528 THE NERVOUS SYSTEM 



nosis in nervous and other disorders. They are termed respectively cutaneous 

 reflexes and muscle reflexes. Cutaneous reflexes are set up by a gentle stimu- 

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

 Although these cutaneous reflex actions may be demonstrated almost any- 

 where, 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, 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 ex- 

 posure 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 esaily 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 is the best known of this variety of re- 

 flexes. 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, 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 ex- 

 tended by the contraction of the stretched muscles. It appears, however, 

 that the tendon reflexes are not exactly w r hat their name implies. The in- 

 terval between the tap and the contraction is said to be too short for the pro- 

 duction of a true reflex action. It is suggested that the contraction is caused 

 by local stimulation of the muscle, but that this would not occur unless the 

 muscle had previously been stimulated by the tension applied, and placed in 

 a condition of excessive irritability. It is probable that the condition on which 

 it depends is a reflex change in the spinal irritability acting on the muscle or 

 exaggerated muscular tone, which is admitted to be a reflex phenomenon 

 in the spinal cord. 



Conduction in the Spinal Cord. With the differentiation of the 

 central nerve axis in vertebrates the conduction in the spinal cord becomes 

 of increasing importance, reaching its maximum in man. It is evident that 

 the cord is the path by which all nerve impulses arising in the trunk or in the 

 arms and legs must reach the brain, or vice versa. Impulses of peripheral 

 origin can and do produce reflexes, but they can arouse sensations and be per- 

 ceived only after they have been conducted to the cerebral cortex. Motor im- 

 pulses arising in the brain can reach the anterior-horn cells of the cord only 

 through the cord as a conducting path. The continuity of the cord, therefore, 

 while not necessary for the execution of reflexes, is absolutely necessary for 

 the higher coordinations of the reflexes and for the excitation and controlling 

 influence of the brain. 



Illustrations of this are furnished by various examples of paralysis, but 



