728 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



found, however, by Prof. Valentin, who has followed up and extended Prof. 

 Weber's observations, that a considerable amount of individual variation 

 exists in regard to the " limit of confusion ;" some persons being able to dis- 

 tinguish the points at one-half or even one-third of the distances required 

 by others. Czermak 1 has drawn attention to many curious facts in relation 

 to the "sense of space," or "locality" possessed by the skin; and has par- 

 ticularly shown that two points may be much more closely approximated, 

 and yet distinguished as two, if they are applied one after the other, than if 

 they are applied together. The delicacy of the sense of touch is diminished 

 if the skin be either artificially or naturally stretched ; hence the skin of 

 the belly is less sensitive during pregnancy than under ordinary circum- 

 stances, and this may also in some measure serve to explain the diminution 

 in sensibility which occurs in the passage from childhood to adult age, though 

 the difference is no doubt partially due to the increased thickness of the 

 epithelium. The theory of the sense of touch which has been suggested by 

 Fick, is that each nerve-fibril breaks up into a pencil of fine filaments at 

 the periphery, which are distributed over a certain space, perhaps on the 

 average about 2 'g-th of an inch in diameter. An impression made upon any 



one of these filaments conveys the same sensation 

 to the sensorium, providing no other nerve be dis- 

 tributed to the same space; but this hardly ever 

 occurs, and hence compound sensations arise, by 

 which our perception of the precise spot of the skin 

 touched by a point is accurately determined. Thus, 

 if we suppose the distribution of nerves to be repre- 

 sented by the circles A, B, c (Fig. 252), single sen- 

 sations would be produced if a point were applied 

 at 1, 2, or 3, but a compound sensation would be 

 produced if contact were made at 4 or 5, for the 

 fibrils of two nerves, A and c, or B and c, would be implicated; whilst if 

 the impression were made at 6, a still more complex sensation would arise, 

 for then the peripheral branches of three of these nerves would be affected ; 

 in all instances a difference in the character of the impression capable of 

 recognition by the mind being produced. It is obvious that the closer these 

 "sensory circles" are, and the more intimately the branches of different 

 nerves are intercalated with one another, the greater will be the accuracy 

 of the sense of locality of that part, or, in other words, the greater will be 

 the facility with which minute differences in the precise spot touched will be 

 appreciated. The experiments of Dr. Paulus 2 show that the scnsitivein'ss of 

 the skin of the lower extremity, in accordance with a theory propounded by 

 Vierordt, increases with its distance from the axis of rotation of the limb, 

 or, in other words, from the proximal joint, and with the freedom with 

 which its movements can be executed. The leg is thus more sensitive than 

 the thigh, the foot than the leg, and the toes than the foot, the sensitiveness 

 in each division again augmenting from its own proximal joint, except in 

 the single case of the leg proper, where the minimum acuity is opposite the 

 centre of the tibia; but this, as Paulus points out, really favors Vierordt's 

 theory, since, under different circumstances, sometimes the upper and some- 



omy and Physiology, vol. iv, p. 1169, will bo found a Table including the whole 

 serins of observations made, by Profs. Weber ami Valentin, the nuixlma and mininnt 

 of tin- hitler bi-inir staled, as well as the mcn/ix. 



1 Sitsc.-l.c-r. der Wiener Al<ad., Bd. xvii, p. 563, Bd. xxiv, p. 231; and Mole- 

 Bchott'a CTntersueh., Bd. i, p. 188. 



1 Paiilu^, /.-itsehrift f. Biologic, Bd. vii, p. 237; see al#o Kottenkamp and Ullrich, 

 idem, lid. vi, p. 37. 



