712 THE NERVOUS SYSTEM. 



the floor of the fourth ventricle) found on the dorsum of the pons. Its further 

 consideration -will be postponed until the floor as a whole is described. 



The Restiform Bodies. These are the largest and thickest " columns " found 

 on the medulla. Each is a well-rounded mass of white fibres, and is directed 

 from below upward, outward, and somewhat forward, diverging from its fellow. 

 Its upper extremity is at the widest part of the medulla, where it bends, almost 

 at a right angle, directly dorsally away from the medulla, and immediately enters 

 the cerebellum. Hence a synonym of the restiform body is the inferior peduncle 

 of the cerebellum. Its lower extremity is somewhat tapering, and not so rounded 

 and prominent as are the succeeding portions. This is due to the fact that the 

 upper ends of the gracilis, cuneatus, and Rolandic columns are not quite on the 

 same level, the cuneatus reaching a little higher than the gracilis, and the 

 Rolandic column a little higher than the cuneatus. The fibres of these three 

 columns end here in a manner to be subsequently described. They do not enter 

 the restiform body, which does receive, on the contrary, all the fibres of the direct 

 cerebellar tract, previously mentioned. The " widening-out " of the medulla in 

 its growth explains the divergence and oblique position of the restiform body, as 

 well as the change in direction of the dorso-lateral groove, which separates the 

 restiform body from the olive, and out of which emerge the fibres of origin of 

 the seventh to the eleventh, inclusive, cranial nerves (except the lateral root of 

 the eighth). 



External Arciform Fibres. The external arciform or arcuate fibres are seen 

 on all three surfaces of the medulla. They are small, but vary in number in dif- 

 ferent medullse. They emerge from the anterior median fissure, between the 

 pyramids, and curve doi'sally on both sides. They pass over the pyramid and 

 olive, and then turn upward to join the restiform body. In doing so they often 

 conceal from view the upper part of the cuneate and Rolandic funiculi. Often 

 these fibres are collected into a well-marked bundle which crosses inferior to the 

 olive, thus obscuring the "lateral tract" and portions of the grooves between 

 the pyramid, olive, and restiform body. Sometimes they spread out over the 

 entire surface of the olivary body. 



Internal Structure. 



The internal structure of the medulla includes that of the whole medulla i. e. 

 its various surfaces, already described, as well as the deep portion surrounded and 

 included by these surfaces. 



The deep portion is divided into three bilateral areas, separated by a median 

 raphe or septum, each of which is known respectively as the anterior, lateral, and 

 posterior area of the medulla, and each of which corresponds to, or may be 

 regarded as having for its superficial or " surface " aspect, one of the subdivisions 

 of the surface of the medulla. Thus, the anterior area corresponds to one-half 

 of the decussation of the pyramids and to the pyramid of its own side ; the late- 

 ral area, to the olive and lateral tract ; the posterior area, to the restiform body, 

 floor of fourth ventricle, and the four small columns below viz. the direct cere- 

 bellar tract, the funiculus of Rolando, cuneatus, and gracilis. These areas, 

 observed in transverse sections, are seen to be somewhat wedge-shaped, especially 

 in the lower half of the medulla, and each to be separated from the adjacent one 

 by a line of nerve-jibres running dorso-ventrally through the substance of the 

 bulb (Fig. 417). Furthermore, the two anterior areas have between them the 

 raphe, while the two posterior, in the lower half of the medulla, are separated by 

 the posterior median fissure. The nerve-fibres referred to above are the root- 

 bundles of the hypoglossal nerve on the one hand, and, depending on the level 

 of the section, of either the seventh, glosso-pharyngeal, vagus, or spinal acces- 

 sory on the other ; the root-bundles of these last being, of course, in the same 

 perpendicular plane. These fibres are all proceeding from their various nuclei 

 of origin in the dorsal part of the medulla, to emerge, those of the hypoglossal 



