Chap, xvii.] THE MEDULLA OBLONGATA. 145 



188. (f) In the region of the pyramidal decussation 

 i.e., the lower part of the medulla immediately fol- 

 lowing the cervical portion of the cord the grey 

 matter of the cord is changed in its disposition by 

 the fasciculus of Tiirk or the crossed pyramidal 

 fasciculus passing en masse from the lateral column 

 of white matter through the anterior horns of the 

 grey matter. Hereby the anterior portion of the 

 grey matter of the cord is shut off from the rest of 

 the grey matter, and is found lying near the surface of 

 the lateral column of the lower portion of the medulla 

 as the lateral nucleus of grey matter (Fig. 88). The 

 main part of the anterior horn, however, is repre- 

 sented by the reticular formation of grey matter. 

 This contains in its lateral portion,, at any rate, the 

 same large multipolar motor ganglion cells with axis 

 cylinder processes and ground-network as the anterior 

 horn of the cord ; but in addition there are the 

 numerous bundles of medullated nerve-fibres passing 

 through it in transverse, oblique, and longitudinal 

 directions. Some of these fibres belong to the con- 

 tinuation of the anterior columns of white matter 

 of the cord, others join the fasciculus gracilis and 

 cuneatus, and a third kind pass out from the middle 

 line of the medulla. 



189. (g) The grey matter of the posterior horns of 

 the cord undergoes a change of disposition when passing 

 into the medulla. Its hindmost portion is gradually 

 shifted outwards by the development of the reticular 

 formation of grey matter, and in about the middle of 

 the medulla it is found lying near the surface of the 

 lateral column as the tubercle of Rolando. The rest 

 of the posterior horn remains at first collected around 

 the central canal; but as this gradually approaches 

 the posterior fissure, in order to open as the fourth 

 ventricle above, the grey matter gradually expands 

 laterally into the funiculus gracilis and cuneatus of 



K 1 



