168 



GERM 



but also used in reference to micro- 

 organisms associated with disease (see BACTERIA, 

 &c. ). By 'germ-cells' the reproductive elements, 

 especially the ova, are meant; while 'germ- 

 plasma' is a very common modern word for the 

 most essential parts of the nuclei in the repro- 

 ductive cells. See EMBRYOLOGY, HEREDITY. 



GERM THEORY OF DISEASE, as the name 

 implies, seeks to find the explanation of certain 

 well-recognised conditions of disease in the presence 

 and action of specific living organisms within the 

 affected body. Though comparatively recently 

 introduced as an efficient working hypothesis in 

 the investigation of some hitherto ill-understood 

 pathological phenomena, the correctness of the 

 theory is now generally admitted. The facts which 

 it has aided in establishing and the numberless 

 investigations which it has inspired have created 

 an important department of medical science. The 

 study of bacteriology (see BACTERIA) has awakened 

 fresh interest in almost every branch of medicine ; 

 and the subject possesses a large and extensive 

 literature of its own. 



The evolution of the theory was due mainly to 

 two factors : ( 1 ) The discussions and investigations 

 which circled round the process of fermentation ; 

 (2) the application of more perfect microscopical 

 methods to the study of the lowest forms of plant 

 and animal life. 



(1) The familiar process of Fermentation (q.v. ) 

 gave birth to much debate. The earlier chemists 

 (Gay-Lussac, and more recently Liebig) held that 

 fermentation was merely the result of the process 

 of decay of organic matter. Various modifications of 

 this doctrine, which cannot be considered here, were 

 enunciated, but the general conclusion remained 

 the same. On the other hand, so early as 1812, 

 Appert had demonstrated from the practical side 

 that organic substances capable of fermentation or 

 putrefaction could be preserved intact if kept in 

 closely stoppered bottles which were afterwards ex- 

 posed to the temperature of boiling water. In 1836 

 Cagniard-Latour described an organism, the yeast 

 plant, which he affirmed to be constantly present 

 in the fermenting fluid. Its growth and reproduc- 

 tion he believed to proceed synchronously with the 

 fermentation. Schwann (1837) described this 

 organism independently, and Helmholtz ( 1843) con- 

 firmed the observation. They maintained that the 

 process, in place of being a mere decomposition, 

 was vital and depended on the presence of the 

 organism they hau discovered. This revolution- 

 ary doctrine was further elaborated pre-eminently 

 by Pasteur and by Schultz, Schroeder, Dusch, 

 Lister, Tyndall, and others. Their researches 

 showed that fermentation was caused by the pres- 

 ence of these organisms ; that the exclusion of 

 these from fluids capable of fermentation, by vari- 

 ous methods of sterilisation and filtration of the 

 air in Avhich they were abundantly present, was 

 sufficient to prevent its occurrence ; that the 

 doctrine which attributed the production of fer- 

 mentation to the influence of certain gases e.g. 

 oxygen (Gay-Lussac) was erroneous ; that the idea 

 of the spontaneous generation (see SPONTANEOUS 

 GENERATION) of such organisms within properly 

 sterilised and protected fluids ( Needham, Bastian, 

 Poucliet, Huizinga) was fallacious; and that the 

 so-called putrefaction was but one variety of fer- 

 mentation. 



( 2 ) One result of these discussions was to develop 

 & refinement of the methods of microscopical re- 

 search, more especially with reference to the in- 

 vestigation of the lowest forms of life (see BAC- 

 TERIA). Though bacteria had been recognised and 

 described in the 17th century ( Leeuwenhoek ), it is 

 mainly to the researches of the latter half of the 

 19th century that we are indebted for an approach to 



an accurate knowledge of the life-history of these 

 organisms. By the masterly labours of Cohn, De 

 Bary, Zopf, Van Tieghem, Nageli, Klebs, Koch, and 

 many others, the methods of demonstration have 

 been improved to an extraordinary degree. The 

 elaboration of staining methods alone, in conjunc- 

 tion with the use of perfected lenses, has made 

 possible the detection and examination of minute 

 organisms hitherto unrecognisable. 



It is impossible to say when the idea of an 

 analogy between the familiar phenomena of fer- 

 mentation and those of acute disease first arose. 

 It is certain that before the 19th century there 

 had been prevalent an ill-defined feeling after some- 

 thing of the kind. More than two hundred years 

 ago Robert Boyle ( 1627-91 ), in his ' Essay on the 

 Pathological Part of Physik,' clothes the idea in 

 words which, as Tyndall has said, ' have in them 

 the forecast of prophecy. ' The idea received more 

 definite formulation in consequence of the re- 

 searches into the nature of fermentation just 

 referred to. In 1848 Fuchs stated that he had 

 discovered bacteria in animals which had died of 

 septicremia. In 1850 it was announced (Davaine^ 

 Branell, Pollender) that bacilli had been detected 

 in the carcasses of animals affected with anthrax. 

 The discovery was corroborated by various ob- 

 servers. But it was not till the disease had been 

 induced by the inoculation of healthy animals with 

 a minimal quantity of the organism (Davaine) 

 that the Bacillus anthracis was recognised as the 

 cause of the disease. Thus was afforded the first 

 substantial proof of the germ theory. This success 

 inspired further research on kindred lines. In 

 comparatively quick succession other discoveries 

 were announced, till, in 1882, Koch described the 

 Bacillus tuberculosis as the organism responsible 

 for the scourge of consumption, and in 1883 the 

 bacillus of cholera. 



Emphasis must be laid on the statement that 

 the discovery of an organism in the circulation or 

 tissues of a diseased animal cannot be accepted 

 as proving the causal efficacy of the former. Apart 

 from further experiment, it were perfectly fair to 

 argue that such organism was a mere accompani- 

 ment of the morbid state, flourishing on the dying 

 or diseased tissues. And, in fact, such secondary 

 factors are recognised. It has, moreover, fre- 

 quently happened that competing claims have 

 been advanced in explanation of the same disease. 

 It was necessary, therefore, that there should be 

 formulated ( Klebs, Koch ) certain conditions, since 

 known as Koch's postulates, which must be ful- 

 filled by an organism whose causal relationship 

 with a given disease is maintained. These are as 

 follows : ( 1 ) The organism must be demonstrated 

 in the circulation or tissues of the diseased animal ; 

 (2) the organism, so demonstrated, must be capable 

 of artificial cultivation in suitable media outside 

 the body, and successive generations of pure culti- 

 vation obtained; (3) such pure cultivation must, 

 when introduced into a healthy and susceptible 

 animal, produce the given disease; (4) the organ- 

 ism must again be found in the circulation or 

 tissues of the inoculated animal. The claims of 

 organisms which fail to meet these demands must 

 be set aside to await further proof. 



The number of diseases whose specific origin is 

 now generally admitted is comparatively large, 

 but of few of these can we speaK with the same 

 certainty as may be done regarding consumption 

 (tuberculosis) and splenic fever (anthrax). In 

 other words, the fulfilment of all four postulates, 

 by many of them has not been demonstrated or 

 has been disputed. Besides anthrax and tuber- 

 culosis, the list includes leprosy, cholera ( Asiatic ), 

 relapsing fever, typhoid fever, yellow fever, malaria, 

 diphtheria, dysentery, syphilis, acute pneumonia, 



