THE MEDULLA OBLONGATA. 



309 



Let us next take up a section involving the lower end of 

 the olivary body. We have the following view presented. 

 The section is slightly cordiform. (See Fig. 135.) The decussa- 

 ting fibres at the base of what remains of the anterior fissure, 

 which has all along become shallower, now forms the com- 

 mencement of the rapke, a structure which extends all through 

 the rest of the medulla and pons, separating the two motor 

 tracts. The union of the lateral and anterior columns now 

 nearly complete, forms the anterior 

 pyramids. The fibres here have a 

 general vertical direction, except 

 that a broad band which emerges 

 from the decussation at the bottom 

 of the anterior fissure, curves around 

 the margin of the anterior pyramid, 

 and then, sometimes in the sub- 

 stance, sometimes at the surface of 

 the medulla, almost completely sur- 

 rounds it, the bundle becoming lon- 

 gitudinal on the posterior surface. 

 These bear the name of the arciform 

 fibres. The rest of the white matter 

 is so cut up as to render it hardly 

 divisible into regions. The central canal, which is very long 

 antero-posteriorly, has almost coalesced with the gradually 

 deepening posterior furrow soon to become the fourth ventricle. 



The gray matter originally in the cord is now collected 

 about the central canal. Anterior and external to the central 

 canal there is a small group of multipolar cells. This is the 

 remnant of the anterior horns, which have been continually 

 crowded back by the accumulation of fibres in the anterior 

 pyramids. These cells in every respect are similar to those in 

 the anterior horns. Their processes give origin to fibres which 

 course forward in two or three bundles through the white 

 matter of the anterior pyramids, and emerge at about the 

 junction of the anterior pyramids and the lateral white mass. 



A little farther back in the gray matter, behind the central 

 canal, is a small group of nerve-cells the remains of the spinal 

 accessory nucleus, from which a few fibres run in a straight 

 course outward and slightly backward, through the lateral 

 white matter. Additional collections of gray matter now begin 



FIG. 135. One half of section at lower 

 end of the olives : 11, upper spinal acces- 

 sory root ; 12, lower hypoglossal roots. 



