530 



THE NERVOUS SYSTEM. 



and cold respectively. Thus if a heated object is moved about 

 over the skin, at some points tactile sensibility alone will be 

 excited, while at others the object will feel distinctly hot. In 

 like manner cold spots will be recognized by the application of a 

 cold object. This test applied to the skin of the forearm has 

 resulted in such a chart as is shown in Fig. 308. 



Sense of Pain. When the stimuli that call out the sense of 

 touch or of temperature are excessive, the sense of pain is pro- 

 duced, and the other sensations are abolished ; thus when a piece 



I 



FIG. 308. Cold and hot spots from the same part of the anterior surface of the 

 forearm : a, cold-spots ; 6, hot-spots. The dark parts are the more sensitive ; the 

 hatched the medium ; the dotted the feeble ; and the vacant spaces the non- 

 sensitive. 



of iron very much heated burns the hand, the sensation is the 

 same as when the iron is very cold. 



Some authorities regard the sense of pain as being a distinct 

 sensation, others as simply an exaggeration of other sensations. 



Sense of Smell. In the consideration of the respiratory 

 processes the nose was described as being a part of the respiratory 

 tract (p. 353). This is true of the lower portion of the nasal 

 cavity, the entrance or regio vestibularis and the regio respiratoria, 

 the rest of the lower part of the nasal cavity; the upper part, 

 however, is more especially concerned with the function of smell, 

 and is therefore called regio olfactoria. This portion of the mucous 

 membrane is sometimes denominated olfactory membrane, and may 

 be defined as that portion of the Schneiderian membrane which 

 covers the superior and middle turbinated bones and the upper 

 part of the septum nasi. The Schneiderian membrane lines the 

 nasal fossae. Before the time of Schneider, from whom the mem- 



