510 MEDULLA OBLOXGATA. 



enlarged. Between the central canal (f) and the bottom of the anterior 

 median fissure are the decussating fibres. A little higher (fig. 357, B) the 

 caput cornu has increased in size and is connected with the neck only by 

 a network. The central grey substance is encroached upon by the fibres 

 fi'om the lateral columns which course forwards and inwards to the 

 decussation {p) and separate the anterior cornua fi'om the rest of the 

 grey substance. The substance of the posterior pyramid l:)ecome6 filled 

 with cells and fibres which are connected with the posterior cornu near 

 its origin. This constitutes the post-pyramidal nucleus or gang- 

 lion (jjpn). A little further out, on each side, a projection backwards 

 from the cervex cornu into the restiform body constitutes the rudiment 

 of the restiform nucleus {r n). Still higher up the medulla (fig. 357, C) 

 these masses of grey matter have increased in size, so that on a level with 

 the lower part of the olivary bodies, that which was the caput cornu 

 posterioris approaches the surface, behind the restiform l3ody, and 

 becomes known as the grey tubercle of Rolando (/ r C, D and E). Still 

 higher up, the post-pyramidal and restiform ganglia coalesce, and both 

 they and the grey tubercle become connected with the nuclei of origin 

 of certain nerves. 



The olivary nuclei which appear on the outer side of the anterior 

 pyramids are unconnected with the system of grey matter prolonged 

 from the spinal cord. 



The anterior pyramids are free fi'om grey matter in their interior, and 

 are separated from the rest of the medulla by strong septa of connective 

 tissue, and from one another by a raphe, which extends back to the 

 grey matter surrounding the central canal, and which contains mesial 

 horizontal fibres, named septal. 



Nerve nuclei. — A continuous series of collections of grey matter, 

 which extends from beneath the corpora quadrigemina, downwards along 

 the floor of the fourth ventricle and the centre of the medulla oblongata 

 as far as the decussation of the pyramids, constitutes the nuclei of origin 

 of the cranial nerves from the third to the hypoglossal. Those nuclei con- 

 tained in the medulla oblongata proper give origin to the hypoglossal, 

 the spinal accessory, the vagus, and the glosso-pharyngeal nerves. Just 

 above the decussation the nan-owed central grey matter which sur- 

 rounds the canal contains numerous very large multipolar cells, arranged 

 in definite groups in front and behind the canal, and symmetrical on 

 the two sides. Those in front of the canal (fig. 257, D xii') give 

 origin to the fibres of the hypoglossal nerve, which may be seen extend- 

 ing from them through the olivary body to emerge at its inner side. 

 From the cells behind the canal (xi'), the upper roots of the spinal 

 accessory nerve arise and pass out through the lateral column (xi). At 

 the point of the calamus scriptorius this nucleus becomes divided into 

 two parts as the central canal of the cord opens into the fourth ven- 

 tricle, and it increases in size at the expense of the adjacent part of the 

 posterior pyramid. Its cells then give origin to the fibres of the 

 pneumogastric or vagus nerve and constitute the vagal nucleus. Higher 

 up (E and F) the hypoglossal nucleus comes to the surface, and dividing, 

 pushes apart the two halves of the vagal nucleus, which have increased 

 in size, partly at the expense of the adjacent post-pyramidal nucleus. 

 Their cells frequently contain brown pigment and lie in a dense net- 

 work of fine nerve-fibres. Still higher the hypoglossal nuclei become 

 smaller, the vagal nuclei also lessen in size, and a group of cells appears 



