THE NERVOUS SYSTEM 497 



sisting principally of the rubro-spinal tract and uncrossed Deitero-spinal fibres, 

 lies mesial to, or partly mingled with, the spino-thalamic and ventral spino-cere- 

 bellar tracts. The mesial part of the U, consisting principally of crossed and 

 uncrossed Deitero-spinal fibres and fibres from other nuclei in the reticular 

 formation, and of fibres from the nucleus of the medial longitudinal fasciculus, 

 now forms the medial longitudinal fasciculus dorsal to the fillet. Near this 

 bundle, or united with it, is the coUiculo-spinal tract (predorsal tract). When these 

 tracts have passed down to below the formation of the fillet and the olives, they 

 assume I he positions noted in the lower levels of the medulla. 



Efferent Suprasegmental Neurones. — The pyramids are the same. Small 

 bundles of more lightly stained fibres present in the fillet here and in higher 

 levels (Weigert stain, not indicated in the figures) are efferent pallial fibres 

 detached from pes or pyramids. They are aberrant fibres which rejoin the pyra- 

 mids or are fibres innervating motor cranial nuclei. The colliculo-spinal tract 

 (see above). 



4. Transverse Section of the Medulla through the Middle of the Ohvary 



Nucleus 



Such a section is so similar to 3 and 5 that its detailed description may be omit- 

 ted. The nucleus cuneatus has disappeared; the fillet increased somewhat; 

 fasciculus solitarius and descending vestibular root have increased; also their 

 terminal nuclei. The olivary nucleus, olivo-cerebellar fibres, and the restiform 

 body have greatly increased. The formatio reticularis has increased in extent. 



5. Transverse Section of the Medulla through the Entrance of the Cochlear 



Root of Nerve VIII (Figs. 332 and 337) 



Efferent Peripheral Neurones. — The dorsal vagus nucleus is not present, 

 but the nucleus ambiguus is usually present and probably sends some axones to 

 nerve IX, passing out with the afferent fibres (see below). The nucleus XII 

 has disappeared and also its root fibres. 



Afferent Roots, their Terminal Nuclei and Secondary Tracts. — Usually 

 the afferent root fibres of nerve IX are present. They enter on the lateral aspect 

 of the medulla ventral to the restiform body, traverse the spinal V, and pass to 

 the fasciculus solitarius or its terminal nucleus. The fasciculus solitarius 

 is smaller, and just above the entrance of the IX consists of only a comparatively 

 few descending afferent root fibres of the VII. 



The fibres of the cochlear nerve enter the extreme lateral angle of the medulla, 

 where many or, according to some, all of them terminate in two masses of cells 

 enveloping externall}^ the restiform body and known as the ventral (or accessory) 

 and dorsal (or lateral) cochlear nuclei. ]Most of the axones of the dorsal nucleus 

 pass across in the floor of the ventricle {strice medullares) to form a part of the 

 opposite secondary tract {lateral lemniscus) of the cochlear nerve. The axones 

 of the ventral nucleus also decussate, but by a more ventral route, (trapezius), 

 and also form a part of the lateral lemniscus. This latter decussation takes 

 place at a higher level (see next section). 



