164 THE NERVOUS SYSTEM. 



parts below the section are violently contracted. Hence, 

 we conclude that, in respect to the movements ordered by 

 the will, the spinal cord is not their source; but that it 

 acts only as a conductor, and is, accordingly, the great 

 motor nerve of the body. 



31. Direction of the Fibres of the Cord. If one 

 lateral half of the spinal cord be cut, or injured, a very sin- 

 gular fact is observed. All voluntary power over the mus- 

 cles of the corresponding half of the body is lost, but the 

 sensibility of that side remains undiminished. This result 

 seems to show that the motor fibres of the cord pursue a 

 direct course, while its sensory fibres are bent from their 

 course. And this has been proved to be the fact; for im- 

 mediately after the posterior roots the conductors of sen- 

 sory impressions join the posterior columns, they enter 

 the gray matter of the cord, and passing over, ascend to the 

 brain on the opposite side. Accordingly, the sensory fibres 

 from the right and left sides interlace each other in the 

 gray matter ; this arrangement has been termed the decus- 

 sation, or crossing of these fibres. This condition serves 

 to explain how a disease or injury of the cord may cause a 

 paralysis of motion in one leg, and a loss of sensation in 

 the other. 



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 

 decussate, or cross from side to side in a mass ; and not 

 separately, as in the case of the posterior fibres just men- 

 tioned. This arrangement is termed the decussation of 

 the anterior columns of the medulla. 



33. From this double interlacing of fibres results a 



3 1 . What singular fact is noticed ? What doe? the result show ? 



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

 medulla oblongata ? The decussation ? 



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



