CH. XLIV.] TRANSVERSE SECTIONS OF BULB 635 



of the fourth ventricle. The central canal has now opened out into 

 the fourth ventricle, and the grey matter on its floor contains the 

 nuclei of the twelfth and tenth nerves ; bundles of the fibres of these 

 nerves course through the substance of the bulb, leaving it at the 

 places indicated in the diagram. 



The nucleus gracilis, nucleus cuneatus, are pushed into a more 

 lateral position, and higher up*- are replaced by small masses of grey 

 matter mingled with nerve-fibres (nucleus posterior) ; the restiform 

 body (C.r.) now forms a well-marked prominence, and the olivary 

 body is well seen with its dentate nucleus ; from the open mouth of 

 this corrugated layer of grey matter a large number of fibres issue, 

 and passing through the raphe, course as internal arcuate fibres to 

 the opposite restiform body, and thus to the cerebellum ; some pass 

 to the restiform body of the same side; the continuation of the 

 direct cerebellar tract of the cord also passes into the restiform body. 

 Its fibres terminate by arborisations round Purkinje's cells in the 

 vermis of the cerebellum. The continuation of the tract of Gowers 

 lies just dorsal to the olivary body. The funiculus solitarius and 

 nucleus ambiguus, also seen in this section, will be more fully con- 

 sidered in our^account of the origin of the ninth and tenth cranial 

 nerves. 



Fourth section (fig. 464). This is taken through the middle of 

 the pons, and shows much the same kind of arrangement as in the 

 upper part of the bulb. The general appearance of the section is, 

 however, modified by a number of transversely coursing bundles of 

 fibres, most of which are passing from the cerebellar hemi- 

 spheres to the raphe, and form the middle cerebellar peduncles. 

 Intermingled with these is a considerable amount of grey matter 

 (nuclei pontis). 



From the cells of the nuclei pontis, many of the fibres of the middle 

 peduncle take origin, and many fibres and collaterals of the pyramidal 

 tract arborise around them. The continuation of the pyramids (py) is 

 embedded between these transverse bundles. The pyramidal fibres 

 which terminate in the pons are situated poster o -laterally, and are 

 spoken of as cortico-pontine in contradistinction with those of the 

 pyramidal tract proper (cortico-spinal) which pass down through the 

 bulb to the cord. 



The pyramidal bundles are separated from the reticular formation 

 by deeper transverse fibres, which constitute what is known as the 

 trapezium (t). These fibres belong to a different system, and form 

 part of the central auditory path ; some of them connect the auditory 

 nuclei of the two sides together. The larger olivary nucleus is no 

 longer seen, but one or two small collections of grey matter (as.) repre- 

 sent it and constitute the superior olivary nucleus. These as well as a 

 collection of nerve-cells in the trapezium (nucleus of the trapezium) 



