METENCEPHAL ON 



7'2d 



not visil^le to the unaided eye, and therefore will be dealt with very l)riefly here. 

 It is usually classified into two categories: (a) Representatives of the grey crescents 

 of the spinal cord; and (6) nuclei or aggregations of grey matter not represented 

 in the spinal cord. As will be seen on consulting the section on the Spinal Cord, 

 the grey matter of the latter consists of a pair of bilaterally symmetrical grey cres- 

 cents which are united in the middle line b}' a grey commissure. In the centre 

 of this commissure is the central canal of the cord. The central canal extends 

 u])wards into the lower or distal part of the medulla, l)ut, in the upper part of the 

 last-named structure, the posterior columns diverge from one another so that th(^ 

 central canal opens out and expands into the floor of the fourth ventricle. There- 

 fore the ependyma which surrounds the central canal, together with the grey 

 masses at the bases of both anterior and posterior horns, is spread out on the floor 



Fig. 481.— Transverse Section of the Medulla a little above the Lower Extremity 

 OF THE Fourth Ventricle. (After Krause.) 



FUNICUL us SOLITA RIUS NUCLE CV OF A UDITOR Y 



yUCLEUS 

 FUNICULI 

 GRACILIS ET 

 CUNEA TI 



SUBSTANTIA 



GEL A TINOSA 



ASCENDING 



ROOT OF 



FIFTH 



VAGUS 



NUCLEUS 

 LATERALIS 

 VA G US 



EXTERNA L 



ACCESSORY 

 OLIVE 



ARCIFORM 

 FIBRES 



OLIVARY 

 NUCLEUS 



Nl CLEUS OF VAGUS 



SULCUS LONG I- 

 TUDINALIS 

 MELIUS 

 NUCLEUS OF 

 HYPOGLOSSAL 



IIYPOGI0SS\L 



PYRAMIDAL BODY 



INTERNA L 

 ACCESSORY 

 OLIVE 



RAPHE 



FISSURA 

 LONG ITU DI- 

 NALIS 

 ANTERIOR 



of the fourth ventricle. From the nerve-cells in this grey matter some of the 

 cranial nerves, which take their origin in the medulla, arise. The grey matter 

 of the base of the anterior horn suffers but little displacement, and lies in a 

 column on each side of the sulcus longitudinalis medianus, and from this a 

 motor nerve (the twelfth) arises. The sixth nerve arises in series with the twelfth 

 in the tegmental region of the pons, and the nuclei of the fourth and third 

 nerves continue this motor column of cells brainwards in the floor of the aqueduct 

 of Sylvius. The grey matter of the base of the i)osterior horn is displaced 

 laterally, so that it underlies the surface markings which have been described above 

 as ala cinerea and tuberculum acusticum, and from tliis grey matter the sensory 

 parts of the tenth and ninth nerves and part of the eighth nerve arise. The heads 

 of both of the horns of the grey crescents are also represented in the medulla. At 



