384 C. JUDSON HERRICK 



rone and the apparatus for the sensory analysis of peripheral 

 stimuli is greatly simplified as compared with the mammalian 

 condition, though more perfect than that in the spinal cord of 

 urodeles. 



Nevertheless a certain degree of functional localization in the 

 gray substance is evident. The fasciculus solitarius is the most 

 distinctly separate bundle of root fibers and the related neurones 

 of the lobus visceralis are evidently under the exclusive or domi- 

 nant physiological influence of this system of root fibers. 



In the somatic sensory area the secondary neurones related with 

 the V root fibers are tolerably distinct, though in all cases related 

 also with other systems, particularly the VIII root, the secondary 

 visceral tract and the underlying tegmentum. The neurones 

 related with the VIII and lateral line roots form a fairly distinct 

 group, the area acustico-lateralis, the more dorsal members of 

 this system of neurones being related exclusively with the various 

 lateral line roots (always with more than one of these roots) and 

 the more ventral neurones having more diversified connections. 

 In the adult the functional localization of specific terminal nuclei 

 within the area acustico-lateralis is far more completely elaborated 

 than in the larva. 



Though the physiological segregation of the secondary neurones 

 into specific sensory centers is incomplete in these larvae and few 

 of the tracts of the second order carry pure sensory impulses 

 derived from a single functional system of peripheral root fibers, 

 yet in most cases some one of these peripheral systems evidently 

 dominates the pathway. In the further evolutionary history of 

 these secondary pathways the dominant system in each case 

 appears to have persisted to the exclusion of the subordinate 

 connections, and the functions of correlation are thus transferred 

 from the primary receptor centers to the reticular formation for 

 the simple bulbar reactions and to the higher cerebral centers for 

 the more complex reactions. We are, accordingly, able torecog- 

 nize the mammalian equivalents of most of the amphibian corre- 

 lation tracts, though the homology in all of these cases is incom- 

 plete owing to the imperfect segregation of the amphibian primary 

 terminal nuclei of the cranial nerve roots. 



