304 THE HUMAN BODY. 



from the absence, in such cases, of sensation in the part 

 whose nerves have been injured; and from 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 unconscious movements serves also to 

 transmit nervous impulses to and from the brain; this is 

 confirmed 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, the skin and muscles, contains 

 many fibres which unite it to other 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. 

 They take heed, some consciously and some unconsciously, 

 of the impulses carried to them by the ingoing nerve-fibres, 

 and then send out impulses along outgoing nerve-fibres; 

 these impulses call into action the proper organs for the 

 safety and well-being of the body in general. The centres 

 do not merely transmit ami reflect, they also co-ordinate. 



Classification of Nerve-Fibres. The nerve-fibres of the 

 body fall into two great groups corresponding to those 



'Far what is the spinal cord a centre? Wliat else does it do? 

 How does histology support the belief that the spinal cord is hoth 

 a nerve-centre and a conductor of nervous impulses? 



What is the function of nerve-fibres? What is done by nerve- 

 centres in addition to conducting nerve-impulses? 



Into what main groups may nerve fibres be classified? 



