THE NERVOUS SYSTEM 217 



point on the foot its length cannot be less than one hundred 

 thousand times greater than its diameter, and yet it performs 

 its work with as much precision as fibres that are compara- 

 tively much stronger, and less exposed. (Head Note 2.) 



17. The Sympathetic System. The sympathetic system of 

 nerves remains to be described. It consists of a double chain 

 of ganglia, situated on each side of the spinal column, and 

 extending through the cavities of the trunk, and along the 

 neck into the head. These ganglia are made up for the most 

 part of small collections of gray nerve-cells, and are the nerve- 

 centres of this system* From these, numerous small nerves 

 are derived, which connect the ganglia together, send out 

 branches to the cranial and spinal nerves, and form networks 

 in the vicinity of the stomach and other large organs. A con- 

 siderable portion of them also follows the distribution of the 

 large and small blood-vessels, in which the muscular tunic 

 appears. Branches also ascend into the head, and supply the 

 muscles of the eye and ear, and other organs of sense. 



2. How Bodily Sensations are Located. ' ' A nervous fibre which ends 

 in the skin forms, as far as its union with the brain or cord is concerned, 

 one long, fine, unbroken thread. The fibres, thus ending in the skin, 

 very soon join to form small branches, and finally in thick nerve trunks, 

 but in no case do two nerve fibres coalesce so as to lose their identity. 

 Every part of the skin has its own separate connections with the centre 

 of the nervous systems, which unite there just as telegraph wires unite 

 at a terminus. The brain is the terminus of these lines of nerves, and, 

 as it were, receives and explains the messages sent to it. It distinguishes 

 very clearly by what particular fibre such a message has come, and just 

 as the clerk in a telegraph office, where a great many wires meet from all 

 sides, knows by experience from what direction each wire brings its 

 message, so the brain also knows by experience what part of the skin 

 is involved when a sensation reaches it along a certain nerve fibre. . It is 

 probable that the brain, by its imaginative faculty, has formed a complete 

 picture of the surface of the body a kind of chart slowly made, and 

 always being more highly perfected, by means of which, with each im- 

 pression from without, there arises in the brain a picture of the spot upon 

 the skin where the irritation has taken place. Now, if an irritation were 

 to pass from one nerve fibre to another, it is very plain, the brain could 

 not tell the place from which it came, and could not localize impressions 

 received from the world about us." Bernstein's Five Senses of Man. 



17. The sympathetic system of nerves ? Of what does it consist ? 



