448 C. JUDSON HERRICK 



edly remained on a low level of organization, as do the modern 

 mudfishes, passing a sluggish and uneventful existence. But 

 with the elaboration of an efficient pulmonary respiratory 

 mechanism (which indeed is only imperfectly realized in many 

 living urodele .Amphibia), these limitations were removed; the 

 oxygen supply of the brain was adequate, and the evaginated 

 type of cerebral hemisphere, acquired during a period when the 

 vital currents were at lowest ebb, develops possibilities of further 

 evolutionary advance forever denied to those types of fishes which 

 diverged in other and (for fishes) more favorable lines of 

 specialization. 



THE CORRELATION CENTERS OF THE FOREBRAIN 



Having now reviewed some of the various forms exhibited by 

 the forebrains of different groups of Ichthyopsida and suggested 

 some of the physiological factors which may have been operative 

 in the establishment of the fully evaginated type of cerebral 

 hemispheres found throughout the Amphibia and all higher 

 forms, it remains to consider briefly some other functional influ- 

 ences which have played a very different role in shaping the 

 anatomical configuration of this region. I refer to the directions 

 taken by the internal conduction pathways and the connections 

 which these make among themselves in the various correlation 

 centers. Here we can base our conclusions on the firm ground 

 of comparative anatomical facts, with satisfying confidence in 

 their vaUdity. 



Without here going into the details of these connections, they 

 obviously comprise two great systems of tracts, first, the descend- 

 ing olfactory system, and, second, the non-olfactory ascending 

 systems. 



The olfactory system comprises: 1) peripheral neurons of 

 the first order arising in the nasal epithelium and ending in the 

 glomeruh of the olfactory bulb; 2) neurons of the second order, 

 the mitral cells of the bulb, whose axons form the olfactory 

 tracts ending in the nuclei of the secondary olfactory area (often 

 called the olfactory lobe); 3) neurons of the third order arising 



