REFLEX ACTIONS. 15& 



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, 

 the plantar reflex, which consists in a flexion of the toes when the 

 sole of the foot is stimulated by tactile or painful stimuli. Under 

 pathological conditions which involve a lesion of the pyramidal 

 tracts in the cord this reflex is altered, the great toe being 

 extended instead of flexed (Babinski's phenomenon). The 

 cremasteric reflex consists in a contraction of the cremasteric 

 muscle which raises the testis. It follows from stimulation of 

 the skin on the inner side of the thigh at the level of the scrotum. 

 The location of the motor centers of these and other similar 

 reflexes is shown in the accompanying illustration (Fig. 72). 



