MESENCEPHALIC ROOT 179 



medium to either the collodion or paraffin alone. It has all the 

 advantages of collodion plus the rapidity of the paraffin method. 

 For this study I had four complete series of guinea-pig brains, 

 each having the left trigeminal roots severed behind the semilunar 

 ganglion in the operation, but retaining proximally their attach- 

 ment to the brain stem. The drawings were made from one 

 series (experiment no. 58). They were outlined with the aid of 

 a home-made drawing and projection apparatus (for description 

 see the Anatomical Record for August, 1918). Every attempt 

 was made to portray accurately the amount of nerve-fiber 

 degeneration. 



2. Results 



If the left trigeminal roots from series no. 58 are examined 

 caudad (centrally) to the lesion (fig. 11), jt will be seen that the 

 motor and sensory roots are sharply differentiated, not by con- 

 nective tissue, but for the reason that there is no interchange of 

 fibers. The sensory root (S.R.V.) is much the larger, more 

 dorsal and lateral in position, and is completely filled with de- 

 generated sensory fibers taking origin from cells in the semilunar 

 ganglion. At this level the much smaller motor root (M.R.V.) 

 is median. and ventral to the sensory root. It contains a few 

 degenerated fibers, but insufficient to be of any importance. I 

 doubt if any of these are ascending mesencephalic root fibers 

 taking origin from cells in the semilunar ganglion. Tracing 

 these trigeminal roots caudad (centrally) to the exit of the motor 

 fibers from the pons (fig. 9) shows no change in the sensory root 

 {S.R.V. ). The motor root (M.R.V.) has become separated from 

 the sensory root and is embedded within the pons, but no more 

 degenerated fibers have appeared among the motor fibers than 

 were present more peripherally, demonstrating that no ascending 

 mesencephalic root fibers have joined this root. 



A more caudal section (fig. 8), at the cephalic end of the tri- 

 geminal motor nucleus, shows both trigeminal roots within the 

 pons. The sensory root (S.R.V.) sends off many fibers and fine 

 collaterals to the trigeminal sensory nucleus or substantia 

 gelatinosa (Sub.G.). If an examination of the trigeminal motor 



