|oHNSTON, Tlje Radix Mesencephalica Trigrniiiii. 621 



probably found in the collateral branches from the mesencephalic 

 libers which enter the motor nucleus. 



In the pig embryos the course of the mesencephalic bundle has 

 been found to be identical with that in the human embryo described 

 above. 



Through the courtesy of Professor Harlow Gale I have had the 

 opportunity to study preparations of fa^tal brains of 27, 37 and 42 

 cm. These are Weigert preparations made in His's laboratory at 

 Leipzig in 1897 for the embryological study of liber tracts after the 

 method of Flechsig. In these series every other section was mounted, 

 the alternate sections being mounted in series which remained in 

 His's laboratory. If those series are still available it will be possible 

 for some one now at Leipzig to review the following description and 

 confirm or correct the results. 



Human fcetus of 27 cm., transverse sections. In a foetus of this 

 stage the medullation of the nerve roots and of the fasciculus longi- 

 tudinalis medialis is much stronger than that of any other fibers in 

 this region. At the level of the trigeminus roots a few internal 

 arcuate fibers are seen and some fibers in the brachium pontis. The 

 sensory and motor roots of the trigeminus are therefore quite clear 

 from confusion mth any other kind of fibers. Furthermore, the 

 l)rain is somewhat embryonic in its form and owing to the slight 

 development or absence of numerous fibers which later appear, there 

 is as yet little of the crow^ling and density of structure which makes 

 it so difficult to thread one's way in sections of the normal adult 

 brain. That this is so will be evident from Fig. 19, which fortu- 

 nately shows the whole length of the mesencephalic root through the 

 region in which its course and relations are at all in doubt. The 

 section passes through the sensory root near its exit and through the 

 motor nucleus. As is well known, the bundles of the motor root, 

 starting from their nucleus, are directed more cephalad than the 

 sensory root and so pass beneath the sensory root to emerge on the 

 surface of the pons cephalad from it. In this section the sensory 

 root is cut lengthwise as it pierces the pons in nearly the transverse 

 plane. In sections just behind this the sensory root turns caudad as 

 the spinal trigeminal tract. Since the motor root runs diagonally 



