3»4 



INDUCTION. 



dependently of the will. There are 

 many experiments which prove that 

 irritation of a nerve in one part of 

 the body may in this manner excite 

 powerful action in another part ; for 

 example, food injected into the sto- 

 mach through a divided oesophagus 

 nevertheless produces secretion of 

 saliva ; warm water injected into the 

 bowels, and various other irritations 

 of the lower intestines, have been 

 found to excite secretion of the gastric 

 juice, and so forth. The reality of 

 the power being thus proved, its 

 agency explains a great variety of 

 apparently anomalous phenomena, of 

 which I select the following from Dr. 

 Brown-S^quard's Lectures on the Ner- 

 vous System : — 



The production of tears by irrita- 

 tation of the eye, or of the mucous 

 membrane of the nose. 



The secretions of the eye and nose 

 increased by exposure of other parts 

 of the body to cold. 



Inflammation of the eye, especially 

 when of traumatic origin, very fre- 

 quently excites a similar affection in 

 the other eye, which may be cured 

 by section of the intervening nerve. 



Loss of sight sometimes produced 

 by neuralgia ; and has been known 

 to be at once cured by the extirpation, 

 for instance, of a carious tooth. 



Even cataract has been produced 

 in a healthy eye by cataract in the 

 other eye, or by neuralgia, or by a 

 wound of the frontal nerve. 



The well-known phenomenon of a 

 sudden stoppage of the heart's ac- 

 tion, and consequent death, produced 

 by irritation of some of the nervous 

 extremities, e.g., by drinking very 

 cold water, or by a blow on the ab- 

 domen, or other sudden excitation 

 of the abdominal sympathetic nerve, 

 though this nerve may be irritated 

 to any extent without stopping the 

 heart's action if a section be made of 

 the communicating nerves. 



The extraordinary effects produced 

 on the internal organs by an extensive 

 burn on the surface of the body, con- 

 sisting of violent inflammation of the 



tissues of the abdomen, chest, of 

 head, which, when death ensues from 

 this kind of injury, is one of the most 

 frequent causes of it. 



Paralysis and anaesthesia of one 

 part of the body from neuralgia in 

 another part ; and muscular atrophy 

 from neuralgia, even when there is 

 no paralysis. 



Tetanus produced by the lesion of 

 a nerve ; Dr. Brown- Sequard thinks 

 it highly probable that hydrophobia 

 is a phenomenon of a similar nature. 



Morbid changes in the nutrition of 

 the brain and spinal cord, manifest- 

 ing themselves by epilepsy, chorea, 

 hysteria, and other diseases, occa- 

 sioned by lesion of some of the ner- 

 vous extremities in remote places, as 

 by worms, calculi, tumours, carious 

 bones, and in some cases even by very 

 slight irritations of the skin. 



§ 4- From the foregoing and similar 

 instances we may see the importance, 

 when a law of nature previously un- 

 known has been brought to light, or 

 when new light has been thrown upon 

 a known law by experiment, of ex- 

 amining all cases which present the 

 conditions necessary for bringing that 

 law into action ; a process fertile in 

 demonstrations of special laws pre- 

 viously unsuspected, and explanations 

 of others already empirically known. 



For instance, Faraday discovered 

 by experiment that voltaic electricity 

 could be evolved from a natural mag- 

 net, provided a conducting body were 

 set in motion at right angles to the 

 direction of the magnet ; and this he 

 found to hold not only of small mag- 

 nets, but of that great magnet, the 

 earth. The law being thus estab- 

 lished experimentally that electricity 

 is evolved by a magnet and a con- 

 ductor moving at right angles to the 

 direction of its poles, we may now 

 look out for fresh instances in which 

 these conditions meet. Wherever a 

 conductor moves or revolves at right 

 angles to the direction of the earth's 

 magnetic poles, there we may expect 

 an evolution of electricity. In the 



