870 



THE NERVE SYSTEM 



pia over this portion coalesce to form a delicate membrane the tela choridea 

 attached along the caudolateral boundary line of the rhomboidal fossa. Along 

 this attachment there is another intrusion of nerve substance between the ependymal 

 and pial layers; this reenforced lamina is usually termed the ligula and may be 

 traced to the clava and cuneate tubercle, thence laterad over the restiform body 

 to bound the lateral recess. The structure is probably a vestige of the secondary 

 rhomboidal lip and has actually been found to be a part of the pontobulbar body 

 referred to above. Another small semilunar lamina of nerve tissue bridges the 

 caudal apex of the fourth ventricle and is called the obex. This structure is 

 often devoid of nerve tissue, and is then a mere membranous lamina. 



Except in rare instances, the tela choroidea is perforated a short distance from 

 the calamus region. The opening is of variable shape and size; it permits of 

 communication between the ventricular cavity and the subarachnoid space and is 

 termed the foramen of Majendie (metapore; apertura medialis ventriculi quarti). 



POST-OEMINUM 



N. V 

 STRIA PONTIS 



N. V 



LOCUS CCERULEUS 

 FOVEA MCDIANA 



FOVEA TRIGEMIN 



EMINENTIA 

 ABDUCENTIS 



AREA VESTIBULARIS 

 STRIAE ACUSTICAE 



AREA NUCL. 

 INCERTI 



AREA N. 



'VESTIBULARIS 

 AREA NUCL. 

 FUN 1C. TERET. 



N. VIII 

 N. IX AND X 

 EMINENTIA HYPOGLOSSI 



NUCL. INTERCALATUS 



FOVEA VAGI 



LIGULA 



AREA POSTREMA 



NUCL. CUNEATUS 



OBEX 



NUCL. GRACILIS 



NTERCALA-I* 



AREA N. HYPOGLOSSI 



FIG. 634. Surface markings and topography of the principal nuclei of the floor of the fourth ventricle. 



(Modified from Streeter.) 



Similar apertures at the extremities of the lateral recesses, and called the 

 foramina Luschkae (apertura lateralis ventriculi quarti) also permit of a tidal flow 

 of the cerebrospinal fluid. 



The choroid plexuses of the fourth ventricle (metaplexuses) are highly vascular 

 infoldings of the tela choroidea, one on either side of the median plane, from each 

 of which offshoots extend laterad into the lateral recesses. As the choroid plexuses 

 of the brain are always formed by infoldings or invaginations of the membranous 

 portions of the brain tube, the ependymal continuity upon them is nowhere 

 interrupted. 



Internal Structure of the Medulla Oblongata. While the spinal cord remains 

 a closed tube with centrally situated gray, the medulla oblongata opens out on the 

 dorsal aspect so as to uncover its part of the neural canal as the "floor" of the fourth 



