NERVE 



133 



sensations Avhich may be experienced, but they may roughly 

 be divided into four main groups : — 



1. Sweet. 3. Acid. 



2. Bitter. 4. Saline. 



2. NASAL MECHANISM- 

 Sense of Smell. 



The olfactory organs are the most fundamental of all 

 distance receptors, and they play a most important part in 



Fig. 59. — The Connections of the Olfactory Fibres. A, olfactory cells ; B, 

 synapses in the olfactory bulb I. ; II., olfactory tracts ; III., olfactory 

 centre ; IV., decussation of fibres. (Howell.) 



the life of the lower animals in guiding them to their food 

 and repelling them from danger, in causing positive and 

 negative chemiotaxis. 



Smell, as Sherrington puts it, is taste at a distance. 

 Just as the taste organs are stimulated by substances taken 

 into the mouth, so the olfactory organs are stimulated in 



