THE CENTRAL NERVOUS SYSTEM 701; 



out in the absence of consciousness ; not necessarily, how- 

 ever, in the absence of general consciousness, but in the 

 absence of consciousness of the particular act itself. But 

 the term is often used so as to embrace all kinds of actions 

 which are not directly voluntary, whether the individual is 

 conscious of them or not. For example, when the sole of 

 the foot is tickled, the leg is irresistibly and involuntarily 

 drawn up by reflex contraction of its muscles ; yet the 

 person is perfectly cognisant both of the movement and of 

 the sensation which accompanies the afferent impulse. Then 

 there is a class of reflex actions in which consciousness is 

 entirely in abeyance ; during sleep most of the ordinary 

 reflexes can be elicited. 



Normally, it is believed that reflex movements are governed 

 by impulses descending from the higher centres, for (a) it is a 

 matter of common experience that a reflex movement may be to a 

 certain extent controlled, or prevented altogether by an effort of the 

 will, and it is worthy of remark that only movements which can 

 be voluntarily produced can be voluntarily inhibited ; (b) an animal 

 like a frog responds to stimuli by reflex movements more readily 

 after the medulla oblongata has been divided from the spinal cord ; 

 (c) long-continued muscular contractions may be caused in animals 

 after removal of the cerebral hemispheres by stimulation of sensory 

 nerves, for example by scratching the mucous membrane of the 

 mouth in a c brainless ' frog or Menobranchus ; (d) by stimulation 

 of certain of the higher centres reflex movements which would other- 

 wise be elicited may be suppressed or greatly delayed. If the 

 cerebral hemispheres are removed from a frog, and one leg of the 

 animal dipped into dilute acid, a certain interval, the (unconnected 

 reflex time, will elapse before the foot is drawn up (p. 768). If 

 now a crystal of common salt be applied to the optic lobes or the 

 upper part of the spinal cord, and the experiment repeated, it will be 

 found that either the interval is much lengthened, or that the reflex 

 disappears altogether. Strong stimulation of any afferent nerve will 

 also abolish or delay a reflex movement. 



Dependence of the Spinal Reflexes on the Brain. That the 

 brain, in man and the higher animals at least, exerts more than 

 a merely inhibitory influence on the production of reflex move- 

 ments is suggested by many facts. The knee-jerk, for example, is 

 increased or ' reinforced,' if an instant before the tendon is struck 

 the patient makes a voluntary movement or is acted on by a sensory 

 stimulus. It often disappears in pathological lesions, situated high 

 up in the cord in man, and is markedly impaired after high section 

 of the cord in dogs. In hemiplegia (paralysis of one side of the body, 

 caused by disease in the brain) the cutaneous reflexes on the paralyzed 

 side may sometimes be absent for years. Some observers have even 



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