12 LOMBARD :
large series of experiments, and conld obtain records of the resulting movements of the leg, one could definitely determine the limits of the normal knee- jerk and the variations which it undergoes under normal conditions. With these thoughts in mind the writer entered upon the research recorded in this paper.
The Apparatus Employed.
1. A hammer by which it was possible to strike a blow of any desired force. (See Plate I, Fig. 1.) — Several methods suggested themselves by which one might strike the ligamentum patellae a blow of any de- sired force. Of these, the one which upon trial commended itself most highly, was to suspend a hammer by an axis passed through its handle. The hammer could then be made to fall like a pendu- lum, and would strike a blow, the force of which would depend on the weight of the hammer and on the height from which it fell.
Two instruments of nearly the same construction were employed, the one in the first series of experiments, the other in the second series. 1 The construction of these instruments is shown in Fig. 1, and was in general as follows :
The head of the hammer, a, which was made of iron, was 10.5 cm. long, 2.5 cm. wide, and 2.5 cm. thick, and weighed 346.5 gms. It was narrowed at either end to a smooth rounded edge, one of these edges being vertical and the other horizontal.
The handle of the hammer, b, a steel rod 22 cm. long and 1 cm. in diameter, weighing 100 gms., passed through a hole which was bored vertically through the middle of the head of the hammer, and protruded a few mm. from the lower side of the head, c. The head was fastened to the handle by a screw ; and it was so placed that its middle point was just 20 cm. distant from the middle of the axis, which supported the hammer.
The axis, d, passed through the handle of the hammer as close as possible to its upper end. It was a steel rod, 5 cm. long and 5 mm. in diameter, and it was pivoted at either end on steel points. The handle was fastened to the axis at about 1 cm. from its inner end.
The screws, e e, on which the axis was pivoted, were held by two heavy pieces of brass, //, which extended downward from the horizontal steel rod, g g, which supported the whole apparatus, and which was itself clamped at any desired height, on a substantial standard, with a heavy iron base.
A brass plate, h, 2.5 mm. thick, was fastened by its upper left hand corner to the supporting rod and to the back of the heavy piece of brass which held the inner pivot, in a plane parallel to that cut by the handle of the hammer when it fell, and with its face
- One of these hammers was made, through the kindness of Prof.
H. Newell Martin, by the mechanic at the Biological Laboratory, the other by Lehman, instrument maker, Baltimore.
