796 



THE NERVOUS SYSTEM 



medulla oblongata ; its intermediate portion is the dorsal surface of the pons region, 

 while its superior portion belongs to the isthmus of the rhombencephalon. Its tri- 

 angular lower extremity terminates as the opening of the central canal of the spinal 

 cord. This portion is deepened at the obex and shows furrows which point down- 

 wards and converge median wards, giving the appearance known as the calamus 

 scriptorius. The mid-line of the ventricle is sharply distinguished by the well- 

 marked median sulcus, which becomes shallower above than below. Throughout 

 the length of the floor on either side of the median sulcus is a continuous ridge, the 

 medial eminence, which is bounded laterally by the limiting sulcus. Underlying 

 the floor of the ventricle is a layer of grey substance of varying thickness, which is 

 continuous with that surrounding the central canal of the cord. The medial emi- 

 nence is subdivided into portions of unequal width and elevation, and the limiting 

 sulcus accordingly shows fovese of different depths. 



FIG. 591. DORSAL SURFACE OP THE BRAIN-STEM SHOWING THE ANATOMY OF THE FLOOR OF THE 

 FOURTH VENTRICLE. (Modified from Spalteholz.) 



MEDIAN SULCUS 



\ 



SUPERIOR FOVEA 



LIMITING SULCUS 



c"'> 



EMINENCE "*-[ 







STRIJK MEDULLARES 



INFERIOR FOVEA 



NUCLEUS FUNICULI CUNEATI 



T.EXI.l OF FOURTH VEN 1 . 



AREA POSTREMA X 



NUCLEUS FUNICULI GRACILIS 



(CLAVA) 



POSTERIOR MEDIAN FISSURE 



A <l I '.-T.D UCTUS CEREBEI 



NUCLEUS INCERTUS 



LOCI'S CJKBCLEUS 



EMIXKXTIA 



t xrci.Krx 



I:"/' 



KlTLEl'S N. 

 COCHLEARIS 



.~*'^. (ri'iu:i:<Ti.i'M 



ACVBUCVJt) 

 AREA ACI'STICA (XI'- 

 CLEf'N VEST1BULARIS) 



'-- NUCLEUS INTERCALATUS 



~-~EMIXEXTIA (TRIC(IM'M) X. 

 BYPOOLO881 



TRIGOXl'M VAGI (ALA CIXEREA) 





FUXICULVS SEPARAKS 

 \ OBEX 



Beginning at the calamus scriptorius the following areas of the floor of the fourth 

 ventricle are usually distinguished: 



The area postrema of Retzius is a superficial vascular structure bounded in- 

 feriorly by the tsenia and overlying the terminal portion of the nucleus funiculi gracilis 

 (clava) and a portion of the nucleus of the vagus nerve. The funiculus separans, 

 a short oblique fold of the floor, composed chiefly of neuroglia, separates the area 

 postrema from the ala cinerea (triqonum ragi), which is an oblique, grey-coloured 

 eminence indicating the position of the middle third of the nucleus of termination 

 (recipient nucleus) of the vagus and glosso-pharyngeal nerves. At the superior ex- 

 tremity of the ala cinerea is a well-marked triangular depression of the limiting 

 sulcus known as the inferior fovea. Mesial to and extending above the ala cinerea 

 is a narrow eminence lying close to the median sulcus, which represents the nucleus 

 of origin of the twelfth nerve, the hypoglossal eminence (trigonum hypoglossi). 

 The nucleus intercalatus of Van Gehuchten is a wedge-shaped portion very slightly 

 demarcated from the hypoglossal eminence, and lying between it and the inferior 

 fovea. This nucleus is considered by some observers as an inferior medial exten- 



