THE PONS VAROLII.
The pons \Tarolii is a. broad and thick band of transverse fibres crossing in front of the upper continuation of the spinal cord and the medulla oblongata, interrupting the anterior median fissure, and connecting the opposite hemis pheres of the cerebellum. It is about one inch and a half broad from above downward, and nearly two inches in a lateral direction. It is contracted upon each side as it approaches the cerebellum, where it forms the middle pedun cles of that body. Its general shape is oval, and its. anterior surface is convex in every direction. A shallow depression crosses its surface, from above downward in the inedian line, called the basilar groove, which lodges the basilar artery. This portion of the pons is perforated by numerous openings for the passage of blood vessels. The upper margin of the. pons is bounded in the middle by the posterior perforated space and laterally by the fibres of the crustm. The third nerves arise from the inner sides of the crustm above this margin. its lower margin is bounded in the middle line by- the anterior median fissure and raphe of the medulla oblongata, and suc cessively- on each side of the fissure, from within outwards, by the anterior pyramid, olivary body, lateral tract, and the restiforrn body. Behind the pons Varolii is the floor of the fourth ventricle. The flocculi rest upon the outer extremities of its lower border in front of the restiform bodies. The sixth nerve emerges from between the pyratnid and the olive, the seventh. and eighth from internal to the flocculus and resti form body, and each of these nerves is directed forward beneath the lower border of the pons. The fifth nerve emerges from the middle of the anterior surface of the mid dle peduncle of the cerebellum.
The lower fibres of the superficial layer of the pons, which form the lower third of its anterior surface are trans verse in direction, and are overlapped at their outer extrem ities by those from above which arch downwards near their ter mination. The upper transverse fibres are arched upward in the middle and terminate in the lower lobes of the cerebellum on each side. The uppermost fibres of the pons are seen to proceed from the posterior perforated space, and winding close to the erns cerebri on each side are directed backwards to enter the cerebellum above the processes. These latter fibres do not properly belong to the pons Varolii, but are derived from the floor of the fourth ventricle, and passing forward through the raphe connect the gray matter of the fourth ventricle with the cerebellum. They are similar in function to the rest iform body of the medulla oblongata and are distributed to the corpus dentatum in conjunction with the inferior pe duncle of the cerebellum. Other fibres, derived from the floor of the fourth ventricle, enter the raphe and pass down wards beneath the lower margin of the pons. These emerge in front from the anterior median fissure of the medulla oblongata and winding over the pyramid and olive, in con junction with the arciform fibres, join the restiform bodies to terminate in the corpus deutatum of the cerebellum. Doubtless, also, there are associated with these, other fibres that correspond to the direct cerebellar tract of the spinal cord, and which will be better understood after this tract has been described.