THE NERVOUS SYSTEM 491 



(c) tract from nucleus of medial longiLutlinal fasciculus (mesial), (d) colliculo- 

 spinal (mesial), (e) shorter descending and ascending tracts which may be re- 

 garded as the equivalent of the ground bundles of the cord comprising shorter 

 reticulo-spinal and spino-ret'cular fibres. The shortest of these fibers, which in 

 the cord were next the lateral gray, arc now mingled with the gray, the combina- 

 tion constituting the gray reticular formation. Other short intra- and interseg- 

 mental tracts lie in and adjoining the dorsal horn, as in the cord. 



Descending suprasegmental paths include certain of the above long descend- 

 ing intersegmental tracts as previously explained. Besides these there are the 

 efferent suprasegmental neurones known as the pallio-spinal or pyramidal tracts 

 and the colliculo-spinal tracts. Bundles of fibres are seen crossing {pyramidal 

 decussation) from the anterior pyramid of one side to the opposite dorso-lateral 

 column, where they turn downward as the crossed pyramidal tract. In their 

 passage through the gray matter, they cut off the ventral horn from the rest of 

 the gray matter. These fibres, as already noted in the cord, are descending 

 axones from motor cells situated in the precentral cerebral cortex. In the pyram- 

 idal decussation most of these fibres cross to the opposite dorso-lateral region 

 to pass down the cord as the crossed pyramidal tract (p. 470, and Fig. 326; 

 Fig. 330, F). The remaining fibres stay in their original anterior position and 

 continue down the cord as the direct pyramidal tract (p. 471, and Fig. 326; 

 Fig. 330, F). A few pass to the ventral tract in the same side, thus becoming 

 uncrossed fibres in the lateral tract. The bundles of fibres do not cross in a trans- 

 verse plane, but take a downward direction at the same time. For this reason 

 transverse sections show these fibres cut rather obliquely. Because of the fact 

 that the fibres cross in alternate bundles, the number of decussating fibres seen 

 in any one section is greater on one side than on the other (Fig. 332). 



2. Transverse Section of the Medulla through the Decussation of the Fillet 

 or Lemniscus (Sensory Decussation) (Figs. 332 and 334) 



The most conspicuous features are the appearance of the nuclei cuneatus and 

 gracilis, the decussation and formation of the medial lemniscus or fillet, and the 

 increase of the gray reticular formation. 



Peripheral Efferent Neurones. — In the lateral part of the central gray is the 

 dorsal nucleus of the X {nucleus alee cinerecB) . In the ventral part of the central 

 gray is the nucleus hypo gloss i and, passing ventrally and emerging lateral to the 

 pyramids, may be seen the axones of its cells — the root fibres of the XII. 



In the nucleus XII can be distinguished (Weigert stain) coarse fibres which 

 are the root fibres, and fine fibres which are terminals of other fibres ending in 

 the nucleus. Among these have been distinguished collaterals from secondary 

 vagoglossopharyngeal and trigeminal tracts (three-neurone reflex, and from 

 various parts of the reticular formation. Whether pyramidal fibres reach the 

 nucleus directly or via intercalated neurones is uncertain. 



Afferent Roots, their Terminal Nuclei and Secondary Tracts. — Entering 

 afferent root fibres are usually not present. 



The funiculi or fascicuH cuneatus and gracilis have diminished, and internal 

 to them have appeared large masses of gray. These are the nuclei of the columns, 



