SENSE OF SMELL. 841 



A. THE SENSE OF SMELL. 



By the sense of smell animals are to a certain extent facilitated in 

 their search for food, in the avoidance of danger, and in seeking the 

 opposite sex. 



By the sense of smell is meant the sensation that is created when 

 certain substances in a gaseous form are inhaled by the nostrils. It will be 

 shown that the sense of smell is only excited under certain definite condi- 

 tions and only when the odorous body comes directly in contact with the 

 organs of sense. Here, as in the case of taste, the sensation is locally 

 excited, probably through some chemical influence, and the result is an 

 entirely specific sensation. It is, therefore, unwarrantable to include 

 the sense of smell among the other senses, since it is quite as different 

 from the sense of touch or of taste as from sight or hearing. The action 

 of the organ of smell is, therefore, due to a special nerve, the olfactory 

 nerve, the first cranial pair, which differs from the others in origin r 

 position, extension, and mode of distribution. 



Under the name of olfactory nerves are usually described the masses 

 of gray matter which arise from the anterior portion of the frontal lobe r 

 and in many animals exist in such bulk as to project beyond the frontal 

 lobes. From the structure of these masses, as well as from comparative 

 anatomy and from their developmental history, it is evident that these 

 parts are not to be regarded as identical with the peripheral branches of 

 other nerves of sense, or the cranial nerves, but are to be regarded as 

 distinct parts of the brain. In many animals the olfactory lobes are 

 hollow, their ventricles communicating with the other ventricles of the 

 brain. As olfactory nerves only are to be described the fibres which 

 originate from these olfactory bulbs and pass through the cribriform 

 plate of the ethmoid bone to be distributed to the olfactory portion of 

 the nasal cavities. 



Fibres from the olfactory lobes have been traced in three bundles 

 backward into the cerebral hemispheres, and the centre for smell in the 

 cortex of the brain has been located in the tip of the uncinate gyrus on 

 the inner surface of the cerebral hemisphere. Of these three roots, the 

 inner one is small, the middle one is large and curves inward to 

 the anterior commissure around the head of the caudate nucleus and 

 decussates through the anterior commissure to the extremity of the 

 opposite temporo-sphenoidal lobe. The outer root passes transversely 

 into the pyriform lobe and ends in the anterior extremity of the optic 

 thalamus. 



Microscopic examination of the olfactory fibres, by which are meant 

 the fibres passing through the cribriform plate, shows that they are thin, 

 transparent fibres, included in a nucleated connective-tissue sheath. 



