268 THE BRAIN OF THE TIGER SALAMANDER 



bulbar formation, except for the accessory bulb, the structure is 

 nearly homogeneous, with little evidence of localization of function. 

 The sense of smell lacks any provision for localizing in external space 

 the source of odorous excitations. In the retina there is very compli- 

 cated mechanism for analysis of the components of visual excitation 

 (Polyak, '41). The analysis of olfactory sensibility for discrimination 

 of odors is evidently a much simpler process. Judging by analogy with 

 Polyak's description of the retina, there is little provision for this in 

 the olfactory bulb. It is possible that the periglomerular cells may 

 perform this function, but the structural organization of the bulbar 

 formation gives clear evidence that the dominant activity here is not 

 analysis but summation and intensification. The correlation of olfac- 

 tion with other sensory systems is effected throughout the cerebral 

 hemisphere, hypothalamus, and epithalamus, beginning in the nu- 

 cleus olfactorius anterior. 



Anterior olfactory nucleus. — This nucleus was first defined in Am- 

 bly stoma ('10, p. 497) as undifferentiated olfactory tissue of the sec- 

 ond order, closely associated with the olfactory bulb and extending 

 backward a longer or shorter distance between the bulbar formation 

 and the more specialized parts of the hemisphere. It is of large extent 

 in the amphibian brain, and to it considerable attention has been 

 given ('246; '31; '27, p. 288; '336, p. 133; '34, p. 99). In higher brains 

 it shrinks in size as progressively more of this generalized tissue is 

 specialized. Its comparative anatomy was discussed in connection 

 with a detailed description of it in the Virginia opossum ('24c?). In 

 the amphibian brain it is a broad ring of gray bordering the bulbar 

 formation on all sides. This cylinder is divided topographically into 

 ventral, medial, dorsal, and lateral sectors, each of which has its own 

 distinctive connections with other parts of the hemisphere. The ven- 

 tral sector and the lower part of the medial contain the primordium 

 of the tuberculum olfactorium. The arrangement of these sectors in 

 Necturus is shown in figures 111 and 112, and their structure and 

 connections have been described in detail ('336, p. 133). Transverse 

 sections through this region of Ambly stoma are in the paper of 1927 

 (p. 288 and figures 2-5). The neurons and neuropil of this nucleus are 

 illustrated in figures 105, 108, and 109 and in 1934, figures 1 and 2. 



Typical neurons of the anterior nucleus have widely spread thorny 

 dendrites, and axons which enter the olfactory tracts. Many other 

 forms of cells are seen, some of which are transitional to those of the 

 olfactory bulb. The axons of some of its cells are directed peripheral- 



