T}IE XERXUUS SYSTEM 



457 



is more conspicuous in the thoracic cord, but extends from the seventh or eighth 

 cervical to about the third lumbar segment and is also present in the sacral cord 

 (especially the third segment), (pp. 462, 463 and 464.) 



From the above it is seen that all the cells of the dorsal and intermediate 

 gray, except the intermedio-lateral group, are column cells, either tautomeric, 

 heteromeric or hecateromeric. 



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(C) Cells of the Ventral Horx. — These fall into two categories: (i) 

 Column Cells. These may be tautomeric, sending axones to the adjoining white 

 matter or heteromeric, the axones of which cross in the ventral commissure to the 

 white matter of the opposite side. Among the latter may be a well-marked group 

 in the dorso-mesial part of the horn (commissural nucleus). Hecateromeric 

 cells may be present. (2) Root cells. Two main divisions may be distinguished: 

 (a) Mesial group, present throughout the cord above the fifth sacral segment 



