exerted on its point, were it not for a fine counteracting spring. This spring attached to the rod and to the upper brass support exactly balances the weight, and the needle exerts no pressure. But, if the instrument is pressed on the skin, this spring is no longer completely extended and the weight exerts a pressure in proportion to the amount, to which it is no longer counterbalanced by the coiled spring. This is read off on a scale attached to the bar, that unites the two guiding arms of the instrument.
Six divisions of this scale correspond to the pressure of 1 grm.; but readings below 15 are of little value as the weight hardly comes into action owing to the friction of the rod. Thus the corrected readings are as follows:—
| 25 | scale | divisions | = | 5 | grm. |
| 31 | „ | „ | = | 6 | „ |
| 37 | „ | „ | = | 7 | „ |
| 43 | „ | „ | = | 8 | „ |
| 49 | „ | „ | = | 9 | „ |
No such apparatus can give anything but approximate readings. For the moment at which pain is produced by the needle depends on the rapidity with which it is applied. If it is placed on the normal skin with the slightest jar, pain is produced at once, before the spring has begun to be affected. Moreover, the sensation of a point merges gradually into the pain of a prick over normal parts. Throughout the stage of recovering sensibility to prick, no pain was produced until considerable pressure was applied, when the characteristic pain made its appearance. Thus, the readings of the algesimeter formed a useful record of the progress of returning protopathic sensibility within the affected area; but we do not recommend it for general use on the normal skin.
All our earlier observations on the pressure necessary to produce deep pain were made by means of Cattell's pressure algometer. More recently we have used the modified form described in the paper by Head and Thompson.
The conditions of our experiment enabled us to investigate thermal sensibility with unusual completeness. Not only was the part explored with tubes containing hot and cold water, but we carefully studied the distribution of the heat- and cold-spots, especially on the back of the hand.
In our preliminary investigations before the operation, we found that different parts of the back of the normal hand were not equally sensitive to thermal stimuli. The skin over the knuckles responded badly, and
