158 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM, 



and in mental conditions of a restful character its extent is relatively 

 small. In conditions of mental excitement or irritation, on the 

 contrar}^, the jerk becomes markedly increased. Lombard ob- 

 served also, in his own case, a daily rhythm, which is represented 

 in the chart given in Fig. 71, It would seem from his experiments, 

 that the extent of the knee-jerk is a sensitive indicator of the 

 relative state of irritabihty of the nervous system: "The knee- 

 jerk is increased and diminished by whatever increases and di- 

 minishes the activity of the central nervous system as a whole," 

 This general fact is supported, especially as regards mental activity, 

 by observations on other similar mechanisms, — such, for instance, 

 as the condition of the nervous centers controlling the bladder. 



Use of the Knee-jerk and Spinal Reflexes as Diagnostic 

 Signs. — The faci 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 lumbar 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 

 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, 14b). 

 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 m 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. 



Other Spinal Reflexes. — Various other distinctive reflexes 

 through the spinal cord may be obtained readily, and since the 

 motor cells concerned lie at different levels in the cord the- 

 presence, absence, or modified character of these reflexes has 

 been used frequently for diagnostic purposes. In the first 

 place there are a number of so-called deep reflexes which may 

 be aroused by sensory stimulation of parts beneath the skin,, 

 such as the tendons, ligaments, and periosteum. Almost any 

 tendon if stimulated mechanically may give a jerk of the cor- 

 responding muscle iust as in the ca.se of the knee-kick. Such 



