30 Dynamic Theory. 



there are not enough brain cells to admit of that. J3ut it must be that 

 in the ease of touch, a single brain cell receives all the stimulations that 

 are made on a definite portion of the skin, the size of which depends 

 wholly on the general average amount of the business done. Thus on 

 the back, which receives but few stimulations, a single brain cell 

 gathers all that occur in a circle two and a half inches across ; while the 

 amount of business at the tip of the tongue is great enough to require 

 at the rate of 3600 times as many cells for an equivalent area, that is, 

 the circles served by a single cell are only ^^ as large. ' When a stim- 

 ulation is made, the brain can probably locate the circle in which it is 

 made, but not the particular part of the circle ; and so, if the tw6 points 

 both touch at once, it gives the impression of only one. But it is ob- 

 vious that if the circles are bounded by definite lines, the two points, 

 when tolerably close together, might straddle a line, and so it would 

 seem ought to give two impressions, which they do not do. The rea- 

 son of this is not settled. But it must be that when an impression is 

 made anywhere on the skin, the stimulation instantly spreads in all di- 

 rections through the cells of the skin, until a nerve fibre is reached 

 which leads to the brain. Without doubt, the skin cells are organs, in 

 which the forms of motion, such as pressure, friction, &c. , are changed 

 to the nervous current, and there are probably no boundaries of circles 

 so rigidly defined that the stimulation cannot spread through the skin 

 laterally. Suppose a and &, fig. 170, to be the 

 two points of the dividers, at a distance apart 

 less than the diameter of a sensory circle each 

 point being in a different sensory circle, fthe ir- 

 ritation spreading from both in all directions, 

 will overlap and reinforce each other at c. This 

 FIG. 170. point lies in the same sensory circle as either a 



or b, and its stimulation will reinforce that of the point lying in the 

 same circle. In other words, the stimulation will be made up at last in 

 favor of the sensory circle in which the most of the stimulation lies, 

 considering the two stimuli together. But there is a residual fraction 

 of stimulation left in the other sensory circle, presumably not enough 

 to excite the brain cell, but which might come to have power to do so 

 after considerable practice. 



Bernstein, in his "Five Senses," says: "Upon the same spot of 

 skin, the size of a sensory circle not only differs in different people, but 

 varies considerably in the same person at different times. The most 

 interesting fact, however, is that constant practice considerably dimin- 

 ishes the limit within which a single impression is produced in certain 

 parts of the skin, in those parts, for instance, which are not naturally 

 very sensitive, and where the sensory circles are large. If, however, 



