AND PHYSIOLOGY. 



349 



-when we are asleep, or the brain is stunned by a blow, the 

 organic functions are carried on as in the state of the activity 

 of the brain, though with far less energy. And in the case 

 already mentioned of animals born without a brain, life (or- 

 ganic) may be sustained for a considerable time merely by the 

 functions of the spinal cord. If this cord be severed near the 

 head, or even if it be compressed, life soon ceases. This is 

 the manner in which death ensues by hanging, or breaking the 

 neck as it is termed ; where one vertebra is slipped from its 

 place (put out of joint), so that by the unequal contraction 

 of the muscles such a pressure is made on the cord that life 

 speedily becomes extinct. And if this cord is compressed in 

 any portion, sensibility and the power of motion in muscles 

 supplied by the part below the point of compression are de- 

 stroyed ; and if the pressure be long continued, or the cord 

 divided, its vitality is for ever destroyed, although the parts 

 above it are only indirectly affected. 



628. Sensation Exists Previously to Motion— Centri- 

 petal and Centrifugal Fibers— Rate of Movement through 

 the Nerves. — From the fact that one portion of each nerve is 

 designed for sensation, and the other for motion, it is probable 

 that sensation must exist previous to the motion of the part. 

 Thus, for example, if the hand be brought in contact with any 

 substance without any previous knowledge of its presence, no 

 matter how soon the hand may seem to grasp it, yet the in- 

 terval must have been long enough for the sensation contact 

 to have passed from the hand to the brain, and the will to de- 

 termine upon the condition of the muscles, and the order to 

 pass down through the white fibers to the hand again before 

 the grasping can take place. Or the same thing may taka 

 place on receiving a shock, or series of shocks, from a gal- 

 vanic battery, the contractions in this case being an instance 

 of involuntary or reflex movement. And although this fact 



What is the effect of pressure npon or division of the spinal cord ? 628. What tvo 

 functions exist in each nerve? Which exists first, motion or sensation? 



