59 6 TEXT-BOOK OF PHYSIOLOGY. 



THE SENSE OF SMELL. 



The physiologic mechanism involved in the sense of smell in- 

 cludes the nasal fossae, the olfactory nerves, the olfactory tracts, and 

 nerve-cells in those areas of the cortex known as the uncinate con- 

 volution and anterior part of the gyrus fornicatus. Peripheral stimu- 

 lation of this mechanism develops nerve impulses which, transmitted 

 to the cortex, evoke the sensations of odor. The specific physiologic 

 stimulus is matter in the gaseous or volatile state. 



The Nasal Fossae. The nasal fossae are irregularly shaped 

 cavities separated by a vertical septum formed by the perpendicular 

 plate of the ethmoid bone, the vomer, and the triangular cartilage. 

 The outer wall presents three recesses separated by the projection 

 inward of the turbinated bones. Each fossa opens anteriorly and 

 posteriorly by the anterior and posterior nares, the latter com- 

 municating with the pharynx. Both fossae are lined throughout by 

 mucous membrane. The upper part of the fossa is known as the 

 olfactory, the lower portion as the respiratory region. In the former, 

 the mucous membrane over the septum and superior turbinated 

 bone is somewhat thicker than elsewhere and covered with a neuro- 

 epithelium which constitutes 



The Peripheral End-organ. This consists of a basement 

 membrane supporting two kinds of cells, the olfactory and the 

 sustentacular. The olfactory cells are bipolar nerve-cells, the center 

 of which contains a large spheric nucleus. The peripheral pole 

 is cylindric or conic in shape and provided at its extremity with 

 several hair-like processes. The central pole becomes the axon 

 process and passes directly to the olfactory bulb. 



The sustentacular cells are epithelial in character and, as their 

 name implies, support or sustain the olfactory cells. 



For the appreciation of odorous particles the air must be drawn 

 through the nasal fossae with a certain degree of velocity. If the 

 particles are widely diffused in the air, they must be drawn not only 

 more quickly but more forcibly into contact with the olfactory 

 hairs, as in the act of sniffing, the result of short energetic inspira- 

 tions. To many substances the olfactory apparatus is extremely 

 sensitive. Thus, it has been shown that a particle of mercaptan 

 the actual weight of which was calculated to be s^ow of a milli- 

 gram gave rise to a distinct sensation. 



The Olfactory Sensations. The sensations which arise in 

 consequence of the excitation of the olfactory apparatus are very 

 numerous and their classification is extremely difficult. For this 

 reason it is customary to divide them into two groups: viz., agreeable 

 and disagreeable, in accordance with the feelings they excite in the 

 individual. As the olfactory sensations give rise to feelings rather 



