380 • • C. JUDSON HERRICK 



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roots (fig. 53), processes arising near the cell body extend outward 

 among the entering fibers of the VIII root, some above and some 

 below the fasciculus solitarius (see fig. 12 of an older larva) and 

 big dendrites extend medialward among the' arcuate fibers and 

 ventralward to reach all of the tegmental tracts. A distinct fas- 

 cicle of descending fibers from the fasciculus longitudinalis medi- 

 alis turns laterally from that tract and terminates about the 

 medial dendrites and body of Mauthner's cell (fig. 54). Mauth- 

 ner's cells and fibers are also present in adult Amblystoma. 



The relations above described are for the most part confirmatory 

 of the observations made by Beccari ('07) upon other species of 

 Urodela. The literature of Mauthner's cells has been fully 

 reviewed by Beccari. I merely call attention to the fact that we 

 have here an apparatus, found in those aquatic Ichthyopsida 

 which swim by means of the trunk and tail musculature, which 

 may be stimulated from the periphery through the VIII and lateral 

 lines nerves or centrally by ascending tracts from the spinal cord 

 (tractus spino-bulbaris system), by descending tracts from the 

 midbrain (tractus tecto-bulbaris and fasciculus longitudinalis 

 medialis) or locally from the motor tegmentum. The general 

 cutaneous and visoeral systems do not appear to participate in 

 this innervation. The axon extends downward throughout the 

 spinal cord and is known to discharge into the somatic motor 

 centers. 



Summary. The nuclei of the motor roots of the cranial nerves 

 can be identified, though they are incompletely separated from 

 the general motor tegmentum. A greatly enlarged portion of the 

 latter region is developed under the cerebellum (eminentia sub- 

 cerebellaris tegmenti) for the reception of descending cerebral 

 tracts, and from the whole motor tegmentum of the oblongata 

 fibers of the tractus bulbo-spinalis system arise. In the marginal 

 zone of the gray substance are found large tangential neurones 

 whose dendrites connect with the sensory field and tegmentum 

 and whose axones enter the fasciculus longitudinalis medialis 

 and tractus bulbo-spinalis. These are the most highly differen- 

 tiated correlation neurones of the oblongata. Two neurones of 

 this system are greatly enlarged and areknownasMauthner's cells. 



