366 ANATOMY OF THE OENTEAL NEETOUS SYSTEM. 



which join the different levels together. Earlier in this work, in the 

 consideration of the brains of lower animals, attention was called to the 

 fact that regularly at the border between the spinal cord and the bulb this 

 great commissural system appears, or strengthens that already existing. It 

 was there designated as the association-field of the medulla. 



Above the level just described begins the rearrangement of fibers, etc., 

 which brings about the changes observed in the cross-section of the bulb. 



We have learned in the cord of two fasciculi cortico-spinales, one of 

 which, lying in the anterior column, conducts toward the brain fibers which 

 are derived on the opposite side from all the different root-regions, and a 

 second, the crossed pyramidal tract, which contains fibers from the cor- 

 responding ventral horn. 



At the upper end of the cord this latter tract enters the anterior column 

 of the opposite side, by large bundles, which break through the ventral horn 

 of the same side. There it meets the anterior or direct pyramidal tract, and 

 beyond this the uncrossed tractus cortico-spinalis passes with the crossed as 

 pyramidal tract to the brain. The dorsal horns move farther forward, just 

 as the place in the lateral column occupied by the crossed pyramidal tract 

 becomes free. 



A few millimeters higher still the pyramidal decussation is complete. 

 There lie, then, now the crossed pyramidal fibers with the direct pyramidal 

 fibers in one large bundle, ventral in the medulla. This is distinctly shown 

 in Fig. 232. It will be seen, too, that the anterior ground-bundle (Fa') is 

 placed dorsal to the pyramids. External to the separated remnants of the 

 ventral horn a small gray focus is seen. It belongs to the lowest point of the 

 olivary body. The olive enlarges materially as one ascends, and fills a large 

 part of the room in the lateral tracts. These latter, from the appearance of 

 the processus reticulares, have become richer in fibers. The association-field 

 develops more and more. However, the paths are only traceable a short dis- 

 tance. Many end in scattered, small groups of ganglion-cells (nucleus of the 

 lateral tract, nucleus reticularis tegmenti, etc.). 



The rearrangement of fibers, the crossing of the lateral pyramidal tract 

 over to the opposite anterior column, is well shown in the accompanying 

 drawings by Henle (Figs. 231 and 232). The separated ventral horns can be 

 traced farther upward, but disappear at about the level of the pons. 



The pyramidal tracts are seen on all the following sections, lying be- 

 tween the olivary bodies anteriorly (see illustrations in following chapter). 

 Later they are covered over and divided into bundles by the transverse fibers 

 of the pons. How they emerge farther on beyond the pons and pass through 

 the pes cerebri into the internal capsule has been repeatedly described in 

 preceding chapters. It has also been explained that the secondary degener- 

 ation, which takes place downward after interruption to the cortieo-spinal 



