STRUCTURE OF MEDULLA OBLONGATA 499 



The medial lemniscus or fillet is placed close to the raphe, on the dorsal 

 aspect of the pyramidal motor fasciculus. 



The anterior column of grey matter is divided in a similar manner, by 

 the fibres of the lateral cerebro - spinal fasciculus as it passes from the 

 pyramid to the lateral funiculus of the opposite side of the medulla spinalis. 

 The basal part of the divided anterior grey column remains near the central 

 canal, but the head is displaced and forced dorso-laterally into the lateral 

 area of the medulla oblongata, where it becomes continuous with strands 

 of grey matter called the nucleus ambigiius and the nucleus laterahs. 



Half- way up the medulla oblongata the central canal> which has been 

 gradually approaching the dorsal surface, opens out into the cavity of the. 

 fourth ventricle, and the remains of the posterior grey columns, which 

 surrounded it at lower levels, are spread out on the floor of the ventricle in 

 such a manner that the portion which corresponds with the basal part of 



Gracile nucleus 



Funiculus 

 gracilis 



Decussation 

 of pyramids 



Funiculus 



cuneatus 



.Tractus spinalis 

 of fifth nerve 



Substantia 

 gelatinosa 

 Rolandi 



Dorsal spino- 



cerebellar 



fasciculus 



Detached head of 

 anterior column 

 of grey matter 

 Fasciculus proprius 

 anterior pushed aside 

 by the decussation 



Antero-median furrow 



FIG. 202. Transverse section through lower end of the Medulla Oblongata of 

 a full-time Foetus, treated by the Weigert-Pal method. The grey matter 

 is therefore bleached white ; whilst the medullated tracts are black. 



the anterior grey column of the spinal medulla is situated close to the 

 median plane, whilst the part which represents the base of the posterior 

 grey column of the spinal medulla occupies a more lateral position ; therefore 

 the hypoglossal nerve, which is a motor nerve, springs from the grey matter 

 near the median plane which is an upward prolongation of the motor or 

 anterior grey column, and the fibres of the glosso-pharyngeal and vagus 

 nerves, which are mainly sensory nerves, terminate in association with the 

 more laterally situated grey matter of the medulla oblongata which is 

 continued upwards from the base of the posterior grey column of the spinal 

 medulla. 



As the central canal opens out into the cavity of the fourth ventricle and 

 the basal parts of the posterior grey columns are forced laterally, the 

 ependymal epithelium of the dorsal wall of the canal is expanded into the 

 extensive epithelial membrane, which forms the roof of the lower part of 

 the fourth ventricle. 



Every section of the upper portion rfjfrthfe InftduHa oblongata is divided 

 into medial, lateral, and dorsa^r^tsWVhV9Jofcii|cf trfJfrBQglossal nerve 

 in 32 a 



