STEUCTUHE OF THE MEDULLA OBLONGATA. 517 



The floor of the fourth ventricle, or space between the medulla and cere- 

 bellum, is formed by that portion of the back of the medulla oblongata 

 which is situated above the divergence of the posterior pyramids. Upon it, 

 the central grey matter of the medulla oblongata, is, as it were, opened out 

 to view. It is marked by a median furrow, ending inferiorly in the calamus 

 scri2Jtorius, and at its lower end is a tubular recess, passing down the centre 

 of the medulla for a few lines. This, which has been sometimes named the 

 ventricle of Arantius, is the upper expanded portion of the central canal of 

 the spinal cord. 



In the upper part of the floor of the fourth ventricle are two longitudinal 

 eminences, one on each side of the middle furrow, greyish below, but 

 appearing white higher up. These are formed by two bundles of white 

 fibres, mixed with much grey matter, the fasciculi teretes of some authors, 

 les faisceaux innomines of Cruveilhier. They seem to be derived from part 

 of the lateral columns of the cord ; Cruveilhier believes, however, that they 

 arise from the grey matter at the lower end of the medulla oblongata. 



Surmounting the free inner margin of the restiform body and posterior 

 pyramid is a thin lamina, the lifjula (smaller pous of Meckel) occupying the 

 angle between the cerebellum and the restiform body, and stretching towards 

 its fellow of the opposite side. It derives a certain interest from indicating 

 how the cylinder, which is closed in the spinal cord, might be completed in 

 this region of the medulla oblongata by the union of the opposite margins. 



Crossing the grey matter in the floor of the fourth ventricle several 

 transverse white lines, or stride, are usually observed, passing outwards from 

 the median fissure, and round the sides of the restiform bodies. Some of 

 these white strise form part of the roots of the auditory nerves, a few run 

 slantingly upwards and outwards on the floor of the ventricle, whilst others 

 again embrace the corresponding half of the medulla oblongata. These 

 transverse lines are sometimes wanting, in which case the white fibres on 

 which they depend probably exist at some depth below the surface. 



Santorini, and subsequently Kolando, described a set of superficial white fibres on 

 the fore part and sides of the medulla oblongata, crossing over it below the olivary 

 bodies, fibrce vel processus arciformes. They belong to a system of white fibres which 

 pass transversely or horizontally outwards, and are probably continuous with the 

 septal fibres about to be noticed. Sometimes the greater part of the pyramidal and 

 olivary bodies is covered by a thin stratum of these transverse fibres, which appear to 

 issue from the anterior median fissure ; but, most commonly, these superficial fibres 

 are found only at the lower extremity of the olive, as the arciform fibres already 

 mentioned. 



Besides the superficial transverse fibres now referred to, the medulla oblongata 

 presents other horizontal fibres in its interior, some of them disposed in a mesial 

 raphe or septum, and numerous others proceeding from that septum transversely 

 outwards. Of these last, the majority, passing through the olivary bodies, and in part 

 the pyramids, enter the corpus dentatum and form the whole of its white substance ; 

 and these fibres, then passing radiately through the grey capsule, turn backwards to 

 the fasciculus cuneatus and lateralis, those of them which pierce the anterior wall of 

 the capsule arching round it to reach their destination. Other fibres pass behind the 

 olivary into the restiform bodies, and seem to terminate in the grey substance of the 

 floor of the fourth ventricle. (See Kolliker's Handbuch der Gewebelehre, 1863, 

 p. 316.) 



A small band of fibres is represented by Eeichert as passing obliquely downwards 

 and backwards from the side of the pons Varolii, descending between the auditory 

 and facial nerves, and crossing over the upper end of the posterior pyramids. He 

 names it the ala pontis. It probably is part of the ligula. (Keichert, Bau des 

 Menschl. Gehirns, part 1st, plate I., 1859.) 



