54 THE BRAIN OF THE TIGER SALAMANDER 



16. OLFACTORY BULB AND ANTERIOR OLFACTORY NUCLEUS 



The olfactory bulb is very large, embracing the anterior end of the 

 lateral ventricle and extending back in the lateral wall for about half 

 the length of the hemisphere (figs. 1, 3, 4, 85, 100, 105, 110; '246, '27). 

 All peripheral olfactory fibers end in the glomeruli of the bulb. 

 Fibers of the second order pass in large numbers to the anterior 

 olfactory nucleus, and they enter longer olfactory tracts with wide 

 distribution (fig. 6), The olfactory tracts are mixtures of fibers from 

 the bulb and the anterior nucleus, as in Necturus ('336, figs. 6 16). 

 They reach all parts of the cerebral hemisphere, the habenula, and 

 the hypothalamus. Some of these decussate in the ventral part of the 

 anterior commissure and some in the habenular commissure (com. 

 superior telencephali). 



The histological texture of the olfactory bulb is more differentiated 

 than that of Necturus ('31), but more generalized than that of higher 

 vertebrates. I have contrasted this with the mammalian pattern and 

 added a theoretic interpretation of probable differences in physiologic 

 properties of the tissue ('246). In brief, this tissue is interpreted as 

 illustrating several transitional stages in the differentiation of polar- 

 ized nervous elements from an unpolarized or incompletely polarized 

 matrix. In Necturus ('31) the transitional character of this tissue is 

 still more clearly evident. The granule cells, in particular, give no 

 structural evidence of physiological polarity, i.e., of differentiation 

 of dendrites from axon, though the connections of these cells in 

 Amblystoma suggest that they have a transient and reversible polar- 

 ity. In connection with this description ('246, pp. 385-95) there are 

 some speculations regarding possible phylogenetic stages in the dif- 

 ferentiation of permanently polarized neurons from an unspecialized 

 nonsynaptic nerve net or neuropil. 



In Amblystoma there is a moderately developed accessory olfac- 

 tory bulb, but no other evidence of local specialization in the primary 

 olfactory center. (There are hints of this in some mammals, e.g., the 

 mink, Jeserich, '45, and references there cited). In 1921, I described 

 the peripheral and central connections of the accessory bulb of 

 Amblystoma and compared these with the more specialized struc- 

 tures of the frog. The anatomical connections there described are, I 

 believe, correct, but the theoretic interpretation of the relationships 

 in vertebrates generally between the vomeronasal organ, accessory 



