CHAPTER X 



INTERNAL STRUCTURE OF THE PONS 



THE pons consists of two portions which differ greatly in structure and sig- 

 nificance. The dorsal or tegmental part resembles the medulla oblongata, of 

 which it is the direct continuation. The ventral or basilar portion contains 

 the longitudinal fibers which go to form the pyramids; but except for these it is 

 composed of structures which are peculiar to this level. It is a recent phyletic 

 development and forms a prominent feature of the brain only in those mam- 

 mals which have relatively large cerebral and cerebellar hemispheres, as might 

 be expected from the fact that it forms part of a conduction path uniting these 

 structures. 



THE BASILAR PART OF THE PONS 



The basilar portion of the pons is the larger of the two divisions. It is 

 made up of fascicles of longitudinal and transverse fibers and of irregular masses 

 of gray substance, which occupy the spaces left among the bundles of nerve- 

 fibers and which are known as the nuclei pontis. 



The longitudinal fasciculi of the pons consist of two kinds of fibers: (1) those 

 of the corticospinal tract, which are continued through the pons into the pyra- 

 mids of the medulla oblongata; and (2) those which end in the nuclei of the pons 

 and are known as corticopontine fibers (Fig. 106). As they pass through the pons 

 the corticospinal fibers give off collaterals which also end in these nuclei. The 

 longitudinal fibers enter the pons at its rostral border from the basis pedunculi. 

 At first they form on either side a single compact bundle; but this soon becomes 

 broken up into many smaller fascicles, which are separated from each other 

 by the transverse fibers and nuclei of the pons (Fig. 108). At the caudal border 

 these bundles again become assembled into a compact strand, which is con- 

 tinued as the pyramid of the medulla oblongata (Fig. 107). It is evident, how- 

 ever, that the volume of the bundles is much greater at the rostral than at the 

 caudal border. This is to be explained by the fact that the corticopontine 

 fibers have left these bundles during their passage through the pons and have 

 come to an end by arborization within the nuclei pontis. 



The transverse fibers are designated as fibres pontis and are divisable into a 

 superficial and a deep group (fibrae pontis superficiales and fibrae pontis pro- 



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