THE NERVOUS SYSTEM. 165 



crossed action between the original and terminal extremity 

 of all nerve-fibres which pass through the medulla ; 

 namely, those of all the spinal nerves. Consequently, if 

 the right hand be hurt, the left side of the brain feels the 

 pain ; and if the left foot move, it is the right hemisphere 

 which dictates its movement. For the same reason, when 

 a loss of sensation and power of motion aifecting the right 

 side of the body alone is observed, the physiologist under- 

 stands that the brain has been invaded by disease upon its 

 left side. This affection is termed hemiplegia, or the " half- 

 stroke." The full-stroke, which often follows the rupture of 

 a blood-vessel in the brain, is commonly called paralysis. 



34. The Reflex Action of the Cord. We have 

 already considered the cord as the great motor and sensory 

 nerve of the body, but it has another and extremely impor- 

 tant use. By virtue of the gray matter, which occupies 

 its central portion, it plays the part of an independent 

 nerve centre. The spinal cord not only conducts some 

 impressions to the brain, but it also arrests others; and, as 

 it is expressed, " reflects" them into movements by its own 

 power. This mode of nervous activity is denominated the 

 Reflex Action of the cord. 



35. A familiar example of this power of the cord is 

 found in the violent movements which agitate a fowl 

 after its head has been cut off. The cold-blooded animals 

 also exhibit reflex movements in an astonishing degree. 

 A decapitated centipede will run rapidly forward, and will 

 seemingly strive to overturn, or else climb over obstacles 

 placed in its way. A frog similarly mutilated will sustain 

 its headless body upon its feet, in the standing posture, 

 just as it might do if it were still alive. If pushed over, 

 it will regain its feet ; and if the feet are irritated, it will 



when the right hand is hurt ? The moving of the foot ? Loss of sensation in one 

 side of the body ? 



34. What other important use has the cord? What is the activity denomi 

 nated ? 



35. Example of the fowl ? Centipede ? Prog ? What dc they prove ? 



