368 ANATOMY OF THE CENTEAL NERVOUS SYSTEM. 



Probably all the fibers of the posterior eolumns ultimately end in these 

 nuclei. From them, in turn, spring masses of fibers through the gray matter 

 forward, and cross over to the other side anterior to the pyramidal crossing. 

 These fibers later make up the fillet, hence this crossing is called the decus- 

 sation of the fillet. 



It is not very easy to be convinced of the existence of a fillet-crossing in a 

 fully-developed brain. But there remains no doubt at all, when one examines sec- 

 tions of the bulb in a fetus of seven months. There the medullary fibers of the 

 pyramidal decussation do not yet disturb the picture, the fibers of the posterior tracts 

 being more distinct through being earlier provided with medullary sheaths. At first 

 one sees principally only fibers coming from the nucleus cuneatus; toward the ninth 

 month, however, a little higher up, one may recognize the decussating fibers from 

 the nucleus gracilis. 



Comparing Fig. 233 with the two figures preceding it, one notices 

 behind the central canal the gray matter, much wider than before. In 

 the column of Goll is seen its nucleus; also in the column of Burdach 

 its nucleus; both are continuous with the gray matter. External to them, 

 surrounded by a thin layer of medullated fibers (spinal root of the tri- 

 geminal nerve), lies the substantia gelatinosa of the dorsal horn. In front 

 of it, in the space which in Fig. 232 is occupied by the dark pyramidal 

 fibers, is a clear place, because these fibers have not yet received their 

 medullary sheaths. The anterior ground-bundle and the cerebellar tracts 

 lying at the periphery of the lateral column are already medullated. 



From the nuclei of the posterior columns may be seen issuing fibers 

 which pass in curves (fibrce arciformes internal) through the gray matter, 

 decussate in front of the central canal, and dispose themselves in a thick 

 layer dorsal to the previously decussated pyramids. The territory occupied 

 by them corresponds to the anterior ground-bundle of the cord. The major 

 portion of the fibers ascending in these already decussated sensory tracts are 

 hereby pushed backward and outward by these newly arrived fibers. There- 

 fore, the united, crossed, sensory paths of the second order (secondary 

 neurons) gradually fill up all the space lying between the two new gray 

 masses, which arise in this level of the medulla oblongata, the olives {olivce 

 inferiores). The higher one goes in the oblongata, the poorer in fibers be- 

 come the posterior columns. Gradually they all pass by the arcuate fibers 

 into the fillet-crossing, and so to the opposite side, near the median line, 

 where they form the interolivary bundle, or, as we from now on may know it, 

 the fillet, for its fibers pass upward to the fillet of the midbrain (lemniscus). 



In Fig. 235 is presented a scheme of the course of the sensory fibers. 

 One may trace the direction of each bimdle by it, beginning at the entrance 

 of the root-fibers, and finally demonstrate where each one ends in the 

 medulla oblongata. Especially noteworthy is it that the fibers which have 



