458 THE NERVOUS SYSTEM. 
The posterior longitudinal bundle (fasciculus longitudinalis medialis) is thus 
largely formed out of fibres, which in the cord constitute the anterior basis-bundle. 
These fibres are thrust back by the two decussations: the first decussation pushing 
them behind the pyramids, and the second decussation displacing them still further 
backwards to a position behind the fillet. 
Olivary Nuclei—The most conspicuous of the isolated clumps of gray matter 
in the medulla are the inferior olivary nucleus and the two accessory olivary 
nuclei. The inferior olivary nucleus (nucleus olivaris inferior) lies subjacent to the 
olivary eminence, and constitutes a very striking object in transverse sections 
through this region. It presents the appearance of a thick wavy or undulating line 
of gray matter, folded on itself, so as to enclose a space filled with white matter. 
It is in reality a crumpled lamina arranged in a purse-like manner, with an open 
mouth or sht, which is called the hilum (hilum nuclei olivaris), directed towards the 
Vago- Vacus ‘le 
glossopharyngeal agus nucleus 
roots Nucleus of the Fasciculus solitarius 
fasciculus solitarius Descending root of vestibular 
Ligula nerve (VIII.) 
Restiform body 
N“ 
Vago-glossopharyngeal roots 
Posterior longitudinal 
fasciculus 
3] Substantia gelatinosa 
7 Rolandi 
7 Spinal root of fifth nerve 
j Nucleus ambiguus 
>+Cerebello-olivary fibres 
Dorsal accessory olivary nucleus 
Anterior superficial arcuate fibres 
Fillet 
j 
a 
Mesial accessory olivary nucleus 
Inferior olivary nucleus 
2G LY 
Pyramid 
ll 
Le 
~ _ Arcuate nucleus 
Anterior superficial arcuate fibres 
Fic. 341.—TRANSVERSE SECTION THROUGH THE MIDDLE OF THE OLIVARY REGION OF THE HUMAN MEDULLA 
oR BULB. 
The floor of the fourth ventricle is seen, and it will be noticed that the restiform body on each side has 
now taken definite shape. 
mesial plane. The hilum does not reach either extremity, so that in transverse 
sections through each end of the nucleus the gray lamina is seen in the form of a 
completely closed capsule. Into and out of the open mouth of the olivary capsule 
streams a dense crowd of fibres. These constitute what is called the olivary 
peduncle. 
The accessory olivary nuclei are two band-like lamine of gray matter, which 
are respectively placed on the dorsal and mesial aspects of the main nucleus. In 
transverse section each of these nuclei presents a rod-like appearance. 
The mesial accessory olivary nucleus (nucleus olivaris accessorius mesialis) extends 
lower down in the medulla than the main nucleus, and it is much larger in its lower than 
its upper part. It begins immediately above the decussation of the pyramids, where it ds 
seen lying immediately on the outer side of the pyramidal tract and the anterior basis- 
bundle (Fig. 338). Higher up it lies across the mouth of the main nucleus and on the 
outer side of the fillet. The dorsal accessory olivary nucleus (nucleus olivaris accessorius 
dorsalis) is placed close to the dorsal aspect of the main nucleus. It begins at the level 
at which the main olive first presents a hilum, and it comes to an end at the level at 
which the central canal opens into the fourth ventricle. The two accessory nuclei fuse 
together before they finally disappear. 
The gray matter forming the three inferior olivary nuclei is crowded with small round 
cells, each of which is provided with one axon and numerous dendrites. It is traversed 
