THE NERVOUS SYSTEM 225 



32. The direction of the anterior, or motor columns of the 

 cord, is downward from the brain. In the cord itself, the 

 course of the motor fibres is for the most part a direct one ; 

 but in the medulla oblongata, or upper extremity of the cord, 

 and therefore early in their career, these fibres cross from side 

 to side in a mass, and not separately, as in the case of the 

 posterior fibres just mentioned. This arrangement is termed 

 the decussation of the anterior columns of the medulla. 



33. From this double interlacing of fibres results a cross 

 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 move- 

 ment. For the same reason, when a loss of sensation and 

 power of motion affecting the right side of the body alone is 

 observed, the physiologist understands 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 con- 

 sidered the cord as the great motor and sensory nerve of the 

 body, but it has another and extremely important 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 



32. Direction of the anterior or motor columns? In the cord itself? In th medulla 

 oblongata ? The decussation ? 



33. Result of the double interlacing of fibres ? Where is the seat-of pain when the right 

 'land is hurt ? The moving of the foot ? Loss of sensation in one side of the body 'i 



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



35. Example of the fowl ? Centipede ? Frog ? What do they prove ? 





