THE SKIN AND COMMON SENSATION. 985 



have been cut close, is examined with bristle aesthesiometers (see 

 " Touch Spots "), a certain number, e.g. fifteen touch spots are found, 

 all easily detected by using pressure intensities of less than 40 

 (3-33) grms. per sq. mm. No further touch spots are then re- 

 vealed by increasing the intensity of stimulation even three- or four- 

 fold. But when the intensity is increased to something over 200 grms. 

 per mm. (occasionally as low as 150). there appear a large number of 

 other " spots," at each of which smart reaction occurs. These spots are 

 scattered pretty evenly, and, unlike touch spots, bear no obvious 

 relation to the position of the hair follicles. 1 The sensation produced 

 at each is pricking and painful, and in many of them the pain caused is, 

 in the opinion of some observers, 2 quite unattended by any sensation 

 of pressure. From each of these spots, when pricked, very lively, in 

 some cases " hardly bearable," pain is evoked. These spots are called 

 " pain spots " (Goldscheider, v. Frey). 



Among the advantages of the hair-aesthesiometer (Hensen, 3 v. Frey 4 ) are— 

 (1) Exactitude of local application, i.e. "stigmatic stimulation"; (2) impossi- 

 bility of exceeding with it a certain easily measured intensity, which is its 

 maximum capacity. In practice it is perfectly easy, when the hair is mounted 

 properly, to ensure employing its maximal power at each application. Hensen 

 and v. Frey use a chemical balance to calibrate the hair instruments, 

 v. Frey takes as index of the stimulus the pressure exerted by the hair 

 into its cross-section, expressed in fractions of a sq. mm. Nagel objects 5 that 

 a truer index would be the force that has to be exerted to bend the hair. 

 He says that stimuli of unequal pressure but equal force are not perceived — 

 in using the hair-assthesiometer — to be different. 



There are many forms of algesimeter besides the " hair " pattern. A simple 

 needle is the most generally used, and offers many advantages for the clinician. 

 If the needle has actually to enter the skin to evoke pain, that is of itself a 

 sign of hypalgesia in any cutaneous region. The pinching up of a fold of 

 skin will often successfully evoke pain when needle-pricks, however deep, 

 do not. Quantitative results are the object of algesimeters. Bjornstrom's 6 

 instrument is a form of pincher, the pressure of which can be read from a 

 scale. Hess's 7 and Motschutkowsky's 8 instruments are standard needles, the 

 degree of depth of insertion of which into the skin can be read off on a 

 scale. Bernhardt's method is by faradisation of the skin, the measure of the 

 stimulus being the distance of the secondary coil from the primary. 



Differences between the reactions obtainable from pain spots and 

 touch spots are found as follows: — 



1. The liminal sensibility of mechanical stimuli (pressure) is 

 higher for pain spots than for touch spots. 9 This the above-given 



toif Cfi It 7)1 P 01 



example illustrated. The ratio • >• — varies in different regions 



of the skin ; thus it is £ for the arm, T ^ for the finger-tips. Hence, 

 when both are equally tightly squeezed, a fold of palmar skin gives less 

 pain than a fold from the fore-arm. 



1 v. Frev, loc. cit. 2 v. Frey, loc. cit. 



3 " Gegen den sechsten Sinn," Kiel, 1893 ; and Arch. f. Ohrenh., 1894, Bd. xxxv. S. 161. 



4 Op. cit. 



5 Arch.f. d. gcs. Physiol., Bonn, 1895, Bd. lix. S. 595. 



6 "Algesimetriu," Upsala, 1877. 



7 Vcrhandl. d. X. internal, mcd. Cong., Berlin, 1890 ; Deutsche mcd. JFchnschr., Leipzig, 

 1892, Bd. x. * 6 



8 Neurol. Centralbl., Leipzig, 1895, Bd. xiv. S. 146. 



!l Goldscheider, op. cit. i» v . Frey, op. cit. 



