878 THE NERVE SYSTEM 



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The longitudinal fibres consist chiefly of the pyramidal tracts, which are solid 

 strands at their entrance to and exit from the pons, but are broken up into lesser 

 bundles at its middle. A certain number of the pyramidal fibre's, as well as 

 other cerebropontile fibre tracts, terminate in relation with the cells of the nuclei 

 pontis, as well as certain of the efferent cranial nerve nuclei. This fact accounts 

 for the demonstrable diminution in bulk of the pyramidal tract in its course 

 through the pons. 



The nuclei pontis are small aggregations of gray substance (which in serial 

 sections show them to be continuations of the arcuate nuclei) diffusely scattered 

 among the fibre systems of the pons proper. They are intercalated in the course 

 (1) of tracts passing from one cerebellar hemisphere to the other, and (2) of de- 

 scending cerebropontile tracts. The cells of the pontile nuclei send their axones 

 chiefly to the opposite cerebellar hemisphere and play an important part as links 

 in the complex chain of the neurone systems which make the cerebellum such an 

 important organ of sensomotor coordination. 



In the contact zone of pons proper and the pontile tegmentum lies a group of 

 transversely decussating fibres with interspersed gray masses with large cells 

 called the trapezium. This body will be more fully described in connection with 

 the central auditory paths. 



The tegmental part of the pons is of much smaller bulk than the pons proper, 

 as seen on trans-sections. On the dorsal surface is spread a layer of gray substance 

 covered by ependyma, which forms the floor of the cephalic part of the fourth 

 ventricle. Beneath this gray substance lies the formatio reticulans divided into 

 symmetrical halves by the raphe continued from the medulla oblongata. Em- 

 bedded in the formatio reticularis are various isolated masses of gray substance 

 and various more or less compact fibre tracts. Among the gray masses are several 

 of the cranial nerve nuclei, to be described in a separate section, and the following: 



The superior olivary nucleus (nucleus olivarius superior) is a small gray mass 

 or aggregation of several smaller masses situated laterad of the trapezium, inter- 

 calated in the path of the trapezial fibres and forming a link in the central acoustic 

 chain (Fig. 649). 



The nucleus incertus (Streeter) is an aggregation of gray substance in the floor of 

 the fourth ventricle near the median sulcus and forming a slight, rounded elevation 

 which extends to the aqueduct. Its functional relations are unknown. 



Fibre Tracts in the Pars Dorsalis Pontis. Among the fibre tracts in the tegmental 

 part of the pons the chief ones are (1) the medial lemnisci, (2) the lateral lemnisci, 

 (3) the medial longitudinal bundle, and (4) the superior peduncles of the cerebellum. 



Each medial lemniscus, or medial fillet, in its passage through the tegmental 

 part of the pons is. gathered into a compact, ribbon-like bundle along the contact 

 zone of the tegmentum and pons proper, lateroventrad of the trapezium, some 

 fibres of which traverse it on their way toward the raphe. The medial lemniscus 

 has been described in the medulla oblongata as occupying the field between raphe 

 and inferior olivary nucleus (the interolivary stratum); in its ascent the medial 

 lemniscus gradually trends laterad, so that it almost reaches the surface (Figs. 

 639 and 645). 



The lateral lemniscus is a constituent of the central auditory path, and will be 

 described more fully on pages 883 and 901. In trans-sections above the level of 

 the trigeminal nuclei the lateral lemniscus is seen as a flattened band spreading 

 over the surface (externally the trigonum lemnisci) ectad of the superior peduncle. 

 Its fibres are interrupted by an intercalated nucleus of the lateral lemniscus. 



The medial longitudinal bundle (posterior longitudinal bundle) maintains its 

 position just ventrad of the central gray, close to the raphe. 



The superior peduncle of the cerebellum or prepeduncle is seen in trans-sections 

 to be a very compact bundle of crescentic outline with the concavity turned 



