THE CENTRAL NERVOUS SYSTEM. 



6 2 



horn, is pushed towards the lateral aspect of the medulla, and is con- 

 tinued upwards as the nucleus ambiguus, which is the nucleus of origin 

 of the cerebral fibres of the spinal accessory nerve and of the motor 

 fibres of the vagus, glossopharyngeal, facial, and trigeminal nerves. The 

 portion of grey matter which is continuous with the base of the anterior 

 horn lies behind the decussating pyramids, and in the upper part of 

 the medulla oblongata lies close to the floor of the fourth ventricle, 

 where it forms the nuclei of the hypoglossal nerves. The further 

 upward continuation of this part 

 forms the nuclei of the sixth, 

 fourth, and third nerves in the 

 mid-brain. 



The grey matter which is 

 continuous with the posterior 

 horn of the spinal cord lies 

 nearly transversely in a section 

 of the lower part of the medulla 

 oblongata, and two outgrowths 

 appear on its dorsal aspect, one 

 projecting into the funiculus 

 gracilis, the other into the funi- 

 culus cuneatus. These out- 

 growths form the nucleus gracilis 

 and the nucleus cuneatus respec- 

 tively, and in them the fibres of 

 the corresponding funiculi ter- 

 minate by arborisation. Most of 

 the axons of the cells of the 

 two nuclei pass forward, and cross the middle line as internal 

 arcuate fibres to form the lemniscus or fillet of the opposite side. This 

 decussation takes place just above that of the pyramids (fig. 17). The 

 fillet runs upwards dorsal to the pyramid to terminate in a cell station 

 in the thalamus. It is joined in the medulla oblongata by the spino- 

 thalamic fibres, which have already decussated in the spinal cord. 



The sensory decussation separates the base from the apex of the 

 posterior horn of grey matter. The base forms a column of grey 

 matter in which are found the sensory nuclei of the vagus and 

 glossopharyngeal nerves, and which is connected with the nuclei of 

 the vestibular nerve and of the sensory root of the facial nerve. The 

 head forms the spinal nucleus of the fifth nerve. 



The upper portion of the medulla oblongata is characterised mainly 

 (1) by the fact that the central canal comes to the surface posteriorly 



8 ) 10 



FIG. 17. Transverse section passing 

 through the sensory decussation. 

 (Schematic.) (Testut.) From Gray's 

 Anatomy. 



1, Anterior median fissure ; 2, posterior median 

 sulcus ; 3, 3', head and base of anterior horn 

 (in red) ; 4, hypoglossal nerve ; 5, bases of 

 posterior columns ; 6, gracile nucleus ; 7, 

 cuneate nucleus ; 8, 8, lemniscus ; 9, sensory 

 decussation ; 10, pyramid. 



