THE CENTRAL NERVOUS SYSTEM 



811 



Examples of superficial reflexes are the plantar reflex (the drawing- 

 up of the foot when the sole is tickled), the cremasteric reflex (retrac- 

 tion of the testicle when the skin on the inside of the thigh just 

 below Poupart's ligament is stroked, especially in boys), the gluteal, 

 abdominal, epigastric, and interscapular reflexes (contraction of the 

 muscles in those regions when the skin covering them is tickled). 

 The behaviour of the toes, especially of the great toe, is of consider- 

 able diagnostic^ importance. Normally, on tickling the sole,Qthe 

 toes are flexed towards the planta ; 

 but when a lesion of the pyramidal 

 tract exists, as in hemiplegia, 

 there is dorsal instead of plantar 

 flexion, most marked in the case of 

 the great toe, and the toe moves 

 more slowly than in the healthy 

 person (Babinski's sign). In chil- 

 dren during the first few months 

 of life stimulation of the sole 

 causes normally a dorsal flexion of 

 the big toe. Examples of deep re- 

 flexes are the knee-jerk (a sudden 

 extension of the leg by the rectus 

 femoris and vastus medialis com- 

 ponents of the quadriceps muscle 

 when the ligamentum patellae is 

 sharply struck), the heel-jerk or 

 foot-jerk (a movement of the foot 

 caused in most healthy persons, 

 though not in all, by tapping 

 the tendo Achillis), and the peri- 

 osteal radial reflex (a movement 

 of flexion and slight pronation 

 of the forearm and hand elicited 

 by tapping the lower end of the 

 radius) . 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 con- 

 dition which impairs the conduct- 

 ing power of the afferent or effer- 

 ent fibres of the reflex arc neces- 

 sarily diminishes or abolishes the F 'G. 338. DIAGRAM OF REFLUX 

 reflex movement, even if the cen- CENTRES IN CORD (AFTER HILL). 

 tral connections are 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. 

 Where the presence of the knee-jerk is doubtful, it is necessary 



