172 WILLIAM F. ALLEN 



of several fish, Amphibia, Mammaha, and a 15.5-min. human 

 embryo. In addition, he studied the cells of origin of this root 

 in the mesencephalon from Cajal and Bielschowsky preparations. 

 The author found these globular cells to be mainly unipolar, 

 though some were bipolar or even multipolar; the bipolar cells 

 were said to be more common in the lower vertebrates and in 

 embryos. He also called attention to the resemblance of these 

 cells to spinal ganglion cells, and regarded the large descending 

 process of one of these cells as comparable to the peripheral proc- 

 ess or dendrite of a spinal ganglion cell, and the more slender 

 process of a bipolar cell he took to be the axone. In adult 

 mammals Johnston suggests that the true axones may have been 

 lost and the collaterals, described by Cajal as going from the 

 peripheral processes to form networks about the trigeminal motor 

 cells, may function as axones. From his Weigert series Johnston 

 traced the mesencephalic root fibers into the sensory root of the 

 trigeminal nerve. He was the first investigator to call attention 

 to the fact that the mesencephalic root cells in the midbrain lie 

 in the alar rather than in the basal plate and he is of the opinion 

 that they represent neural crest cells which were not extruded 

 when the medullary folds of the midbrain rolled up to form a 

 tube. The author's conclusion is that the mesencephalic root is 

 sensory rather than motor. 



So far as can be learned. May and Horsley have given us the 

 first important experimental work on the mesencephalic root. 

 They experimented on cats and monkeys and studied several 

 clinical cases, approaching the problem from the standpoint 

 of chromatolysis and Marchi staining of medullary sheath 

 degenerations. 



As a result of a lesion of the mesencephalic root at various 

 intervals in different animals from its passage through the inferior 

 colliculus to a region immediately proximal to the gasserian gan- 

 glion, together with severing the ophthalmic, maxillary, the motor 

 portion of the mandibular, and the mandibular nerves in other 

 animals, the authors found decided chromatolytic changes in 

 the globular cells of the mesencephalon in every instance, except 

 the experiments. in which the ophthalmic and maxillary divisions 



