160 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



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 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. 147) 

 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. 



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, just as in the case of the knee-kick. Such 

 reactions have been described and used in the case of the wrist- 

 jerk, the jaw-jerk, the Achilles-jerk, etc. The last named is 

 obtained by putting the foot into a position of dorsiflexion and 

 then tapping the tendo calcaneus (Achillis). The result is a 

 contraction of the gastrocnemius, causing plantar flexion of the 

 foot. A variation of this reflex is the phenomenon known as 

 ankle clonus. This is obtained by giving a quick forcible 

 dorsiflexion to the foot thus putting the tendon and muscle 

 under a sudden mechanical strain. In some cases there results 

 a rhythmical series of contractions of the gastrocnemius. A 

 second group of reflexes may be obtained by stimulation of 

 special points on the skin, the cutaneous reflexes. For example, 



