Its under surface presents a broad transverse band of white fibres, which arches like a bridge across the upper part of the medulla, extending between the two hemispheres of the cerebellum. This surface projects considerably beyond the level of these parts, is of a quadrangular form, rests upon the basilar groove of the occipital bone, and is limited before and behind by very prominent margins. It presents along the middle line a longitudinal groove, wider in front than behind, which lodges the basilar artery; numerous transverse striae are also observed on each side, which indicate the course of its superficial fibres.

  Its upper surface forms part of the floor of the fourth ventricle, and at each side it becomes contracted into a thick rounded cord, the crus cerebelli, which enters the substance of the cerebellum, constituting its middle peduncle.

  Structure. The pons Varolii consists of alternate layers of transverse and longitudinal fibres intermixed with grey matter (fig. 240).

  The transverse fibres connect together the two lateral hemispheres of the cerebellum, and constitute its great transverse commissure. They consist of a superficial and a deep layer. The superficial layer passes uninterruptedly across the surface of the pons, forming a uniform layer, consisting of fibres derived from the crus cerebelli on each side, which meet in the median line. The deep layer of transverse fibres decussate with the longitudinal fibres continued up from the medulla; they also connect the hemispheres of the cerebellum.

  The longitudinal fibres are continued up through the pons. I. From the anterior pyramidal body. 2. From the olivary body. 3. From the lateral and posterior columns of the cord, receiving special fibres from the grey matter of the pons itself.

  1. The fibres from the anterior pyramid ascend through the pons, embedded between two layers of transverse fibres, being subdivided in their course into smaller bundles; at the upper border of the pons they enter the crus cerebri, forming its fasciculated portion.

  2. The olivary fasciculus divides in the pons into two bundles, one of which ascends to the corpora quadrigemina; the other is continued to the cerebrum with the fibres of the lateral column.

  3. The fibres from the lateral and posterior columns of the cord, with a bundle from the olivary fasciculus, are intermixed with much grey matter, and appear in the floor of the fourth ventricle as the fasciculi teretes, they ascend to the deep or cerebral part of the crus cerebri.

  Septum. The pons is subdivided into two lateral halves by a median septum, which extends through its posterior half. The septum consists of antero-posterior and transverse fibres. The former are derived from the floor of the fourth ventricle and from the transverse fibres of the pons, which bend backwards before passing across to the opposite side. The latter are derived from the floor of the fourth ventricle, they pierce the longitudinal fibres, and are then continued across from one to the other side of the medulla, piercing the antero-posterior fibres. The two halves of the pons, in front, are connected together by transverse com-misural fibres.