122 WALTER NEUHAUS 



forebrain, for example in the Epistriatum. The pathways to the brain stem 

 are more highly developed, that is the tractus olfacto-habenularis, ascend- 

 ing to the diencephalon. As the olfactory pathways to the forebrain are 

 better developed in reptiles and particularly in mammals (Fig. 4) it is 

 probable that they have reduced in birds, while the connections to the 

 diencephalon have not. In the higher vertebrates the power of association 

 is mainly restricted to the forebrain. Optical and acoustical pathways and 

 nuclei are also well developed in birds, in accordance with their good 

 learning power in these respects. As the histological structures necessary 

 for olfactory learning are poorly developed in the bird, it seems probable 

 that learning experiments based on olfaction would be difficult to perform 

 successfully. Even in man, with his relatively poor sense of smell, the 

 power of olfactory learning is probably smaller than in the dog with its 

 much more sensitive olfaction. Because of the well-developed connections 

 to the brain stem, on the other hand, spontaneous, particularly vegetative 

 reactions of the bird seem quite possible. 



These ideas are quite in accordance with our experimental results. It 

 was not possible for us to teach birds of different species to respond to 

 olfactory stimuli, which is in agreement with many other authors. On the 

 other hand, our three geese spontaneously responded to strong olfactory 

 stimuli with a changed respiratory rhythm. It is quite possible that there 

 also occur other, less easily measured, vegetative reactions. These effects 

 are surely a matter of nonconditioned reflexes, which are not, however, 

 of the preciseness of the patellar reflex, because of the many synapses in the 

 reflex chain. Presumably they have a biological purpose, which, however, 

 is not clearly known at present. All in all it can be stated that in birds 

 olfactory reactions are possible, corresponding to the available substrate of 

 nerves and receptors, but that contrarily to most mammals olfactory res- 

 ponses of birds are inferior to visually, auditorily or mechanically con- 

 ditioned responses. 



SUMMARY 



With methods, partly intentionally in conformity with other authors, it 

 was not possible to obtain any positive olfactory learning in pigeon, robin 

 redbreast, blue titmouse, thrush, and goose. 



Sometimes seemingly positive results were obtained, which always, 

 however, could be attributed to optical perception. This result implies that 

 perhaps the earher authors have not sufficiently obviated the possibility of 

 optical orientation. In spite of this a perception and elaboration is shown 

 by the changed respiratory rhythm in response to an olfactory stimulus. 



The difference in the results, on one hand the impossibility of obtaining 

 olfactory learning response, while there are, on the other hand, the 



