414 



PHYSIOLOGY 



nucleus gracilis, while those of the postero- external column, or 

 column of Burdach, of which the majority is formed by fibres from 

 the upper limbs, terminate in the 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 posterior aspect of 

 the medulla, bending round, as the arcuate fibres, to the deeper parts 



Posterior longitudinal fas 



Substantia gelatinosa Rolan< 

 Spinal root of fifth nerve 

 Nucleus ambiguus 

 Cerebello-olivary fibres 

 Dorsal accessory olivary nucleus 

 Anterior superficial arcuate fibres 

 Fillet 

 Mesial accessory olivary nucleus 



Inferior olivary nucleus 



FIG. 184. 



Pyramid 

 Arcuate nucleus 



Anterior superficial arcuate fibres 



Transverse section through the middle of the olivary region of 

 the human medulla. (CUNNINGHAM.) 



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

 columns on the posterior aspect of the cord. With the disappearance 

 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. 184) 

 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 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, con- 

 sisting of a thin layer shaped like a flask with its orifice directed 

 inwards, is developed in the lateral part of the medulla, between the 



