peripheral nerve to the skin, its steady increase leads to the production of pain. But when the lesion is situated within the spinal cord, this is not the case, and it is therefore necessary to have some means of measuring the amount of pressure capable of causing pain. For this purpose we have used a modification of Cattell's algometer suggested by Dr. Rivers. The end of the instrument is placed on the part to be tested, and pressure is exerted on the round knob which fits into the palm of the observer's hand. This compresses a spring in the handle. On the rod slides a scale, which is pushed down as the shaft is driven upwards by the increasing pressure (fig. 3).
Immediately the patient calls out that the pressure has become painful, the instrument is removed, the rod springs out of the handle again, carrying with it the scale, which remains at the point where the rod emerged from the handle at the moment of maximum pressure reached during that observation. A line drawn round the rod acts as an indicator, and the amount of pressure applied can be read off at leisure from the relation of this line to the measure on the scale, which is graduated in kilograms.
We have found an algometer constructed on these principles satisfactory. It will glve different readings in the hands of each observer according to variations in the manner and rapidity with which it is applied. But although the actual amount of pressure necessary to cause pain varies according to this personal equation, a comparison of the records on the normal and abnormal sides in the same patient shows a remarkable similarity with different skilled observers.
At least three or more readings must be taken over every part examined, as the answers vary considerably according to the state of expectation in the patient's mind.
A.—Shows the algometer before use. The zero of the scale corresponds to the horizontal line on the rod. B.—Shows the algometer after use. The horizontal line on the rod now corresponds to 10 divisions on the scale. At this point the patient complained that the pressure caused pain. The graduations correspond to kilograms.
