INTERNAL STRUCTURE OF MEDULLA OBLONGATA AND PONS. 557 



TxENIA VENTRICULI 



QUART! 



olivary nucleus of that side or sweep round it. Ultimately, on the dorsal aspect 

 of the olivary nucleus, they are gathered together in the form of a conspicuous 

 group of arcuate fibres, which curve backwards to take up a position in the deep 

 part of the restiform body. In passing back, they traverse the tractus spinalis of 

 the trigeminal nerve and break it up into several separate bundles. The 

 olivo-cerebellar fibres thus connect the inferior olivary nucleus of one. side with 

 the opposite side of the cerebellum. Each part of the inferior olivary nucleus is 

 connected with a definite part. of the cerebellum. 



Decussation of the Pyramids and the Changes produced thereby. As we 

 examine, under the microscope, a series of successive transverse sections through the 

 inferior end of the medulla oblongata and the upper end of the spinal medulla, the 

 most striking change which meets the eye is the decussation of the lateral cerebro- 

 spinal tracts. From their position alongside the anterior median fissure of the 

 medulla oblongata most of the fibres of the pyramid cross the median plane 



and, after passing 

 through the anterior 

 column of gray mat- 

 ter, bend downwards 

 in the lateral funi- 

 culus of the oppo- 

 site side of the spinal 

 medulla. Strands 

 from the right lateral 

 cerebro-spinal tract 

 alternate with cor- 

 responding strands 

 from the left side, 

 and the interval be- 

 tween the bottom of 

 the anterior median 

 furrow and the gray 

 matter surrounding 

 the central canal be- 

 comes filled up with 

 a great mass of inter- 

 crossing bundles of 

 fibres. 



As a rule the 

 medial three -fourths 

 of the pyramid are 

 composed of fibres 

 which, lower down in 



lateral funiculus of the spinal medulla, form the fasciculus cerebro- 



whilst the lateral fourth of the pyramid proceeds downwards in the anterior 



iilus of the spinal medulla of the same side, as the fasciculus cerebrospinalis 



A considerable amount of variation, however, occurs in the proportion of fibres 



s allotted to the formation of these two tracts. Sometimes the lateral cerebro- 



tract is much larger than usual, and then the anterior cerebro-spinal tract suffers 



esponding diminution in size. Cases, indeed, occur in which the entire pyramid 



into the decussation, and in these there is no anterior cerebro-spinal tract. 



is not uncommon to meet with variations of an opposite kind which lead to 



sase of ^the anterior cerebro-spinal tract at the expense of the lateral cerebro- 



Sometimes the decussation is asymmetrical, and the corresponding cerebro- 



J on opposite sides of the spinal medulla are then unequal in size. One 



: often comes into play and causes asymmetry is the prolongation downwards 



pyramid on one side (usually the left) of some of the cerebro-pontine fibres. In 



iese fibres soon leave the pyramid and form the fasciculus circumolivaris. 



itions indicated above receive an additional interest when viewed in the light 



itiye anatomy. It would appear that only in man and the anthropoid apes is 



xtion of the pyramids in the inferior part of the medulla oblongata incomplete. 



XII. 

 [HYPOCLOSSAL] 



3- DIAGRAM OF THE FASCICULUS OLIVOCEREBELLARIS (CEREBELLO- 

 OLIVARY FIBRES). 



(This diagram has been constructed from the specimen figured on p. 555.) 

 X., Vago-glossopharyngeal nucleus. N.XIL, Hypoglossal nucleus. 



