144 A FEW POINTS IN CONNECTION WITH 



placed closer and closer to one another until they can no longer 

 be distinguished as two distinct points. The seat of most acute 

 tactile sensation is the tip of the tongue, where two points, only 

 I mm. apart, may be distinguished. Next comes the inside of the 

 finger-tips, where, at 2 mm. apart, they are felt distinct ; whilst on 

 the back of the hand they are only distinguished as two when 

 30 mm. apart ; and on the neck and back of the trunk when from 

 50 — 70 mm. distant. 



The Temperature sense is somewhat more complicated to 

 investigate, inasmuch as some parts of the skin seem to feel low 

 temperature more readily, and some to feel high temperature more 

 acutely ; but, speaking broadly, the seats of most acute tactile 

 sensibility do not correspond to those of most acute temperature 

 sense. For instance, the shoulders and back are very sensitive to 

 changes of temperature, not of touch. I stated a short time ago 

 that our knowledge of our -surroundings is the combined result of 

 the manner in which they stimulate our various special senses, 

 coupled with our memory of past stimuli. 



Thus, we find that whenever we see, hear, smell, taste, or touch 

 anything, a mental picture is produced which is far more elaborate 

 than the stimulus alone really warrants us in forming ; so that in 

 analysing our impressions we must very carefully distinguish what 

 we actually see, hear, smell, taste, or touch from what we think we 

 do. Familiar examples of this occur every moment of our lives. 

 We pass a cookshop and smell certain odours, and immediately 

 infer that certain things are being cooked, from our previous expe- 

 rience. We see the rays of light from this table with certain 

 shadowed and bright lines, and infer that the table is made of 

 wood covered with leather and solid. But the actual rays which 

 are stimulating our eyes do not carry any such information them- 

 selves, so that a clever drawing of a table might entirely deceive 

 us. 



Now, this distinction between what we really see, to take this 

 sense for our example, and what we think we see, which is a 

 process of cerebration, comes home very strongly to all who take 

 up microscopy; for in microscopy the size of the objects with 

 which we are dealing prevents us from making any use of our 

 sense of touch, and we are iu consequence unable to correct the 



