208 THE HUMAN BODY. 



physiological character of a nerve-fibre is its conductivity 

 its power of propagating a disturbance when once its mo- 

 lecular equilibrium has been upset at any one point it is 

 obvious that through the nervous system any one part of the 

 Body, supplied with nerves, may react on all other parts 

 (with the exception of such as hairs and nails and cartilages, 

 which are not known to possess nerves) and excite changes in 

 them. Pre-eminently the nervous system forms a uniting 

 anatomical and physiological bond through the agency of 

 which unity and order are produced in the activities of differ- 

 ent and distant parts. We may compare it to the Western 

 Union Telegraph, the head office of which in New York 

 would represent the brain and spinal cord, the more impor- 

 tant central offices in other large cities the sympathetic 

 ganglia, and the minor offices in country stations the sporadic 

 ganglia; while the telegraph-wires, directly or indirectly 

 uniting all, would correspond to the nerve-trunks. Just as 

 information started along some outlying wire may be trans- 

 mitted to a central office, and from it to others, and then, 

 according to what happens to it in the centre, be stopped 

 there, or spread in all directions, or in one or two only, so 

 may a nervous disturbance reaching a centre by one nerve- 

 trunk merely excite changes in it or be radiated from it 

 through other trunks more or less widely over the Body and 

 arouse various activities in its other component tissues. In 

 common life the very frequency of this uniting activity of the 

 nervous system is such that we are apt to entirely overlook 

 it. We do not wonder how the sight of pleasant food will 

 make the mouth water and the hand reach out for it; it 

 seems, as we say, "natural," and to need no explanation. 

 But the eye itself can excite no desire, cause the secretion of 

 no saliva, and the movement of no limb. The whole com- 

 plex result depends on the fact that the eye is united by the 

 optic nerve with the brain, and that again by other nerves 

 with saliva-forming cells, and with muscular fibres of the 

 arm; and through these a change excited by light falling 

 into the eye is enabled to produce changes in far-removed 

 organs, and excite desire, secretion, and movement. In cases 

 of disease this action exerted at a distance is more apt to ex- 

 cite our attention: vomiting is a Tr ery common symptom of 

 certain brain diseases, and most people know that a disordered 

 stomach will produce a headache; while the pain consequent 



