188 THE HUMAN BODY. 



vents the farther transmission of nervous impulses (show- 

 ing 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 rt flex actions serves to transmit nervous im- 

 pulses to and from the brain; a fact which 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 peri- 

 pheral parts, contains many which unite it to other centres; 

 and others which connect the various centres in it, as those 

 for the arms and legs, together. 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 impulses 

 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 }ris spinal cord 

 cut or diseased in the thoracic region will kick violently if the 

 soles of his feet be tickled, but will feel nothing of the tick- 

 ling, and if he did not see his legs would not know that they 

 were moving. Reflex centres moreover do not act, as a rule, 

 indifferently and casually, but rearrange the impulses reach- 

 ing them, so as to produce a protective or in some way advan- 

 tageous result. In other words, these centres, acting in 

 health, commonly co-ordinate the incoming impulses and give 

 rise to outward-going impulses which produce an apparently 

 purposive result. The burnt hand or the tickled foot, in the 

 absence of all consciousness, is snatched away from the irri- 

 tant; and food chewed in the mouth excites nerves there 

 which act on a centre which causes certain cells in the salivary 

 glands to make and pour into the mouth more saliva. In 

 addition to the reflex centres we have others, placed in the 



