182 THE HUMAN BODY. 



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 stimula- 

 tion of its skin after such injury to the spinal cord as pre- 

 vents the transmission of nervous impulses farther on 

 (showing that the cord is a reflex centre), and from the 

 absence, in such cases, of sensation in the part whose nerves 

 have been injured, and the loss of the power of voluntarily 

 causing its muscles to contract. 



6. Finally we conclude that the spinal cord in addition 

 to being a centre for reflex actions serves to transmit nerv- 

 ous impulses to and from the brain ; a fact which is con- 

 firmed by the histological observation that in addition to 

 the nerve-cells, which are the characteristic constituents of 

 nerve-centres, it contains the simply conductive nerve- 

 fibres, many of which pass on to the brain. In other 

 words the spinal cord, besides containing fibres which enter 

 it from, and pass from it to, peripheral parts contains 

 many which unite it to other centres ; and connect the 

 various centres in it, as those for the arms and legs, to- 

 gether. This is true not only of the spinal cord but of the 

 brain (which contains many fibres uniting different centres 

 in it), and probably of all nerve-centres. 



The Functions of Nerve-Centres and Nerve-Trunks. 

 From what has been stated in the previous paragraphs it is 

 clear that we may distinctly separate the nerve- trunks from 

 the nerve-centres. The fibres serve simply to convey im- 

 pulses either from without to a centre or in the opposite 

 direction, while the centres conduct and do much more. 

 Some, as the spinal cord, are merely reflex centres, and have 

 nothing to do with states of consciousness. A man with 

 his spinal cord cut or diseased in the dorsal region will 

 kick violently if the soles of his feet be tickled, but will 

 feel nothing of the tickling, and if he did not see his legs 

 would not know that they were moving. Eeflex centres 



