THE NERVOUS SYSTEM OF AMPHIBIA 253 



throuQ;h the medulla oblongata and to be inclined ventro-laterad 

 from the middle line on the side viewed in fgure 22. This plane 

 of section reveals characteristic ventricular pits opposite the 

 entrance of the nerves of the trigeminal and facial groups. The 

 extent of the ascending trigeminal tract is probably fully shown 

 in figure 22 (Tr.asc.V.). It extends only a short distance rostrad 

 immediately beneath the external limiting membrane. The de- 

 scending trigeminal tract (Tr.des.V) is considerably longer and 

 of a more complicated structure. As illustrated in figure 22, 

 this tract runs from the trigeminal root to the level of the middle 

 of the auditory vesicle immediately next to the external limiting 

 membrane. The development of the tract is even slightly exag- 

 gerated in the figure. The trigeminal root fibers themselves 

 appear to extend only about to the level indicated in the figure 

 by the reference line Tr.Des.V. Beyond this the tract appears 

 to be made up of short processes of neuroblasts along its course, 

 corresponding, presumably, to neurones of an incipient sub- 

 stantia gelatinosa. The fibrillar element of this tract can be 

 recognized only in favorable longitudinal sections. Its full de- 

 scription belongs to a later pai^er. 



A gaint ganglion cell component of the trigeminal nerve exists 

 also in this stage, but nothing noteworthy aj)iiears concerning 

 either its central or peripheral relations. 



C. THE COILED-REACTION STAGE 



The ophthalmic ganglion has receded along the root till its 

 proximal portion has become massed against the brain. Its rela- 

 tion to the eye, also, is more intimate than in the earlier stages 

 (figs. 3 and 76, G.oph.). The n. ophthalmicus profundus is per- 

 ceptibly more developed than in the early-flexure stage and the 

 distribution of its fibers to the skin is more obvious. In addition 

 to the two main divisions branching out in front of the eye there 

 are now fibers that go directly from the ganglion to the overlying 

 skin. The root fibers can be traced into the brain and differen- 

 tiated from the root fibers of the Gasserian ganglion, the latter 

 entering in a slightly more dorsal position. 



