THE SENSE OF PRESSURE. 839 



It will be observed that accuracy of localization and sensitiveness to pressure 

 find their most perfect manifestations in widely separate regions of the skin. 



Tactile areas are found to have a general oval form with the long axis 

 parallel with the long axis of the member investigated. If the compass-points, 

 separated, say, half an inch apart, be passed over the skin of the palm from 

 the middle of the hand to the finger-tips, the sensation will be that of a single 

 line gradually separating into two diverging lines. The result, of course, 

 depends on the compass-points passing successively through areas of finer 

 localization. If an area be marked out on a part of the skin where localiza- 

 tion is poor, within which area two points simultaneously applied appear to be 

 one, a single point moved within it is still perceived to change its place, and 

 two points successively applied may be perceived to occupy diiferent positions. 

 The mental fusion or separation of the two compass-points cannot depend 

 altogether on their being placed over the terminal twigs of the same or of two 

 adjoining nerve-fibres, for, were this the case, the points could be discriminated 

 when separated by a very small distance across the line drawn between the 

 endings of adjoining nerve-fibres, while on either side the points would have 

 to be much more widely separated in the area of distribution of a single fibre. 

 The important factor in the mental separation of two stimulated points is, that 

 between such points there shall be found a certain number of sensory elements 

 which are unstimulated. 1 Practice in such experiments greatly increases the 

 power to localize impressions. This improvement is evidently due not to 

 the establishment of new nerves, but to a more perfect discrimination of sen- 

 sations in the nerve-centres. When, by practice, the localizing power of 

 the skin of a finger of one hand has been increased, it is found that the 

 same improvement has been acquired by the corresponding, but untrained, 

 finger of the other hand ; in other words, the localizing power is central, 

 not peripheral. 



Pressure-points. It has been found that if a light object, such as a lead- 

 pencil, be allowed to rest by a narrow extremity successively on different parts 

 of the skin, its weight will appear very different according to the part which 

 is touched. If the spots on which the weight appears greatest be marked with 

 ink, they will be found to have a constant position, and the skin may therefore 

 be mapped out in areas of pressure-points, which are believed to indicate the 

 place of ending of pressure-nerve filaments. 



The Importance of the End-organ. The sense of touch or pressure is a 

 special sense ; that is, any irritation conveyed to the nerve-centres in which 

 the nerves of pressure terminate gives rise to a feeling of touch, just as dis- 

 turbance in the visual or the auditory centre is recognized in consciousness as 

 a sensation of sight or of sound. The complex anatomical structures known as 

 sense-organs may be considered as instruments each of which is differentiated 

 in a manner to make it particularly irritable toward some special form of 

 energy. Thus, the retina is most sensitive to the luminiferous ether ; the organ 

 of Corti, to waves of endolymph, etc. To this differentiation of structure the 



1 Weber: "Tastsinn und Gemeingefuhl," Wagner's Handworterbuch der Pkysiologie, 1846. 



