366 



PHYSIOLOGY 



grey matter of the nucleus cuneatus. The cells of these two masses of 

 grey matter of course give off axons, which can carry on the impulses 

 brought to them by the fibres of the posterior columns. These axons speedily 

 leave the dorsal aspect of the medulla, bending round, as the arcuate fibres, 

 to the deeper parts of its structure. Thus nothing is left to take the place 

 of the posterior columns on the posterior aspect of the cord. With the dis- 

 appearance of these columns and the development.of the pyramids we get a 

 practical obliteration of the anterior fissure and a displacement of the central 

 canal towards the dorsal surface. A little higher up (Fig. 183) the canal 

 opens out altogether, forming the fourth ventricle, covered on its dorsal 

 surface only by a thin layer of ependyma, a simple epithelium representing 



sterior longitudinal fasciculi 



bstnntia gelatinosa Eolandi 

 pina! root of fifth nerve 

 XucJeus ambiguus 



Cerebdlo-olivary fibres 

 Dorsal accessory olivary nucleus 



Anterior superficial arcuate fibres 



Fillet 



Mesial accessory olivary nucleus 



Inferior olivary nucleus 



Pyramid 

 Arcuate nucleus 



Anterior superficial arcuate fibres 



FIG. 183. Transverse section through the middle of the olivary region of the human 

 medulla. (CUNNINGHAM.) 



all that is left of the dorsal wall of the primitive cerebral vesicle. The 

 appearance of the section is now modified 'by two structures. In the first 

 place, a new mass of grey matter, consisting of a thin layer shaped like a 

 flask with its orifice directed inwards, is developed in the lateral part of the 

 medulla, between the pyramids in front and the tubercle of Rolando behind. 

 This is the olivary body, and has on its inner and dorsal sides two little grey 

 masses which are the accessory olivary bodies. The other feature is the new 

 relay of sensory fibres which start from the dorsal nuclei, the nuclei gracilis 

 and cuneatus. , These lil>n-< run out \\anls ;m <l ionvards from the nuclei 



