INHIBITION. 95 



between the dipping of the toe into very dilute 

 sulphuric acid and the resulting withdrawal of the 

 foot. If, however, the optic lobes or optic thalami 

 be stimulated, as by putting a crystal of sodium 

 chloride on them, it will be found, on repeating the 

 experiment while these structures are still under 

 the influence of the stimulation, that the time 

 intervening between the action of the acid on the 

 toe, and the withdrawal of the foot, is very much 

 prolonged. That is to say, the stimulation of the 

 optic lobes has caused impulses to descend to the 

 cord, which have there so interfered with the action 

 of the nerve-cells engaged in reflex action as greatly 

 to retard the generation of reflex impulses ; in other 

 words, the stimulation of the optic lobes has inhibited 

 the reflex action of the cord." 



The effects of light have been referred to several 

 times as producing marked effects upon the brain- 

 centres, and exerting a stimulating or inhibitory 

 influence. It may then be interesting to review a 

 few of the known effects of light, and the absence 

 of light, upon living things. 



It is generally admitted that light does produce 

 marked effects upon the human body. We see 

 different phenomena occur in daylight and in dark- 

 ness. Sunlight is most useful in the cure of ansemia, 

 and partial darkness often helps the cure of an acute 

 case of chorea ; in megrim and in ophthalmia the 

 patient is often unable to bear the effects of light. 

 Some of the effects of the' exclusion of light from 

 cave-living animals have been well demonstrated 

 by the author of the "Origin of Species" (see 



