measure of light touch. For this purpose he divides the pressure in milligrammes by the radius of a circle of the same area as the elliptical crosssection of the hair. The result expressed in grm./mm. represents the tension of the hair.[1]
Throughout the observations on my arm we were careful to bear this difference in mind, not only in the pressure exerted per unit area given in grm./mm.2, but the hair is also spoken of as "No. 3," "No. 6," etc., which expresses the tension in grm./mm. For clinical purposes these refinements are unnecessary, and, whenever the tactile hairs are employed in pathological cases, the measurements are recorded in grm./mm.2, the pressure per unit area.
In the following table we give the necessary data for determining the force exerted by the battery of test hairs we have used in our researches. But it is unnecessary for the clinical observer to provide himself with many hairs; those exercising a pressure of about 14, 21, 23, 35, 70 and 100 grm./mm.2 are sufficient for practical work. Of these the first is useful for testing tactile sensibility on such parts as the palmar aspect of the fingers. Hairless parts, such as the palm and sole of the foot, which respond to cotton wool, will usually be found to be sensitive to a hair of 21 grm./mm.2. If cutaneous sensibility is completely absent, but the deep parts remain highly sensitive, they may respond to a hair of from 23 to 35 grm./mm.2, which is well above the threshold for light touch over normal areas.[2]
A lesion of the cerebral cortex may produce a peculiar uncertainty in the response to measured tactile stimuli; the patient may be able to appreciate the contact of a certain hair at one time, but not at another. This is not confined to a small range of difference in the pressure exerted per unit area, as is the case with normal sensibility; but the uncertain responses may be equally evident with 21 and 100 grm./mm.2. In such cases sixteen contacts with the same hair were made in one minute; this rate allowed us to vary the intervals between any two touches, so as to avoid the tendency to rhythmical replies, so common over areas of defective sensibility. On the affected side, the first hair selected is one which can be easily appreciated over similar normal parts; then hair after hair of increasing strength is applied, at a rate
- ↑ Thus on the following table the hairs which have a tension ot 4 grm./mm. and 5 grm./mm. both happen to exercise a pressure per unit area of 21 grm./mm.2; and yet, from the point of view of tactile sensibility, 5 grm./mm. is undoubtedly a stronger stimulus.
- ↑ The actual hairs we have used were made for us by Professor von Frey. They are kept in a metal box with the handles supported on a rack, so that the hair remains entirely free from contact when at rest. The force required to bend them varies according to use and to the condition of the atmosphere; but, at the end of six years' continuous work, 14 grm./mm.2 turned the scale at 0·21 grm., 21 grm./mm.2 at 0·32 grm. and 23 grm./mm.2 at a little over 0·8 grm. We wish to protest against a common variation of von Frey's apparatus, which consists of a single hair in a metal sheath, so arranged that it can be protruded or withdrawn to a varying extent. The condition of such a hair changes greatly, and the pressure necessary to bend it varies from time to time, even when it is extruded from its sheath to the same amount. Moreover, in order that the hair may not suffer by the extension and withdrawal, it must of necessity be thicker and coarser than when the test is made with a set of hairs, each permanently affixed to its own handle.
