CAUSES OF DISEASE. ^ 215 



spontaneous movement and possessing the power of 

 ingesting, and digesting when suitable, particles of 

 matter presented to it. The essential difference between 

 the simple amoeba and the most complex animals is 

 that the latter are compound amcebse in which individual 

 cells perform separate duties ; there is a differentiation 

 of labour ; some persist in virtue of their contractility, 

 others for digestive functions ; some secrete, others 

 serve for reproduction, and so forth. Most complex 

 organisms are pervaded by a corpusculated fluid, which 

 may circulate throughout the organism by travers- 

 ing lacunar spaces, or by means of narrow tubular 

 passages possessing distinct walls. This circulating 

 fluid, named blood, serves as a living medium of com- 

 munication between the various parts of an organism. 

 The blood in higher metazoa contains two kinds of 

 corpuscles ; the more numerous are circular or elliptical 

 microscopical discs, tinged of a pale red colour, and, in 

 some vertebrates, furnished with a nucleus. The second 

 variety are nucleated, irregular, colourless, and exhibit 

 amoeboid movement ; they change their shape, and can 

 escape from the confines of the capillary vessels when 

 such are present. Like an amoeba they can ingest, and 

 when suitable digest, particles of matter presented to 

 them. In all vertebrata, with the exception of amphi- 

 oxus and ascidians, red and colourless corpuscles are 

 present. The invertebrata possess, with very few 

 exceptions, only the colourless corpuscles. These 

 corpuscles, or leucocytes as they are called, fulfil some 

 very extraordinary functions. Should a portion of an 

 animal die, the leucocytes will attack it and, if it be 



