THE CENTRAL NERVOUS SYSTEM 707 



cially in boys), the knee-jerk (a sudden extension of the leg 

 by the rectus femoris muscle when the ligamentum patellae 

 is sharply struck), the gluteal, abdominal, epigastric, and inter- 

 scapular reflexes (contraction of the muscles in those regions 

 when the skin covering them is tickled). The jaw-jerk (a 

 movement of the lower jaw when, with the mouth open, the 

 chin is smartly tapped) and ankle-clonus (a series of spasmodic 

 movements of the foot, brought about by flexing it sharply 

 on the leg) are phenomena of the same class, which can 

 be elicited only in disease. Any condition which impairs 

 the conducting power of the afferent or efferent fibres of 

 the reflex arc necessarily diminishes or abolishes the reflex 

 movement, even if the centre is intact. E.g., in locomotor 

 ataxia the disappearance of the knee-jerk is one of the 

 most important diagnostic signs. This disease involves 

 the posterior roots and the fibres that continue them in the 

 posterior column. The anterior nerve-roots are perfectly 

 healthy. The grey matter of the cord at least, in the earlier 

 stages of the disease is unaffected. The weak link in the 

 chain is the afferent path. In anterior poliomyelitis (p. 686) 

 the afferent link is intact, but the other two are broken, and 

 the reflexes also disappear. Certain lesions which cut off 

 the spinal cord from the higher centres without affecting the 

 integrity of the reflex arcs increase the strength of reflex 

 movements and the facility with which they are called forth. 

 In paraplegia, e.g. (paralysis of the legs and the lower portion 

 of the body), caused suddenly by accident to the cord, or 

 more slowly by acute or chronic transverse myelitis, or in 

 hemiplegia, the knee-jerk can usually be elicited with start- 

 ling promptitude and exaggeration, and ankle-clonus may 

 also be obtained. In primary spastic paraplegia, which is 

 associated with degenerative changes in the lateral columns, 

 a similar increase in the true and pseudo-reflexes may be 

 seen, due either to the cutting off of inhibitory impulses or 

 to an actual increase of excitability in the grey matter of 

 the cord. The position of the centres in the cord for the 

 various simple reflex movements is shown in Fig. 254. 



Myotatic Irritability (Muscle Reflex). Although for con- 

 venience of treatment we have included the knee-jerk (with 



452 



