OH. XLVII.] REFLEX ACTION IN MAN. 66 1 



doing the experiment with acid just described the toes of the 

 other foot are being simultaneously pinched, the response to the 

 acid is delayed. Inhibition, or delay of reflex time is thus pro- 

 duced by other sensations, which, as it were, take up the attention 

 of the cord. 



This influence of the brain on the cord is also illustrated in 

 man, by the fact that a strong effort of the will can control many 

 reflex actions. It is, for instance, possible to subdue the tendency 

 to sneeze ; if one accidentally puts one's hand in a flame, the 

 natural reflex is to withdraw it : yet it is well known that 

 Cranmer, when being burnt at the stake, held his hand in the 

 flames till it was consumed. 



After the spinal cord has been divided by injury or disease in 

 the thoracic region, the brain can no longer exert this controlling 

 action ; hence the part of the cord below the injury having it, 

 as it were, all its own way, has its reflex irritability increased.* 

 The increase of reflex irritability is also seen in the disease called 

 lateral sclerosis; here the lateral columns, including the pyra- 

 midal tract, become degenerated, and so the path from the brain 

 to the cells of the cord is in great measure destroyed. In these 

 patients the increase of reflex irritability may become a very 

 distressing symptom, slight excitations, like a movement of the 

 bed-clothes, arousing powerful convulsive spasms of the legs. 



Reflex time. In the frog, deducting the time taken in the 

 transmission of impulses along nerves, the time consumed in the 

 cord (reflex time) varies from 0*008 to 0-015 second; if the 

 reflex crosses to the other side it is one-third longer. It is 

 lessened by heat, and under the influence of a strong stimulus. 



Reflex Action in Man. 



The reflexes obtainable in man form a most important factor 

 in diagnosis of diseases of the nervous system ; each action is 

 effected through an afferent sensory nerve, a system of nerve-cells 

 in the cord termed the reflex centre, and an efferent motor nerve ; 

 the whole constitutes what is called the reflex arc. The absence 

 of certain reflexes may determine the position in the spinal cord, 

 which is the seat of disease. 



Two forms of reflex action must be distinguished : 

 i. Superficial reflexes. These are true reflex actions, and arc 

 excited by stimulation of the skin. 



* In some injuries to the cord produced by crushing, there is a loss of re- 

 flexes below the injury. These, however, are not simple t ransverse lesions ; the 

 loss of reflex action is dne to extensive injury to grey matter by haemorrhage. 



