide infra); • Genu - this is the most anterior part where it bends sharply backwards; • Trunk (body) - this is the main mass of fibres extending from the genu anteriorly to the splenium posteriorly. It lies below the lower free edge of the falx cerebri. The anterior cerebral vessels run on its superior surface; and • Splenium - this is the thickened posterior end. In cross-section, fibres from the genu that arch forward to the frontal cortex on each side are called forceps minor, and fibres from the splenium passing posteriorly to each occipital cortex are called forceps major. Fibres extending laterally from the body of the corpus callosum are called the tapetum. These form part of the roof and lateral wall of the lateral ventricle.
Fig. 2. 5 MRI scan of brain: midline sagittal image. 1. Frontal lobe
16. Optic chiasm
2. Parietal lobe
17. Midbrain
3. Occipital lobe
18. Interpeduncular
4. Rostrum of corpus callosum 5. Genu of corpus callosum 6. Body of corpus callosum 7. Splenium of corpus callosum
cistern 19. Aqueduct of Sylvius 20. Quadrigeminal plate (superior and inferior colliculi) 2 1 . Quadrigeminal plate cistern 2 2 . Fourth ventricle
8. Septum pellucidum
2 3 . Vermis of cerebellum
9. Foramen of Monro
2 4 . Pons
10. Fornix
2 5 . Tonsil of cerebellum
11. Massa intermedia
26. Prepontine cistern
of thalami 12. Third ventricle 13. Supraoptic recess of third ventricle 14. Suprapineal recess of third ventricle 15. Pineal gland
27. Medulla oblongata 28. Odontoid process 29. Cisterna magna 3 0 . Clivus 31. Pituitary 3 2 . Tentorium cerebelIi 3 3 . Spinal cord
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