SPINAL COED. 479 



center, and C 1 represents a cell in the anterior cornu of the cord, 

 from which passes a motor impulse to the muscle M. 



In the human subject, when the cord is injured or diseased at 

 any point, so as to cut off communication between the brain and 

 extremities, but is still intact below this point, tickling of the 

 soles of the feet will be followed by their withdrawal, although 

 the individual will be entirely unconscious of any sensation. 

 This is also an instance of reflex action. As in the frog, so in 

 man, the three structures mentioned must exist in a state of 

 integrity for the performance of this act. 



It is not essential, however, that the cord be diseased in order 

 to have it manifest reflex action : this property is one which 

 normally resides in the cord. Thus if the hand comes in contact 

 with a flame, it is immediately withdrawn. This is not a voluntary 

 act, for the act of withdrawal takes place before the sensation of 

 pain is felt in the brain. It is a purely reflex act, in which the 

 gray matter of the cord, after being stimulated by an impulse 

 carried to it by an afferent nerve, generates an impulse which is 

 conveyed by an efferent nerve to the muscles concerned in with- 

 drawing the arm. The afferent nerve, nerve-center, and efferent 

 nerve form a reflex arc. If the attention was fixed upon the 

 subject at the time the burn was received, it might be possible 

 to prevent the withdrawal. This would be an instance of inhibi- 

 tion of reflex action. 



Reflex Time. From the moment when the stimulus is 

 applied to the moment when the reflex action takes place is an 

 appreciable interval of time, part of which is occupied by the 

 passage of the afferent impulse to the center, part by the passage 

 of the efferent nerve to the muscle, part by the latent period of 

 the muscular contraction, and part by the reception of the afferent 

 impulse and the generation of the efferent impulse in the center 

 itself; this latter is the reflex time. In the frog it varies from 

 0.008 to 0.015 second. Heat and an increase in the strength of 

 the stimulation lessen it. 



Reflexes in Man. The presence or absence of certain 

 reflexes is made use of to determine the presence or absence of 

 certain diseases in the human subject. They are included in two 

 groups, superficial and deep. 



Superficial Reflexes. Of these, there are many, but the princi- 

 pal ones are : 



1. Plantar. Tickling the sole of the foot causes its with- 

 drawal. 



2. Gluteal. Pricking of the skin over the gluteus causes a 

 contraction of that muscle. 



3. Cremasteric. Stimulating the skin on the inner side of the 

 thigh causes a retraction of the testicle. 



