19(3 WILLIAM F. ALLEN 



from the masseter branch of this nerve go to the semilunar gan- 

 ghon or to the sensory root. It is no more surprising to find 

 muscle sense fibers in the purely motor components of the tri- 

 geminus than to find them in the oculo-motor nerve. 



NEURONS OF THE MESENCEPHALIC ROOT 



In order that the cells of origin of the descending mesen- 

 cephalic root fibers within the inferior colliculus and locus 

 coeruleus might be studied and compared with the sensory cells 

 of the semilunar ganglion and the motor cells of the trigeminal 

 motor nucleus, two guinea-pig brains with their left trigeminal 

 roots and ganglions intact were prepared after a slight modifi- 

 cation of the Cajal-Ranson silver method and were sectioned 

 serially. The technique employed was as follows: Material was 

 placed for two days in a mixture of equal parts of the following 

 solutions: a) 100 cc. absolute alcohol plus 1.5 cc. strong ammonia; 

 b) 100 cc. of 10 per cent formalin plus 3 cc. strong ammonia. Re- 

 moved to running water for twenty-four hours. Placed m 

 pyridin for twenty-four hours, dilute pyridin and running water 

 for twenty-four hours. Stained in 2 per cent silver nitrate for 

 seven days in an oven at 38°C. Reduced with a 2 per cent hy- 

 drochinon solution plus 5 cc. of formalin to every 100 cc. of 

 hydrochinon for twenty-four hours in an oven. Embedded after 

 the collodion-paraffin method and the sections were cut 20/x 

 thick. 



Figure 31a shows one of the ordinary semilunar ganglion cells 

 to be identical with one of the large spinal ganglion cells with 

 a coiled process. In figures 31b and 32 we have a cell from the' 

 locus coeruleus and two mesencephalic root cells from* the in- 

 ferior colliculus, all drawn to the same scale of magnification as 

 figure 31a. It is perfectly evident that the mesencephalic root 

 and locus coeruleus cells are globular unipolar cells of the same 

 size and type as the sensory cells of the semilunar ganglion, the 

 only difference being that their single process is not coiled about 

 itself If, on the other hand, a comparison is made with the 

 much larger multipolar cells of the trigeminal motor nucleus 

 (figs. 33e and 35), little in common will be found between these 



