NORMAL KNEE-JERK. 13
looking toward the hammer. The left edge of the plate was verti- cal, the upper edge horizontal, and the right and lower borders formed an arc whose centre would he cut by a line drawn through the pivots supporting the axis of the hammers. A scale of 90° was engraved on this plate a centimeter from its curved edge, and in such a way that 0° corresponded to the middle of the handle of the hammer when it was hanging in the position determined by the force of gravity.
On the back of this plate and parallel to its surface there swung from an axis, whose centre would be cut by a line drawn through the pivots supporting the axis of the hammer, a heavy strip of brass, i, 3.5 mm. thick, 25.5 cm. long and 2.5 cm. wide.
This swinging arm bore on its face a small brass plate, k, which had a lip which slightly lapped over the curved border of the plate on which the scale was engraved. This small plate was held in
Elace on the arm by two pins and a thumb screw, I, which had its ead on the back of the arm. When the thumb screw was screwed home it pressed the lip, like a clamp, tightly down on the border of the large plate at any desired place. This clamp bore on the middle of its face an index, m, the point of which was directed to the scale engraved on the large plate and determined the position •of the arm.
The free end of the arm terminated in a catch, by means of which the hammer could be held and easily be released whenever it was desired. This catch had the following construction viz. : A heavy block of brass, n, w r as fastened to the end of the arm in such a position that the end of the handle of the hammer which pro- truded beyond the head would just swing clear of its upper surface. A steel spring forced a small steel catch, o, up through a hole in the brass block, so that it projected slightly beyond the surface and ob- structed the fall of the hammer by catching the handle where it protruded from the head. The lower part of the block of brass was cut away, so as to make room for a lever, p, which had the shape of an inverted L, and which was pivoted at the end of its short arm on the lower end of the catch, and again at the place where the two arms of the L meet, to the solid brass block. By means of this lever the catch could be drawn down and the hammer released.
In all the experiments reported in this paper the subject was re- clining, with outstretched leg, (see Fig. 2,) and inasmuch as the ligamentum patellae was horizontal, the blow was struck with the vertical edge of the hammer. In certain other experiments, in which the subject sat with dangling legs, the ligament held a verti- cal position and the head of the hammer had to be turned around and its horizontal edge used.
2. The couch and the supports for the thigh and foot. 1 — See Plate I ; Fig. 2, a.) — The following arrangements were made, first, to insure' the subject an absolutely comfortable position and freedom from all avoidable reenforcing influences ; second, to relieve the quad- riceps muscle from the weight of the foot, and so permit its slightest contraction to produce a visible movement.
The man experimented upon lay on his left side, upon a comfort-
^These arrangements were the same as those employed by the writer in a previous research, viz.: "Is the Knee-Kick a Keflex
Act?"— The Amer. Jour, of Med. Sciences, Jan., 1837.
