THE NERVOUS SYSTEM 237 



tion are paralyzed, since they are supplied by nerves issu- 

 ing from that region; and as a result of this paralysis, 

 the lungs are unable to act, and life is speedily brought to 

 a close. 



33. 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, referable 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 irri- 

 tated, the muscles of the parts below the section are violently 

 contracted. Hence we conclude that, in respect to the move- 

 ments 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. 



34. Direction of the Fibres of the Cord. If one lateral half 

 of the spinal cord be cut, or injured, a very singular fact is 

 observed. All voluntary power over the muscles of the corre- 

 sponding half of the body is lost, but the sensibility of that 

 side lemains undiminished. This result shows 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 immediately after the posterior roots the 

 conductors of sensory 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 

 decussation, 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. 



83. Experiment of cutting the spinal cord of an animal ? What inference is drawn ? 



84. What singular fact is noticed ? What does the result show ' 



