Johnston, The Radix Mesencephalica Trigennni. 615 



that although the bundle traverses the upper surface of the motor 

 nucleus much more closely than in the mole it still passes on to join 

 the spinal trigeminal tract caudal to the motor nucleus. The con- 

 fusion into which previous workers have fallen may easily have 

 arisen from the fact that the bundle comes down from the locus 

 coeruleus upon the dorso-cephalic surface of the motor nucleus close 

 to the motor bundles as they make their exit from the nucleus. There 

 is even some intertwining of the motor fibers with the mesencephalic 

 fibers. Then the latter fibers instead of bending down with the motor 

 bundles turn lightly dorsad and follow over the motor nucleus as a 

 dorso-ventrally compressed bundle, the coarseness of whose fibers 

 makes it possible to trace them with certainty. The bundle then 

 bends down around the caudal surface of the motor nucleus, inclines 

 laterad and pierces the substantia gelatinosa. Here the fibers are 

 gathered in small bundles parallel with and mingled with the ter- 

 minal bundles of sensory trigeminal fibers in the substantia gela- 

 tinosa. Passing through this they enter the spinal trigeminal tract. 

 The diagram in Fig. 12 indicates this peculiar course of the 

 bundle in the adult. ISTo such complex relations exist in the embryo 

 (see below) and the bundle is traced with ease. In the adults, in 

 the beginning of my study, repeated efforts to find the bundle at all 

 at the level of the trigeminal roots were fruitless. It might be 

 thought that sensory fibers of the trigeminus destined to the mesen- 

 cephalon would go directly up in the ascending root through the 

 sensory nucleus or along with the cerebellar fibers and so pass cephalad 

 from the motor nucleus to reach the locus co3ruleus. I wasted much 

 time and effort at first in the attempt to find such fibers, and it was 

 only by tracing the bundle down from above, first in the mole and 

 then in the rat and cat, that its course-was made out. 



In the mole, cat and human embryo of 15.5 mm. there are strong 

 indications that when the bulk of the mesencephalic bundle joins the 

 sensory root of the trigeminus, a few fibers continue on caudad to 

 the level of the vestibular nerve. Wallenberg (1894) saw fibers of 

 this bundle in birds continue caudad to the level of the "Cochlearis- 

 Eckkernes" and he mentions that Probst has seen such a caudal 

 continuation of this bundle in mammals. These caudally directed 



