the patient must usually withdraw the normal hand from behind the screen when control observations are being made upon it.
When the loss of sensibility affects the foot, we employ an exactly analogous method, and the errors of localisation are recorded on diagrams of the foot. The living model, upon which the patient localises the spot touched in himself, consists either of the foot of one of the observers, or, more often, of the corresponding lower extremity of some other patient.
Occasionally we have also used the method in which the patient names the spot stimulated. But accurate results cannot be obtained by this method, and it labours under the serious disadvantage that confusion frequently arises in the terms employed to designate the different parts, such as the fingers or their segments.[1]
The groping method is useless as a means of testing the power of localisation, as the results obtained by it are gravely affected by any coincident disturbance of the power of recognising the position in space of the part tested.
8. The Compass Test.
(a) Simultaneous Application of Two Points.—To test the power of discriminating two points we have usually employed a pair of carpenter's compasses, the points of which had been ground down until they gave no sensation of sharpness. Most of the instruments, called "æsthesiometers," used for this purpose are provided with points so sharp as to be wholly useless.
These large compasses are excellent for observations made in a hospital, but they are clumsy for the daily run of clinical work. A modification, which has been devised by Dr. Gordon Holmes, consists of two flat triangular pieces of steel 10 cm. in length and 1·25 cm. in breadth across the base. Each limb ends in a rounded point which has been twisted out of the horizontal so that it makes an angle of roughly 45° with the axis of the steel bar. The two limbs are hinged together at their broad bases so as to form a small pair of compasses that can be carried in the waistcoat pocket. On the flat surface of each bar, which becomes more and more exposed when the limbs of the compasses are separated from one another, fines are engraved corresponding to the distance separating the points; thus, when they are 1 cm. apart, the edge of the flat bar corresponds to one of these lines, 2 cm. to another, and so on, up to a distance of 10 cm.
For recording our observations we used the method suggested by McDougall (72). The compass points were set at a certain distance from one another; they were then applied to the part to be tested in such a way that sometimes two points, sometimes one point only, touched the skin. The stimuli followed one another in an entirely irregular order, but so that, ultimately, the patient
- ↑ For example, the index is sometimes said to be the "first," sometimes the "second finger"; the little finger may be called the "fourth" or "fifth finger." The "first joint" of a digit may be either the proximal or distal phalanx.
