306 THE SENSES 



the end bulbs of Krause and the tactile corpuscles of Meiss- 

 ner. (See Figs. 71 and 72.) Besides, tactile impressions are re- 

 ceived 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 develop- 

 ment by practice is well illustrated in the case of many blind 

 persons. They learn to read with 'comparative facility by pass- 

 ing 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 por- 

 tions 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 -^ inch on the tip of the tongue to i\ inches in the 

 dorsal region. 



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

 concerned in the reception of temperature impressions, though 

 this has not been definitely proven. Decisions as to tempera- 

 ture 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, approximate how much hotter or 

 colder. The delicacy of the temperature sense agrees with 

 that of touch as regards the thickness 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 to which 

 the olfactory fibers are distributed are delicate spindle-shaped 



