598 THE BRAIN. 



reticular formation, of which we shall speak later on. It is at this level and 

 for some little distance above (Fig. 131, 4, 5) that these nuclei acquire their 

 greatest development. Further forward (Fig. 132, 6), when the fourth ven- 

 tricle has opened out and the nuclei of the cranial nerves are becoming con- 

 spicuous, and the posterior columns have been thrust aside laterally, both 

 these nuclei have diminished in size; still further forward they become still 

 smaller, and toward the pons they gradually disappear. 



The mass of gelatinous substance, forming at the level of the first cervical 

 nerve the swollen caput of the horn close to the surface but separated from 

 it by a band of fibres ( Fa.) of fine calibre, to which we have already referred 

 as belonging to the fifth cranial nerve, increases in bulk at a somewhat 

 higher level (Fig. 132, 2, 3, s. g.) and forms on the surface a slight projec- 

 tion, called the tubercle of Rolando. It soon, however, becomes thrust ven- 

 trally by the divergence of the posterior columns, and more and more covered 

 up by the fibres which are going to form the increasing restiform body (Fig. 

 132, 4, 5, 6, R). Retaining this position the islet of gelatinous substance 

 diminishes in size further forward (Fig. 133, s. g.), and eventually disappears. 



524. The fibres of the bulb. It is obvious, from what has already been 

 said, that the arrangement into posterior, lateral, and anterior columns, so 

 clear and definite in the spinal cord, becomes broken up in the bulb ; indeed 

 it will be best, in treating of the bulb, not to attempt to trace out these 



f!eui. 



Through the Bulb just Behind the Pons. v Sherrington.) Taken in the line 110, Fig. 131. Pt/. 

 pyramids ; R. restiform body ; Cbm. cerebellum ; F. fillet ; /. a. e. external,/, a. i. internal arcuate 

 fibres ; t. bundle of fibres from olive to the lenticular nucleus ; I. posterior longitudinal bundles; 

 n.f. t. nucleus of the fasciculus teres; s. o. superior olive; n. c. e. nucleus centralis (the marks 

 within it are sections of bundles of fibres by which it is traversed); s. g. substance of Rolando; 

 V. a. ascending root of fifth nerve; VII. a. nucleus of the seventh nerve; VIII. auditory nerve, 

 chiefly the dorsal or cochlear root; VIII. <* median nucleus; VIII. ft. lateral nucleus; VIII. y. 

 accessory nucleus of auditory nerve ; IX. fibres of root of ninth nerve passing through ascending 

 root of fifth nerve. 



columns, but to speak of the course of the several tracts into which these 

 columns may be divided. 



The direct and cross pyramidal tracts of the cord unite to form, as we 

 have seen, the pyramid of "the bulb, and so pass on to the pons. We need 

 say nothing more at present concerning this important pyramidal strand, 

 except that, as we trace it down from the pons to the spinal cord, it gives off 

 to the bulb itself fibres which make connections with the motor fibres of the 

 cranial nerves proceeding from this region. 



Concerning the course taken by the other less conspicuous " descending " 



