302 INDUCTION. 



Both these propositions had to be proved by experiment; and for the 

 experiments which prove them, science is also indebted to Dr. Brown-Se- 

 quard. The former of the two — that paralyzed muscles have greater irri- 

 tability than healthy muscles — he ascei'tained in various ways, but most 

 decisively by "comparing the duration of irritability in a paralyzed muscle 

 and in the corresponding healthy one of the opposite side, while they are 

 both submitted to the same excitation." He " often found, in experiment- 

 ing in that way, that the paralyzed muscle remained irritable twice, three 

 times, or even four times as long as the healthy one." This is a case of in- 

 duction by the Method of Difference. The two limbs, being those of the 

 same animal, were presumed to differ in no circumstance material to the 

 case except the paralysis, to the prese^ice and absence of which, therefore, 

 the difference in the muscular irritability was to be attributed. This as- 

 sumption of complete resemblance in all material circumstances save one, 

 evidently could not be safely made in any one pair of experiments, because 

 the two legs of any given animal might be accidentally in very different 

 pathological conditions ; but if, besides taking pains to avoid any such dif- 

 ference, the experiment was repeated sufficiently often in different animals 

 to exclude the supposition that any abnormal circumstance could be pres- 

 ent in them all, the conditions of the Method of Difference were adequate- 

 ly secured. 



In the same manner in which Dr. Bi'own-Sequard proved that paralyzed 

 muscles have greater irritability, he also proved the correlative proposition 

 respecting cadaveric rigidity and putrefaction. Having, by section of the 

 roots of the sciatic nerve, and again of a lateral half of the spinal cord, 

 produced paralysis in one hind leg of an animal while the other remained 

 healthy, he found that not only did muscular irritability last much longer 

 in the paralyzed limb, but rigidity set in later and ended later, and putre- 

 faction began later and was less rapid than on the healthy side. This is a 

 common case of the Method of Difference, requiring no comment. A fur- 

 ther and very important corroboration was obtained by the same method. 

 When the animal was killed, not shortly after the section of the nerve, but 

 a month later, the effect was reversed ; rigidity set in sooner, and lasted a 

 shorter time, than in the healthy muscles. But after this lapse of time, the 

 paralyzed muscles, having been kept by the paralysis in a state of rest, had 

 lost a great part of their irritability, and instead of more, had become less 

 irritable than those on the healthy side. This gives the ABC, a ^c, and 

 BC, 5 c, of the Method of Difference. One antecedent, increased irrita- 

 bility, being changed, and the other circumstances being the same, the con- 

 sequence did not follow ; and, moreover, when a new antecedent, contrary 

 to the first, was supplied, it was followed by a contrary consequent. This 

 instance is attended with the special advantage of proving that the re- 

 tardation and prolongation of the rigidity do not depend directly on the 

 paralysis, since that was the same in both the instances ; but specifically on 

 one effect of the paralysis, namely, the increased irritability ; since they 

 ceased when it ceased, and were reversed when it was reversed. 



2d. Diminution of the temperature of muscles before death increases 

 their irritability. But diminution of their temperature also retards cadav- 

 eric rigidity and putrefaction. 



Both these truths were first made known by Dr. Brown-Sequard himself, 

 through experiments which conclude according to the Method of Differ- 

 ence. There is nothing in the nature of the process requiring specific 

 analysis. 



