PEDUNCULI CEREBRI.
The cerebral peduncles, with the substantia perforata posterior, form the ventral portion of the mid-brain and are bounded by the optic tract in front and by the pons and its peduncles behind (Figs. 15 and 75). Cross-sections of the mid-brain show a subdivision of the cerebral peduncle into a ventral segment, the basis pedunculi, and a dorsal area, the tegmentum. Between these subdivisions lies a grayish black substance In the form of a crescent, the substantia nigra of Summering. Superficially the basis and the tegmentum are separated by two furrows, medially by the sulcus nervi oculomotorii or sulcus mesencephali medialis and laterally by the sulcus mesencephali laterals. Dorsally the tegmentum is overlaid by the quadrigeminal plate.
The cerebral peduncles emerge from the pons as robust striated columns and extend divergingly toward the optic tracts, beneath which they disappear. The course of the fibre-bundles is worthy of note. They exhibit an outward and forward twist (Fig. 75). Between the cerebral peduncles lies the fossa interpeduncularis (Tarini), whose floor is formed by the substantia perforata posterior, penetrated by numerous apertures for the passage of blood-vessels. The posterior part of the fossa deepens toward the
pons into the recessus posterior, while the anterior part, toward the corpora mamillaria, sinks into the recessus anterior. The fossa is divided by a shallow median furrow into two symmetrical halves ; laterally, toward the cerebral peduncle, it is limited by the sulcus nervi oculomotorii, from which emerge the fibre-bundles of the nervus oculomotorius.
A special strand of fibres, the tractus peduncularis transversus, remains to be noted. This springs from the dorsal surface of the cerebral peduncle, between the brachium quadrigeminum posterius and the corpus geniculatum mediate, winds around the peduncle midway between the optic tract and the front border of the pons and dis appears in the sulcus nervi oculomotorii (Fig. 75). According to Marburg, the tractus peduncularis is identical with the basal optic root present in the lower vertebrates. The fibres are supposed to arise in the retina and finally end in the ganglion ektonzamillare located laterally to the corpus mamillare.