HISTOLOGY OF THE CENTRAL NERVOUS SYSTEM OF THE WEDDELL SEAL. 149 



senting the posterior longitudinal bundle : and ventral to this another band, not well 

 defined, which is the anterior longitudinal bundle. 



D. At the middle of the olivary body. (PI. I. fig. 4.) 



The main points in this region of the medulla are : the olivary nucleus, the restiform 

 body, the nuclei of the 12th and 10th cranial nerves, and outside both of these a group 

 of nerve cells which apparently represent the nucleus of the vestibular division of the 

 8th cranial nerve. 



Other well-marked features are the issuing fibres of the 12th, 10th, and 9th nerves : 

 the first of these pass down through the internal arcuate fibres to emerge from the 

 medulla between the olive and a mass of grey matter which has become separated from 

 the substantia gelatinosa ( ? nucleus ambiguus) : the fibres of the 10th nerve pass 

 through the arcuate fibres between the restiform body and the fibres of the 12th nerve 

 to emerge at the inner edge of the restiform : the fibres of the 9th nerve are only seen 

 for a short part of their course and emerge further forward (see next section). The 

 pyramids are now very well defined, and the tract of the fillet and the posterior 

 longitudinal bundle form characteristic features. 



E. At the upper level of the olivary body. (PI. I. fig. 5.) 



In this section, a layer of grey matter appears spreading over the floor of the 4th 

 ventricle, quite distinct from the subjacent nuclear groups ; of the latter groups, two, 

 representing nuclei of the 9th nerve, are to be seen just below and external to the 

 very obvious posterior longitudinal bundle. The fibres of the 9th nerve are seen 

 issuing between the olivary body and the restiform body, whilst external to the latter 

 some fibres of the 8th cranial nerve are to be distinguished. 



Several small groups of grey matter, just mesial to the olivary nucleus, may 

 possibly represent accessory olivary nuclei. 



The anterior longitudinal bundle, the fillet, and the pyramids all form marked 

 features from above downwards ; and the fibres which enclose and arch round the 

 restiform body ( ? continuation of the external arcuate fibres) form also a point worthy 

 of notice. 



III. HISTOLOGY OF THE PONS VAROLII. (PI. 1. figs. 6. 7, and 8.) 



A section across the lower pontine region of the brain shows the 4th ventricle 

 closed over by the superior medullary velum, and the floor of the ventricle lined by a 

 well-marked ependyma ; a thick layer of grey matter lies subjacent to this, and, in the 

 middle line, the posterior longitudinal bundle is one of the most prominent features 

 of the section. In the middle of this region there is to be seen the formatio reticularis, 

 and ventral to this, a fairly wide trapezium makes its appearance ventral to the 



(ROY. HOC. EDIN. TRANS., VOL. XLVIII., 857.) 



