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MANUAL OF HISTOLOGY. 



its peripheral extremity into a bulbous termination (see Fig. 

 133), from which the posterior root emerges. The central gray 

 matter between the two horns is traversed and intersected by 

 the decussating fibres from the lateral columns. Numerous pro- 

 longations from this gray matter spread out into the lateral 

 columns, presenting a coarse reticulum, called iheformatio 



FIG. 133. Diagram of transverse section of 

 human medulla below external decussation of 

 pyramids, showing bulbous posterior horns: 

 F R, formatio reticulnriR ; 11, spinal accessory 

 root and decnssation of the lateral columns. 



Flo. 134. Diagram. Decnssation of the pyra- 

 mids, shows decussation of the lateral columns, 

 the swelling of the posterior horns, the shrink- 

 age of the anterior horns, the spinal accessory 

 root 11, and a partial decussation of the posterior 

 columns behind the central canal. 



reticularis. The gray commissure is very broad, the central 

 canal having its long diameter directed antero-posteriorly. 



In sections at the decussation of the pyramids proper, i.e., 

 where they are seen to decussate externally, a slightly different 

 picture is presented. The lateral columns have nearly disap- 

 peared, having now almost all entered into the decussation, 

 which is here very broad (see Fig. 134), and presents a peculiar 

 zigzag appearance from the interweaving of bundles of fibres 

 from the opposite lateral columns. These fibres, after curving 

 around 'the anterior columns for a short distance, seem to dis- 

 appear by assuming a vertical direction. The club-shaped ex- 

 tremities of the posterior horns remain, while the rest is pushed 

 back into the posterior columns, and contains many large cells. 



The anterior horns are also displaced backward, pushed 

 back by the anterior columns increased in size by the addition 

 of the lateral columns. Hence, the anterior roots have a longer 

 path through the anterior columns and approach the type of 

 the hypoglossal nerve-roots seen a little higher up. (See Fig. 

 135.) The spinal accessory nerve curves out and back from the 

 lateral gray matter where a group of cells is situated. 



