MEDULLA OBLONGATA OF AMBLYSTOMA 363 



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cellularis tecti (figs. 55, 56). These neurones are scattered 

 throughout the length of the tectum mesencephah and at various 

 levels in the substance of the stratum griseum. Very many of 

 these neurones are apparently perfectly impregnated in our Cajal 

 preparations, but in no case have I ev,er seen any indication of 

 dendrites or collateral branches arising either from the cell body 

 or from any portion of the mesencephalic V root fiber within the 

 mesencephalon. 



A diagram of the central relations of the mesencephalic V root 

 of larval Amblystoma, as seen from the dorsal side, is shown in 

 figure 57. 



The morphological and physiological significance of the mesen- 

 cephalic V root have been much discussed. A large number of 

 observations, particularly those of Johnston ('05, '09) and Wil- 

 lems ('11) make it appear probable that this root is concerned with 

 muscular sensations from the trigeminal muscles and that the 

 neurones of the mesencephalic V nucleus are comparable with 

 spinal ganglion neurones. In these Amblystoma larvae the super- 

 ficial resemblance between these neurones and the larger ones of 

 the semilunar ganglion of the Y nerve is indeed very striking 

 (cf. figs. 55 and 56 with fig. 45), though without further compara- 

 tive cytological and embryological study it would be obviously 

 unwise to stress this point. Upon this interpretation of the 

 mesencephalic V nucleus, the single process arising from these 

 unipolar neurones could be compared with that of the neurones 

 of the semilunar and spinal ganglia. Just as the single process of 

 the spinal ganglion cell divides in the form of a T into peripher- 

 ally and centrally directed branches, so the single processes of 

 the mesencephalic V neurones divide just within the superficial 

 origin of the root into peripherally and centrally directed proc- 

 esses, the latter of which terminates in synaptic relation with the 

 motor V and motor VII nuclei. 



A quite analogous relation is presented by the transitory giant 

 ganglion cells of Rohon and Beard in the spinal cords of various 

 larval Ichthyopsida. In some cases, as in Lophius, these giant 

 cells persist up to adult life. In very young larvae of Ambly- 

 stoma these giant cells have recently been shown by Coghill ('14) 



