THE OEIGIN AND PROPAGATION OF DISEASE. 229 



came known, it was perfectly evident liow tliese epidemics originated 

 and exactly how far they might extend. Each one was commenced by 

 the slaughter and preparation for food of a trichinous pig 5 and the pa- 

 tients affected were precisely those who had introduced into their sys- 

 tems ever so small a portion of the infectious food. 



In this instance, also, there was found to be an unexpected relation be- 

 tween two different forms of the same parasite. Trichina sj)iralis had 

 been known since 1830 ; but it had yet been seen only in its quiescent, 

 encysted form, embedded in the muscular tissue, without movement or 

 reproduction. Consequently, though we were familiar with the worm 

 itself, Ave knew nothing of the disease produced by it. Its new growth 

 and active reproduction in the intestinal canal, the swarming emigra- 

 tion of its innumerable progeny, and the constitutional symptoms which 

 followed, were a new revelation, and showed that the whole system, as 

 well as a particular organ or tissue, might suffer from the effects of para- 

 sitic contamination. 



In all the affections which have now been enumerated, the parasite is 

 one of ^n animal nature, with regular generative apparatus and active 

 sexual reproduction. But the last thirty years have seen a very remark- 

 able advance also in our knowledge of the vegetable parasites. This has 

 naturally coincided with a similar activity among scientific botanists 

 in the study of the simpler forms of vegetation, the cryptogamic plants in 

 general, and particularly of the microscopic fungi and algae. A little over 

 a half a century ago the species of flowering plants described by bota- 

 .nists were much more numerous than the cryptogams ; but now the pro- 

 portions of the two classes are reversed. In 1818, according to Mr. 

 Cooke,* an eminent British botanist, '' less than eighty of the more mi- 

 nute species of fungi, but few of which deserved the name of micro- 

 scopic, were supj)osed to contain all then known of these wonderful 

 organisms. Since that x)eriod microscopes have become very different 

 instruments ; and one result has been the increase of the 564 species of 

 British fungi to 2,479. By far the greater number of the species thus 

 added depend for their specific characters upon microscopical exam- 

 ination. At the present time the number of British species of flowering 

 plants scarcely exceeds three-fourths of the number of fungi alone, not 

 to mention ferns, mosses, algse, and lichens." 



A large proportion of these microscopic plants are parasitic upon other 

 organisms; and for the earliest study of them, as connected with dis- 

 ease in the human subject, we are indebted to the dermatologists. 



The first discovery of parasitic vegetation in cutaneous affections was 

 by Schonlein, in 1839,t who found, in the crust oifavus, cryptogamic vege- 

 table filaments ramifying in the diseased growth. In 1841 Gruby made 

 a similar observation,!: and described accurately both the mycelium fila- 



* " Introduction to the Study of Microscopic Fungi," London, 1870, p. 45. 



t Muller's Archiv for 1839 ; cited in Robin's " V6g6taux parasites," Paris, 1853, p. 477. 



i "Comptes Eendus derAcad6mie des Sciences," 1841, tome xiii, pp. 72, 309. 



