THE SENSE OF SMELL. 307 



have been described in connection with afferent nerves. 

 They are chiefly the end bulbs of Krause and the tactile 

 corpuscles of Meissner. (See Figs. 71 and 72). Besides, 

 tactile impressions are received by the free extremities of 

 afferent nerves situated over the body at large. Numbness 

 from cold is due to interference with cutaneous circulation 

 upon which the sense of touch is directly dependent. It is 

 almost impossible to distinguish mere touch from pressure. 



Acuteness. How the sense of touch is capable of devel- 

 opment by practice is well illustrated in the case of many 

 blind persons. They learn to read with comparative facility 

 by passing the hand over raised letters ; or they frequently 

 make the sense of touch take the place of the lost sense in 

 other almost incredible ways. The acuteness of this sense 

 in different portions of the body has been made the subject 

 of observation by touching two different parts in the same 

 region with finely pointed instruments and noting how near 

 the points can be brought together and still be recognized as 

 two. This distance is found to vary from ^4 inch on the tip 

 of the tongue to 2^2 inches in the dorsal region. 



(b) It is not improbable that there are special nerve end- 

 ings concerned in the reception of temperature impressions, 

 though this has not been definitely proven. Decisions as to 

 temperature are only relative; the surface temperature of 

 the part upon which the impression is made is the standard, 

 and one can only tell absolutely whether the object is hotter 

 or colder than the skin, and, within certain limits, approxi- 

 mate how much hotter or colder. The delicacy of the tem- 

 perature sense agrees with that of touch as regards the thick- 

 ness or absence of the epidermis. 



2. The Sense of Smell. 



Regarding the mechanism of olfaction it is found that one 

 of the .first conditions necessary is the presence of particular 

 cells. Between the epithelial cells of the mucous membrane 



