506 MEDULLA OBLONGATA. 



fissure. They consist of white fibres, continuous with tliose of the 

 posterior median columns of the cord, and contain much grey matter. 

 They increase in size as they ascend until they reach the point where 

 the medulla opens out to form the floor of the fourth ventricle ; and 

 there, diverging from one another, they appear to taper and become 

 closely applied to the restiform bodies. Their fibres quit these bodies, 

 however, and pass up to the cerebrum. 



The iioor of the fourth ventricle (figs. 355 and 358), or space 

 between the medulla and cerebellum, is formed by that portion of the 

 back of the medulla oblongata w^iich 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 scripforius (fig. 355, v), 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 ventride of 

 Aran fins, 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 longi- 

 tudinal 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 (fig. 358, 1). 



Surmounting the free inner margin of the restiform body and posterior 

 pyramid is a thin lamina, the lit/ula, occupying the angle between the 

 cerebellum and the restiform body, and stretching towards its fellow of 

 the opposite side. It derives a certain interest fi'om 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. 

 Transverse fibres. — Crossing the grey matter in the floor of the 

 fourth ventricle several transverse white lines, or strice meclullares, are 

 usually observed, passing outwards from the median fissure, and round 

 the sides of the restiform bodies (figs. 355 and 358, 2). Some of these 

 white stride 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 arp sometimes wanting, in which case the white 

 fibres on which they depend probably exist at some depth below the 

 surface. 



A set of superficial white fibres on the fore part and sides of the 

 medulla oblongata, crossing over it below the olivary bodies, was 

 described by Santorini and liolando as Mrcd vel processus arciformes. 

 They beloug 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 ex- 

 tremity 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 (fig. 357, E), and numerous 

 others pi'oceeding from that septum transversely outwards. Of these 

 last, the majority, entering the olivary bodies, form the whole of their 



