PONS VAROLII.
We distinguish a pars dorsalis and a pars basalis pontis. The pars dorsalis corresponds to the pars intermedia of the floor of the fourth ventricle. The pars basalis forms a broad white bolster, that expands transversely and is bounded in front by the cerebral peduncles and behind by the medulla oblongata. The lateral boundary is indicated by a line connect ing the points of emergence of the roots of the trigeminal and facial nerves. Lateral to this line, the pons narrows and passes on each side into the brachium pontis, or middle cerebellar peduncle, which extends backward and enters the cerebellum. The ventral surface of the pons is arched in the sagittal and transverse directions and exhibits a distinct transverse striation. These transverse fibres are grouped in three more or less well-defined bundles : The fasciculus superior pontis, which courses in advance of the attachment of the trigeminal nerve.
The fasciculus inferior pontis, in the lower third of the pons.
The fasciculus medius pontis, between the foregoing bundles, which crosses the fasciculus inferior in convex curves and runs towards the places of attachment of the facial and acoustic nerves. On account of this course, the bundle is also called the
fasciculus obliquus pontis or fasciculus arcuatus (Foville).
The ventral pontile surface is modelled in the mid-line by a broad furrow, the sulcus basilaris, in which the basilar artery usually lies. This furrow, however, is not caused by the basilar artery, but by the two adjacent longitudinal ridges, the eminenlia pyramidales, which contain the pyramidal tracts. The sulcus basilaris is present even when the basilar artery pursues an irregular course ; it disappears, however, in degeneration of the pyramidal tracts. The taenia pontis or fibra pontis is a special band of fibres that arises in or medial to the sulcus mesencephali lateralis, runs along the front border of the pons and disappears in the sulcus nervi oculomotorii ; these fibres are also called the fila lakralia pontis (Fig. 73).