THE CENTRAL NERVOUS SYSTEM 61
Each crus extends posteriorly to below the splenium of the corpus callosum, where it connects with that of the other side as the commissure of the fornix. Together the two crura form the body of the fornix. The body of the fornix extends anteriorly around the upper surface of the thalami. It is attached posteriorly to the undersurface of the corpus callosum and anteriorly to the inferior border of the septum pellucidum. Above the interventricular foramen the body divides into two columns, which pass inferiorly between the foramen and the anterior commissure and form the anterior border of the interventricular foramen. The columns then pass to the hypothalamus and the mamillary bodies, in which they terminate. Some fibres from the columns of the fornix, the habenular fibres, turn back over the thalamus to join posteriorly in the habenular commissure. Radiological features of the limbic lobe Magnetic resonance imaging The body of the fornix is seen in midline sagittal MRI (see Fig. 2. 5), extending from the undersurface of the corpus callosum posteriorly to the interventricular foramen anteriorly along the undersurface of the septum pellucidum. Fine cut coronal MRI perpendicular to the axis of the hippocampus and angled axial and sagittal images along its long axis are used to image the hippocampus, especially in the evaluation of epilepsy (see Fig. 2. 13). Ultrasound examination of the neonatal brain (see Fig. 2. 8) The cingulate gyrus can be identified surrounding the corpus callosum. This is absent if the corpus callosum is absent. The body of the fornix can be seen below the septum pellucidum on midline sagittal images. In the coronal plane at the level of the third ventricle the C-shaped echoes of the parahippocampal gyrus are used as a far-field landmark. THE BRAINSTEM (see Fig. 2. 10) The brainstem connects the cerebral hemispheres with the spinal cord and extends from just above the tentorial hiatus to just below the foramen magnum. It is bounded anteriorly by the clivus-basisphenoid above and the basiocciput below. The pons, the widest part of the brainstem, also grooves the apex of the petrous temporal bone on each side. This bony anterior boundary can cause an artefact in CT scanning of the brainstem, although it does not interfere with MRI studies. The brainstem has three parts: from superior to inferior, the midbrain, the pons and the medulla.
