THE SENSE OF SMELL 677 



ind smell that they are extremely difficult of classification. Nevertheless 

 Ax primary tastes can be recognized: bitter, sweet, acid or sour, salt or saline, 

 ilkaline and metallic. Though the contact of any bitter, sweet, acid, salt, 

 jtc., substance with any part of the tongue will, if the substance be present 

 n sufficient quantity or concentration, develop a corresponding sensation, 

 iome regions of the tongue are more sensitive and responsive than others. 

 [Thus, the posterior portion is more sensitive to bitter substances than the 

 .nterior; the reverse is true for sweet substances and perhaps for acids and 



.lines. 



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

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

 lepend on the area affected. 



THE SENSE OF SMELL 



The physiologic mechanism involved in the sense of smell includes the 

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

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

 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 fossa; 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 turbmat* 

 bones. Each fossa opens anteriorly and posteriorly by the anterio 

 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. 1 

 former, the mucous membrane over the septum and superior turbmat 

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



Tne i-enpnerai End-organ.-This consists of a basement membrane 

 supporting two kinds of cells, the olfactory and the sustentacular. T 

 olfactory cells are bipolar nerve-cells, the center of which contains a large 

 spheric nucleus. The peripheral pole is cylindric or conic ir [shape and 

 provided at its extremity with several bur-like Processes ^central 

 pole becomes the axon process and passes directly to the olfact ory buHx 



The sustentacular cells are epithelial m character and, as their nam 

 imnlies suDDort or sustain the olfactory cells. 



Fo^he P appreciation of odorous particles the air must be drawn through 

 the nasal foss* with a certain degree of velocity. If the particles are widely 

 doused in ttelir they must be drawn not only more quickly but more 

 *$& nto contact wi'th the olfactory hairs, as in the act of sniffing the 

 result of short energetic inspirations. To many substances the oHf 

 apples is exttemfly sensitive. Thus, it has been shown that a parUcle 

 of mercaptan the actual weight of which was calculated 

 nf a milligram eives rise to a distinct sensation. 



TheStory Sensations. -The sensations which arise m consequence 

 of the excitation of the olfactory apparatus are very numerous and their 



