640 TEXT-BOOK OF PHYSIOLOGY. 



the tongue will, if the substance be present in sufficient quantity or con- 

 centration, develop a corresponding sensation, some regions of the tongue 

 are more sensitive and responsive than others. Thus, the posterior por- 

 tion is more sensitive to bitter substances than the anterior; the reverse is 

 true for sweet substances and perhaps for acids and salines. 



The intensity of the resulting sensation in any given instance will depend 

 on the degree of concentration of the substance, while its massiveness will 

 depend on the area affected. 



THE SENSE OF SMELL. 



The physiologic mechanism involved in the sense of smell includes the 

 nasal fossae, the olfactory nerves, the olfactory tracts, and nerve-cells in those 

 areas of the cortex known as the uncinate convolution and anterior part of 

 the gyrus fornicatus. Peripheral stimulation 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 vaporous 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 communicating 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 inspirations. 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 i-^nnmrinnr 

 of a milligram gives 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 



