FUNCTIONS OF NERVES AND NERVE-CENTRES. 303 



pass from the centre to muscles as tvell as from tlie skin to 

 the centre. This is confirmed when we find that if the 

 nerves of the limb be divided the Will is unable to act 

 upon its muscles, showing that these are excited to con- 

 tract through their nerves. That the nerve-fibres con- 

 cerned in arousing sensation and muscular contractions are 

 distinct, is shown also by cases of disease in which the 

 sensibility of the limb is lost while the power of voluntarily 

 moving it remains; and by other cases in which the op- 

 posite is seen, objects touching the hand being felt, while it 

 cannot be moved by the Will. We conclude therefore that 

 certain nerve-fibres when stimulated transmit something (a 

 nervous impulse) to the centres, and that these, when ex- 

 cited by the nervous impulse conveyed to them, may radiate 

 impulses through other nerve-fibres to distant parts, the 

 centre serving as a connecting link between the fibres which 

 carry impulses from without in, and those which convey 

 them from' within out. 



5. Further we conclude that the spinal cord can act as 

 an intermediary between the fibres carrying in nervous im- 

 pulses and those carrying them out, but that sensations can- 

 not be aroused by impulses reaching the spinal cord only, 

 nor has the Will its seat there ; volition and consciousness 

 are dependent upon states of the brain. This follows from 

 the unconscious movements of the limb which follow stimu- 

 lation of its skin after such injury to the spinal cord as 

 prevents the transmission of nervous impulses farther on; 



Point out a fact tending to prove that the muscles are normally 

 excited to contraction through their nerves. State facts showing 

 that the nerves of sensation and those governing the muscles are 

 distinct. What purpose does the nerve-centre serve? 



What further conclusions may we draw from the facts already 

 considered in this chapter? Give reasons for your answer. 



