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 amceba and the most complex animals is 
that the latter are compound amcebe 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 amceba 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 
