176 WILLIAM F. ALLEN 



May and Horsley that the locus coeruleus cells have no con- 

 nection \Yith the mesencephalic root. 



I have been unable to obtain copies of any of van Valkenberg's 

 papers and consequently know nothing of their contents. 



In fact, my own studies of the mesencephalic root w^ere com- 

 pleted or nearly completed before I received copies of the three 

 previous papers, so that my results w'ere obtained entirely inde- 

 pendent of their investigations. 



ASCENDING SENSORY FIBERS IN THE MESENCEPHALIC ROOT 



1. Experiments 



In order to establish whether there were any ascending 

 sensory fibers in the mesencephalic root taking origin from cells 

 in the semilunar ganglion, the left trigeminal roots W'Cre severed 

 immediately behind the semilunar ganglion in a number of 

 guinea-pigs. After allowing the animals to live for fifteen days, 

 the brains with the trigeminal roots intact were removed and 

 treated according to Simpson's modification of the Marchi 

 method for demonstrating degenerated medullary sheaths, and 

 were sectioned and mounted serially. The reasons for selecting 

 the guinea-pig for this experiment rather than some larger 

 animal w^ere as follows: 1) The comparative ease with w^hich 

 the trigeminal roots can be reached. It is not necessary to drill 

 through the skull in the guinea-pig to sever the trigeminal roots, 

 because the semilunar ganglion is situated immediately above a 

 large foramen (fig. 4, S.F.) in the alisphenoid bone. 2) The 

 brain stem, which has essentially the same functional arrange- 

 ment as the dog and cat, is much smaller, permitting of a 

 thorough penetration of osmic acid through a piece extending 

 from behind the entrance of the trigeminal roots to the cephalic 

 end of the superior colliculus (corpora quadrigemina) , and in 

 addition requiring the handling of fewer sections to complete a 

 series. 



Operations. All operations were performed under sterile con- 

 ditions, using ether for the anesthetic. The animal was placed on 

 its back, a longitudinal incision was made through the skin 



