MEDULLA OBLONGATA AND TEGMENTUM OF THE PONS. 383 



they are represented by dots, but in the adult the two kinds of fibrse arci- 

 formes intemse are not to be differentiated. The examination of their 

 periods of medullation first rendered them distinct from each other. 



In the median line all these tracts must naturally cross with those from 

 the other side. This line, with its many crossing fibers, is called the raphe. 



The lemniscus, or fillet, contains in this level, besides the ascending 

 paths of the antero-lateral tract, also the largest part of those coming from 

 the nuclei of the posterior columns. The latter extend further forward 

 into the upper lemniscus. 



Notice also in the figure the fibrils passing from the fillet to the oppo- 

 site vagus nucleus; they are analogous to the decussating fibers from the 

 antero-lateral column to the posterior horn, the secondary vagus path. 



Dorsal to the lemniscus we find for the first time again the posterior 

 longitudinal fasciculus, mentioned in Chapter A^II. As low as the first 

 cervical nerves its fibers are found deep in the anterior column. 



On either side, external to the pyramids, lie the lower olives. They are 

 penetrated by the fibrse arcuatse, which, as we have learned, end in it in so 

 far as they come from the cerebellum, but pass through it in so far as they 

 come from the nuclei of the posterior columns. 



Lateral and dorsal to the olive lie the inner and posterior accessory olives, 

 nuclei similarly constructed to the olives themselves, and, like them, are 

 penetrated by the arcuate fibers. Through the first, the internal one, pass 

 the fibers from one olive to the other, while the posterior one is principally 

 pierced by the fibers from the posterior columns, as shown in the figure. 



Dorsal to the olive, in the region of the posterior accessory olive, a field 

 of white matter that from now on remains visible in the middle of the teg- 

 mentum, may be distinctly followed above, beyond the center of the 

 trigeminus. The whole of its fibers {central tegmental path) connects prob- 

 ably the olive with the midbrain (Beehterew). 



The dorsal periphery of the section is occupied by the nerve-centers. 

 Innermost lies the nucleus of the hypoglossus, whose fibers, passing through 

 the olivary region, press forward (c/. Fig. 240). From the raphe many fibers 

 pass to join it. External to it lies the pneumogastric nucleus. A remnant 

 of the lateral cells of the ventral horn lies as ventral or motor vagus nucleus 

 Just anterior to the posterior horn. The fibers arising from it form an angle 

 before their exit, around which roots from the sensory nucleus arborize. 



The thin bundle- of cross-cut nerve-fibers, lying external to the last- 

 mentioned sensory nuclei, is the combined vago-glosso-pharyngeal root, with 

 its nucleus attached. 



External to this common nucleus of the two nerves lie the nuclei of the 

 posterior columns, in front of which is the substantia gelatinosa from the 

 extremity of the dorsal horn of the cord. It is bounded externally by a 



