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 43 LOMBARD :

paper. The apparent contradiction, however, may be explained by the fact that Sternberg's experiments dealt with cases of ex- treme fatigue, while those of the author were confined to a study of an amount of fatigue such as would ordinarily occur in the course of a day. The whole subject of the effect of different kinds and of different degrees of fatigue on the knee-jerk, is worthy of further study.]

Reenforcements of the Knee-Jerk. — As has been said, successive blows of the same force, delivered at like intervals, and on exactly the same part of the ligamentum patellae, called forth knee-jerks of different strengths. Since the stim- ulus was the same in each case, the causes of the variations must be sought within the individual. It immediately suggests itself, that it is possible that the irritability of the muscle is continually under- going change. When one, however, considers how equally a muscle which has been separated from the influence of the central nervous system, by division of its nerve, responds to like stimuli, he is forced to admit that the variations in the knee-jerk must result from changes originating outside of the muscle, and, most probably, in the central mech- anisms with which it is connected. If the knee- jerk be a reflex act, as many suppose, its variations may well be due to alterations in the activity of the reflex centers of the cord ; if it be a peripheral act, it may be that the variations are dependent on changes in the tension of the muscle, resulting from changes of activity of the centers of the spinal cord, which are thought to control its tonus. In fact, whatever the nature of the process resulting in the knee-jerk, one must look to the centers of the spinal cord as the source of the variations which have been noticed. What are the influences which determine the activity of these centers ? It is wisest