MESENCEPHALIC ROOT 197 



two types of cells, even though the locus coeruleus cells may 

 be located directly above the trigeminal motor nucleus. The 

 writer agrees with Johnston that the position of the globular 

 unipolar mesencephalic root cells in the alar (sensory) plate of 

 the midbrain is sufficient reason for classifying the descending 

 mesencephalic root fibers as sensory rather than motor. 



As previously stated, the locus coeruleus cells appear to be 

 nothing more than a caudal extension of the mesencephalic root 

 cells downward into the motor area of the pons. A study of 

 their embryology would likely show that the locus coeruleus 

 cells also came from the alar plate. In these sections numerous 

 fibers and collaterals were seen to go from the region of the 

 mesencephalic root toward the trigeminal motor nucleus; such 

 a collateral is shown in fig. 34, Col. These collaterals appear 

 to be especially numerous in the region where the mesencephalic 

 root fibers bend ventrally around the caudal end of the trigeminal 

 motor nucleus, but it is necessary to search many fibers before 

 one is found that has a collateral. Most of such pictures turn out 

 to be merely the crossing of these fibers over or under the mesen- 

 cephalic root fibers. 



GENERAL SUMMARY, DISCUSSION AND CONCLUSIONS 



The results of the previous experiments confirm May and 

 Horsley's conclusions that the mesencephalic trigemmal root 

 contains both ascending and descendmg fibers. The former take 

 origin from sensory cells in the semilunar ganglion and the latter 

 from globular, unipolar cells in the alar (sensory) plate of the 

 mesencephalon, and from a caudal continuation of these cells, 

 known as the locus coeruleus, which extend downward into the 

 motor area of the pons. 



Upon emerging from the inferior coUiculus the mesencephalic 

 root is composed mainly of descending fibers. They continue 

 caudad through the locus coeruleus above the trigeminal motor 

 nucleus, bend laterally and ventrally around the caudal end of 

 this nucleus, to pursue a cephalic course between the trigeminal 

 motor and sensory (substantia gelatinosa) nuclei, and here inter- 



