of sixteen times in the minute, until either the maximum threshold is passed or the strongest purely tactile hair is reached. Frequently we then go backwards to the hair with which the testing began. No word is spoken throughout such a series of tests, which always end with a final set of contacts on the normal parts. This is necessary in order to be certain that the patient's general powers of attention have not deteriorated during the course of the examination.
Each correct answer is recorded by a vertical stroke and failure to reply by an 0; hallucinatory responses, if they occur, are marked by a broken stroke. From such a record the proportion of correct answers, and the order in which they occurred, can be studied at leisure; thus the condition of tactile sensibility is not a matter of unsupported personal opinion.
| Pressure in grammes. | Measured radii in μ. | Total area in mm.2 | Radius of a circle of the same area in μ. | Pressure per unit area. | Tension. |
| 0·4 0·1 0·21 0·23 0·36 0·88 1·4 1·8 3 3·6 3·5 |
30 × 54 47·5 × 57·5 55 × 90 40 × 80 60 × 90 100 × 120 100 × 130 115 × 125 115 × 115 100 × 130 80 × 140 |
0·005 0·0085 0·015 0·011 0·017 0·0377 0·041 0·045 0·042 0·041 0·035 |
40 52 70 58 73·5 110 114 120 115 114 110 |
8 grm./mm.2 12 grm./mm.2 14 grm./mm.2 21 grm./mm.2 21 grm./mm.2 23 grm./mm.2 35 grm./mm.2 40 grm./mm.2 70 grm./mm.2 90 grm./mm.2 100 grm./mm.2 |
1 grm./mm. 2 grm./mm. 3 grm./mm. 4 grm./mm. 5 grm./mm. 8 grm./mm. 12 grm./mm. 15 grm./mm. 26 grm./mm. 32 grm./mm. 32 grm./mm. |
Hairs exciting a pressure of more than 100 grm./mm.2 usually cause a sensation of pricking, and we have therefore avoided their use in all observations on tactile sensibility. But those ranging from 70 grm./mm.2 up to 260 grm./mm.2 are sometimes useful as a measure of cutaneous painful sensibility.
A camel's-hair brush is not a satisfactory method of testing light touch. For, in the case of my hand, we were able to show that whether a sensation was or was not elicited by such a stimulus, when the skin was entirely insensitive, depended on the way in which the brush was used. If applied suddenly and vertically to the skin, so as to cause a jarring contact, a slight sensation of touch was produced; but when the pressure was made more gradually no sensation was evoked until distinct deformation of the brush occurred; even with these precautions it required slight pressure only to cause a sensation. Thus a camel's-hair brush stimulates both the cutaneous and deep sense organs, and cannot be considered as a test for superficial or light touch.
(b) Pressure touch can be roughly tested by means of some blunt object, such as the unsharpened end of a pencil or the pulp of the observer's finger, so long as its surface temperature does not differ widely from that of the part to be examined.
