584 NERVOUS SYSTEM. 



PONS VAROLII. 



The Pons Yarolii (mesocephale, Chaussier) is the bond of union of the various 

 segments of the encephalon, connecting the cerebrum above, the medulla ob- 

 longata below, and the cerebellum behind. It is situated above the medulla 

 oblongata, below the crura cerebri, and between the hemispheres of the cere- 

 bellum. 



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 trans- 

 verse 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 Yarolii consists of alternate layers of transverse and 

 longitudinal fibres intermixed with gray matter (Fig. 327). 



The transverse fibres connect together the two lateral hemispheres of the cere- 

 bellum, and constitute its great transverse commissure. They consist of a super- 

 ficial and a deep layer. The superficial layer passes uninterruptedly across the 

 surface of the pons, forming a uniform layer, which consists of fibres derived 

 from the crus cerebelli on each side, meeting in the median line. The deep 

 layer of transverse fibres decussates with the longitudinal fibres continued up 

 from the medulla, and -contains much gray matter between its fibres. 



The longitudinal fibres are continued up through the pons. 1. From the an- 

 terior pyramid. 2. From the olivary body. 3. From the lateral and posterior 

 columns of the cord, receiving special fibres from the gray matter of the pons 

 itself. 



1. The fibres from the anterior pyramid ascend through the pons, imbedded 

 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 gray 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. 



Foville believes that a few fibres from each of the longitudinal tracts of the 

 medulla turn forwards, and are continuous with the transverse fibres of the pons. 



Septum. The pons is subdivided into two lateral halves by a median septum, 

 which extends through its posterior half. The septum consists of antero-pos- 

 terior 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 con- 

 tinued 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 comrnissural fibres. 



