560 THE NERVOUS SYSTEM 



child, the impression on the skin of the palm producing a reflex movement of 

 the muscles which close the hand. But when the individual is awake no 

 such reflex is produced. 



Further, many reflex actions are capable of being more or less controlled 

 or even altogether prevented by the will, of which the following may be 

 quoted as familiar examples: 



When the foot is tickled we can, by an effort of will, prevent the reflex 

 action of jerking it aw r ay. So, too, the involuntary closing of the eyes and 

 starting back, when a blow is aimed at the head, can be similarly restrained. 

 Darwin has mentioned an interesting example of the way in which such an 

 instinctive reflex act may override the strongest effort of the will. He placed 

 his face close against the glass of the cobra's cage in the Reptile House at 

 the Zoological Gardens, and, though of course thoroughly convinced of his 

 perfect security, could not by any effort of the will prevent himself from 

 starting back when the snake struck with fury at the glass. 



It can be readily shown, by comparing a spinal frog and a normal unin- 

 jured frog, that stimuli which call forth definite reflexes in the one often pro- 

 duce no movement of the other. 



Cutaneous and Muscle Reflexes as Diagnostic Signs. In the hu- 

 man subject two classes of reflex actions dependent upon the spinal cord are 

 usually distinguished, the alterations of which, either of increase or of diminu- 

 tion, are indications of some abnormality, and are used as a means of diag- 

 nosis in nervous and other disorders. They are termed, respectively, cutane- 

 ous reflexes and muscle reflexes. Cutaneous reflexes are set up by a gentle 

 stimulus applied to the skin. The subjacent muscle or muscles contract in 

 response. Although these cutaneous reflex actions may be demonstrated 

 almost anywhere, yet certain of such actions as being most characteristic are 

 distinguished, e.g., plantar reflex, gluteal reflex, i.e., a contraction of the 

 gluteus maximus when the skin over it is stimulated; cremaster reflex, re- 

 traction of the testicle when the skin of the inside of the thigh is stimulated, 

 and the like. The ocular reflexes, too, are important. They are contraction 

 of the iris on exposure to light, and its dilatation on stimulating the skin of 

 the cervical region. All of these cutaneous reflexes are true reflex actions. 

 They differ in different individuals, and are more easily elicited in the young. 



Muscle reflexes or tendon reflexes consist of a contraction of a muscle 

 under conditions of more or less tension, when its tendon is sharply tapped. 

 The so-called patellar- tendon reflex " knee-jerk" is the best known of this 

 variety of reflexes. If one knee be slightly flexed, as by crossing it over the 

 other, so that the quadriceps femoris is extended to a moderate degree, and 

 the tendon of the patella be tapped with the fingers, the muscle contracts and 

 the foot is jerked forward. Another variety of the same phenomenon is seen 

 if the foot is flexed so as to stretch the calf muscles, and the tendo Achillis is 

 tapped; the foot is extended by the contraction of the stretched muscles. It 



