948 



THE FIBRES OF THE BULB. 



[BOOK in. 



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. 109, 2, 3, s.g., and forms on the surface a 

 slight projection, called the tubwcle of Rolando. It soon, how- 

 ever, becomes thrust ventrally 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. 109, 4, 5, G, R. 

 Retaining this position the islet of gelatinous substance diminishes 

 in size farther forwards, Fig. 110, s.g., and eventually disappears. 



611. 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 



n.f.t. 



FIG. 110. THROUGH THE BULB JUST BEHIND THE PONS. (Sherrington.) 

 Taken in ilie, line 110, Fig. 108. 



Py. Pyramids. R. Kestiform 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 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. n. nucleus of the 7th nerve. VIII. 

 auditory nerve, chiefly the dorsal or cochlear root; VIII. a. 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. 



broken up in the bulb ; indeed it will be best in treating of the 

 bulb, not to attempt to trace out these columns, but to speak of 

 the course of the several tracts into which these columns may be 

 divided. 



The direct and crossed 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. 



