THE ART OF SCIENTIFIC INVESTIGATION 



demonstrate it. However, he persisted in his conviction and 

 finally in desperation he injected the heroic dose of 35 ml. of 

 culture filtrate into a guinea-pig. Rather surprisingly the guinea- 

 pig survived the injection of this volume of fluid and in due 

 course Roux had the satisfaction of seeing the animal die with 

 lesions of diphtheria intoxication. Having established this point 

 Roux was soon able to find out that his difficulties were due 

 to the cultures not having been incubated long enough to 

 produce much toxin, and by prolonged incubation he was able 

 to produce powerfully toxic filtrates. This discovery led to 

 immunisation against diphtheria and the therapeutic use of 

 antiserum.^" 



Following the hypothesis that impulses pass along sympathetic 

 nerves and set up chemical changes producing heat in the skin, 

 Claude Bernard severed the cervical sympathetic nerve in the 

 expectation of it leading to cooling of the rabbit's ear. To his 

 surprise the ear on that side became warmer. He had disconnected 

 the blood vessels of the ear from the nervous influence which 

 normally holds them moderately contracted, resulting in a 

 greater flow of blood and hence warming of the ear. Without 

 at first realising what he had done, he had stumbled on to the 

 fact that the flow of blood through the arteries is controlled by 

 nerves, one of the most important advances in knowledge of 

 circulation since Harvey's classical discovery. An interesting and 

 important illustration of what often happens in the field of 

 observation is provided by Bernard's statement that from 1841 

 onwards he had repeatedly divided the cervical sympathetic 

 without observing these phenomena which he saw for the first 

 time in 1851. In the previous experiments his attention was 

 directed to the pupil; it was not till he looked for changes in the 

 face and ear that he saw them.^* 



Claude Bernard reasoned that the secretion of sugar by the 

 liver would be controlled by the appropriate nerve, which he 

 supposed was the vagus. Therefore he tried puncturing the origin 

 of the nerve in the floor of the fourth ventricle, and found 

 that the glycogenic function of the liver was greatly increased 

 and the blood sugar rose to such an extent that sugar appeared 

 in the urine. However, Bernard soon realised that, interesting 

 and important as were the results obtained, the hypothesis on 



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