THE OLFACTORY APPARATUS 223 



the latter makes up by far the larger part of the human hemi- 

 sphere. The rhinencephalon consists of a reflex part in the brain 

 stem and a cortical part in the archipallium. Smell and taste are 

 both chemically excited senses, but the threshold of excitation is 

 much lower in the case of smell. This is brought about by the 

 suppression of a synapse in the peripheral receptor organ and by 

 a complex mechanism -for the summation and reinforcement of 

 stimuli in the primary olfactory center in the olfactory bulb. 

 The secondary olfactory center is the olfactory area, which has 

 three parts, each of which is a reflex center of distinctive type. 

 The reflex path from the secondary center passes backward to 

 the epithalamus and to the hypothalamus, from both of which a 

 descending path goes to the motor centers in the cerebral 

 peduncle. The secondary olfactory center also discharges into 

 the olfactory cerebral cortex, which is chiefly contained within 

 the hippocampus and from which manifold association pathways 

 connect with all other parts of the cerebral cortex. 



LITERATURE 



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