blow. The argument that the time is too short for a reflex act is inconclusive on account of our lack of knowledge of reflex times in general, and the attempt to prove the existence or non-existence of muscle tonus has thus far proved futile. The fact remains, however, that the existence of the knee-jerk is de- pendent on the integrity of the reflex arc, and, more- over, that the extent of the knee-jerk is greatly influenced by the irritability of the spinal cord.
Causes for Variations in the Extent of the Knee-Jerk. — It is not the intention of the writer to offer the results recorded in this paper as laws applicable to all men. The influences which determine the extent of the knee-jerk are far too numerous and too subtle to be ascertained by a few thousand experiments on any one man. Al- though, as has been said, the nature of the knee- jerk is not thoroughly understood, we know it to be an elaborate physiological process, involving the action of many different organs, for both ex- perimentation and clinical experience have disclosed that the normal activity of the quadriceps muscle, of the corresponding afferent and efferent spinal nerves and their roots, and of a certain portion of the cord are necessary to its completeness. Since every condition which influences the action of these different organs must necessarily have its effect upon the extent of the knee-jerk, it is not strange that the phenomenon is subject to many variations. This becomes the more apparent if one considers how many influences are continually modifying the activity of nerve and muscle tissue, and, still more, of the delicate mechanisms of the central nervous
