i86 PHYSIOLOGY TOR BEGINNERS CHAP: 



connection with the brain has been destroyed by the injury. 

 A movement produced by the spinal cord or brain without the 

 action of the will, and in consequence of sensory impulses 

 brought to it, is said to be produced by reflex action. 

 Reflex actions, though they are observed in any animal and in 

 ourselves, can best be studied in a cold-blooded animal, such as 

 a frog, which has been deprived of its brain by cutting off its 

 head ; such an animal remains still and motionless, no sign 

 of movement, except the beating of the heart, being seen so 

 long as it is left undisturbed. It can feel nothing, cannot 

 move of its own will, and will remain lying on its belly till it 

 dries up ; yet if the toe is pinched the leg will be drawn away. 

 If a small bit of blotting paper moistened with acid, even if 

 the acid is very dilute, be placed on one of the legs or on the 

 flank, one leg or both will be used to rub it away. The acid 

 irritating the skin, however slightly, causes sensory impulses 

 to pass from the spot to the spinal cord, and motor impulses 

 are then sent out from the grey matter of the cord to those 

 muscles which can move the leg in such a way as to rub 

 away the acid. In order to remove the acid the foot has 

 not to be jerked up aimlessly, but has to be brought up with 

 precision to where the acid is, and then has to rub it away. 

 To perform these movements many, different muscles have 

 to be brought into action in proper order and with proper 

 amount of force. So that a few simple sensory impulses 

 passing up, it may be along a single nerve, to the spinal cord 

 lead to the outflow of a large number of carefully- adjusted 

 motor impulses passing out along several nerves. Thus we 

 see that the spinal cord, in addition to being a path for the 

 transmission of impulses to and from the brain, is able to 

 originate complicated and delicate movements ; but this only 

 in response to the impulse of an afferent nerve. 



A large number of the movements we perform are brought 

 about by reflex action. If a strong light is flashed across the 

 eyes they are instantly closed ; if the hand comes in contact 

 with a hot body it is at once drawn away ; if a sudden sound 

 is heard we start : these are instances of reflex actions, the 

 movements being produced by the central nervous system 

 without the action of the will, and in consequence of sensory 

 impulses reaching it. Some of these are, moreover, instances 



