REFLEX ACTIONS. 



151 



Use of the Knee-jerk and Spinal Reflexes as Diagnostic 

 Signs. The fact that the knee-jerk depends on the integrity of 

 the reflex arc in the lumbar cord has made it useful as a diagnostic 

 indication in lesions of the cord, particularly, of course, for the 

 lumbar region. It is mainly on account of its practical value and 

 the ease with which it is ordinarily obtained that the phenom- 

 enon has been studied so extensively. In the disease known as 

 progressive locomotor ataxia the posterior root fibers in the pos- 

 terior columns in the lumba'r region are affected, and, as a con- 

 sequence, the jerk is diminished or abolished altogether according 

 to the stage of the disease. So also lesions affecting the anterior 



April Otll 1887 



**. 



Fig. 66. Lombard's figure to indicate the daily rhythm in the extent of the knee- 

 jerk and the effect of mental stimuli. The ordinates (0-110) represent the extent of the 

 kick in millimeters. Each dot represents a separate kick, while the heavy horizontal line 

 gives the average extent for the period indicated. 



horns of the gray matter will destroy the reflex by cutting off the 

 motor path, while in other cases lesions in the brain or the lateral 

 columns of the cord affecting the pyramidal system of fibers may 

 be accompanied by an exaggeration of this and similar reflexes. 

 This latter fact agrees with the experimental results (see p. 141) 

 upon ablation of the brain. After such operations in the frog 

 and lower mammals at least the spinal reflexes may show a marked 

 increase. Interruption of the descending connections between brain 

 and cord at any point, therefore, may be accompanied by a strik- 

 ing increase in sensitiveness of the spinal reflexes. The explana- 

 tion usually given is that the inhibitory influences of the brain 

 centers upon the cord are thereby weakened or destroyed. The 



