Fiber dissection of the upper brainstem, ventral view. The cerebral peduncles and upper pons have been dissected to show their internal architecture. The ponto-mesencephalic sulcus remains intact to serve as reference. The cortico-spinal tract (CST) occupies the middle 3/5 of the transverse surface of the cerebral peduncles. Within the cerebral peduncles, it relates to the frontopontine tract anteromedially, and to the temporo-parieto-occipital pontine tract posterolaterally. The cerebral peduncles have an oblique trajectory in the coronal plane, travelling from lateral to medial as they descend. The cerebral peduncles are separated by each other by the interpeduncular fossa. Here the third cranial nerves are closely related with the fronto-pontine tract laterally. The mammillary bodies are located at the floor of the third ventricle. The middle cerebellar peduncle has been transected in the upper ventral pontine region to display the underlying CST.  The optic tract covers the CST superiorly as it descends from the central core of the brain. (Image courtesy of JC Fernandez-Miranda)