THE NERVOUS SYSTEM. 163 



anterior and posterior roots of the spinal nerves have sepa- 

 rate functions, so the anterior and posterior columns of 

 the cord are distinct in function. The former are con- 

 cerned in the production of motion, the latter in sensation. 

 If the cord be divided, as before in the case of the nerve, it 

 is found that the parts below the point of injury are de- 

 prived of sensation and of the power of voluntary motion 

 on both sides of the body, a form of paralysis which is 

 called paraplegia. 



29. This form of disease, paraplegia, is sometimes seen 

 among men, generally as the result of a fall, or some other 

 severe accident, by which the bones of the spine are broken, 

 and the cord is crushed, or pierced by fragments of bone. 

 The parts which are supplied by nerves from the cord 

 above the point of injury are as sensitive and mobile as 

 before. The results are similar, whether the division hap- 

 pens at a higher or lower portion of the spinal cord ; but 

 the danger to life increases proportionally as the injury 

 approaches the brain. When it occurs in the -neck, the 

 muscles of inspiration are paralyzed, since they are sup- 

 plied by nerves issuing from that region ; and as a result 

 of this paralysis, the lungs are unable to act, and life is 

 speedily brought to a close. 



30. When the spinal cord of an animal has been cut, in 

 experiment, it may be irritated in a manner similar to that 

 alluded to when considering the nerves. If, then, the 

 upper cut surface be excited, it is found that pain, refer- 

 able to the parts below the cut, is produced ; but when the 

 lower cut surface is irritated, no feeling is manifested. So 

 we conclude that in respect to sensation, the spinal cord is 

 not its true centre, but that it is merely a conductor, and 

 is therefore the great sensory nerve of the body. When 

 the lower surface of the cut is irritated, the muscles of the 



29. Paraplegia? Result and danger to life? When the injury occurs in the 

 neck? 



30. Experiment of cutting the spinal cord ot an animal ? What inference is 

 drawn? 



