The Motor Ganglion Cells of the Frog's Spinal Cord. 57 



lies in the presence of a number of small ganglion cells, ap- 

 parently belonging to the motor center, but whose small 

 size makes them hardly distinguishable from the '' granules " 

 of the cord. In the first frogs investigated, these cells were 

 not counted. Later it was found that they belong to the 

 small motor fibres and they were enumerated. In frog number 

 eighteen, whose large cells only were first counted, the 

 small cells were afterwards determined and about six hun- 

 dred were found on each side. 



The number of cells in the motor group varies in each 

 section from 2-3 to 25-30, according to its thickness and the 

 region of the cord to which it belonged. Single, very thin 

 sections from the center of the cord, occasionally contain no 

 cells. 



There is no difficulty in determining with fair accuracy the 

 number of cells, if all mechanical aids are employed, so that 

 individual attention can be given to counting. One source 

 or error, however, cannot be avoided. There is no natural 

 limit to the motor center toward the medulla oblongata, but 

 the cord gradually passes into that structure. The only 

 fixed point is that where the spinal canal opens into the 

 fourth ventricle, and this arbitrary line was chosen as the 

 only one which could be determined. The following table 

 gives the result of the counting. 



TABLE VII. 



The correspondence of the two sides is so close that it 

 might be supposed that the exactness was attained by uncon- 

 sciously following, on one side of each section, the number 



