SIMPLEST NEURAL ARCS. 37 



levers reduced the latent time to 37 a, with a mean variation within the 

 series of la, or less. Similar differences between the latent time as 

 measured respectively by the movements of the leg and by thickening 

 of the quadriceps muscle are reported by Weyler. 1 Partial explana- 

 tion of these differences is found in the weight of the leg. A certain 

 degree of contraction of the muscle-fibers seems to be necessary before 

 the heavy leg-lever can be set in motion. An entirely new factor in 

 the case was shown by Dodge to be the initial forward motion of the 

 leg immediately after the stimulus blow. This is a purely mechanical 

 effect, and is due to the virtual shortening of the tendon of the extensor, 

 which is produced through its deformation by the stimulus blow. The 

 consequent pendular movement of the leg prevents its showing the 

 exact moment of reaction. This disturbance is more serious than 

 might at first appear. Since the deformation of the tendon increases 

 with the weight of the pendulum hammer, the initial movement of the 

 leg, which has no direct connection with the reflex action, must increase 

 with the weight of the hammer. This may have led to the anomalous 

 data reported by Lombard, 2 who found that, as measured by the leg- 

 movement, the latent time was increased by increasing the weight of 

 the hammer. A further advantage of the muscle-thickening records 

 is that they show the true course of the muscle contraction and disclose 

 any preliminary stiffening of the leg to receive the blow, as well as any 

 arbitrary or voluntary interference with the course of contraction. 

 Finally, quadriceps thickening is uninfluenced by the chance interplay 

 of flexor antagonism. 



STIMULUS. 



In all quantitative studies of the knee-jerk, the usual stimulus has 

 been a sudden blow on the patellar tendon. While this is not the only 

 means of eliciting the knee-jerk, and is not always the best for clinical 

 purposes, for experimental work it is doubtless the most convenient 

 and exact. It should not be forgotten, however, that the receptors 

 in the knee-jerk are intra-muscular, and that their stimulation is a 

 sudden muscle deformation. Consequently, only a perfectly regulated 

 blow on the tendon, with a muscle at uniform tension, is satisfactory 

 for comparative purposes. This demands a uniform percussion ham- 

 mer, striking at a uniform place, with the limb in a uniform position. 



The preliminary study favored the electrically released double, 

 pendulum-percussion hammer which is shown in figure 4. A pendulum 

 is ideally uniform in action under similar circumstances, and the 

 energy of the blow can be easily and simply expressed in c. g. s. units. 

 Its velocity and mass are independently variable. Both factors are 

 directly measurable on the apparatus and may be experimentally 

 changed, easily and with precision. To provide for changes of mass, 



1 Weyler, Zeitschr. f. d. ges. Neur. u. Psychiat., 1910, 1, p. 116. 

 2 Lombard, Journ. Physiol., 1889, 10, p. 139. 



