20 THE MICROSCOPE 



first place, that the muscular fibres of the smaller arteries yield and 

 become flaccid and elongated, and, in consequence of this, the 

 diameter of their tube is increased. A greater quantity of 

 blood, therefore, flows into the capillaries, which become dis- 

 tended, and the movement of the blood gets slower and slower, 

 and at last completely stops ; and if this continues long, it would 

 result in serious structural changes of the inflamed part. But if 

 at this stage the mustard be removed and the web kept moist, it 

 will be found that the movement of the blood re-commences, and 

 the normal condition is re-estabHshed, and the part resumes its 

 natural state. 



One of the most important results of microscopical research 

 has been the development of the germ theory of infectious diseases, 

 by which theory it is supposed that the epidemic diseases are due 

 to minute parasitic germs, which float in the atmosphere, enter the 

 body, and by their growth and development produce various 

 febrile disturbances of more or less gravity, the communication of 

 which germs from one individual to another constitutes infection. 

 It is true that, at present, the actual germs, the development 

 of which cause the various infectious diseases to which man is 

 subject, have not been microscopically demonstrated, although 

 certain German microscopists suppose that they have found the 

 germs of cholera and measles; but disease-germs have been 

 plainly traced in some of the diseases to which the lower animals 

 are subject. The first step in this branch of study was the 

 simultaneous discovery by Cagniard de la Tour and Schwann of 

 Berlin in 1836 of the yeast-plant, a living organism, which, 

 when placed in a proper medium, feeds, grows, and reproduces 

 itself, and in this way carries on the process which we call fer- 

 mentation. Some years subsequently, the great Pasteur discovered 

 that a mysterious disease, which affected the silkworms in the 

 valley of the Rhone, was caused by a living parasite, which 

 consisted of vibratory corpuscles, which took possession of the 

 intestinal canal, and more or less completely occupied the whole 

 organism of the worm. Judging by analogy, he came to the 

 conclusion that many of the zymotic diseases are caused by these 

 lower fungi, especially Bacteria^ and since then further investiga- 

 tion and improved microscopic research have discovered the 



