4:86 HANDBOOK OF PHYSIOLOGY. 



plantar reflex; gluteal reflex, i. e., a contraction of the glutens maximus 

 when the skin over it is stimulated; ere master reflex, retraction 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, aad its dilatation on stimulating the skin 

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

 actions, but they differ in different individuals, and are more easily 

 elicited in the young, (b) Muscle reflexes, or as they are often termed, 

 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 

 patella-tendon-reflex is the most well-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 patella 

 tendon be tapped with the fingers or the earpiece of a stethoscope, the 

 muscle contracts and the knee is jerked forwards. 



Another variety of the same phenomenon is seen if the foot is flexed 

 so as to stretch the calf muscles and the tendo Achilhs is tapped; the 

 foot is extended by the contraction of the stretched muscles. It appears, 

 however, that the tendon reflexes are not exactly what their name im- 

 plies. The interval between the tap and the contraction is too short for 

 the production of a true reflex action. It is suggested that the contrac- 

 tion is caused by local stimulation of the muscle, but that this would not 

 occur unless the muscle had been reflexly stimulated previously by the 

 tension applied, and placed in a condition of excessive irritability. It is 

 further probable that the condition on which it depends is a reflex spinal 

 irritability of the muscle or (exaggerated) muscular tone, which is ad- 

 mitted to be a reflex phenomenon. 



Inhibition of Reflex Actions. The fact that such movements as are 

 produced by irritating the skin of the lower extremities in the human 

 subject, after division or disorganization of a part of the spinal cord, do 

 not follow the same irritation when the mind is active and connected 

 with the cord through the brain, is, probably, due to the mind ordinarily 

 perceiving the irritation and instantly controlling the muscles of the 

 irritated and other parts; for even when the cord is perfect, such invol- 

 untary movements will often follow irritation, if it be applied when the 

 mind is wholly occupied. When, for example, one is anxiously thinking, 

 even slight stimuli will produce involuntary and reflex movements. So, 

 also, during sleep, such reflex movements may be observed, when the 

 skin is touched or tickled; for example, when one touches with the finger 

 the palm of the hand of a sleeping child, the finger is grasped the im- 

 pression on the skin of the palm producing a reflex movement of the 

 muscles which close the hand. But when the child is awake, no such 

 effect is produced by a similar touch. 



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

 trolled or even altogether prevented by the will: thus an inhibitory ac- 

 tion may be exercised by the brain over reflex functions of the cord and 

 the other nerve centres. The following may be quoted as familiar ex- 

 amples of this inhibitory action: 



