CHAP, i.] BLOOD. 47 



bacteria into their substance, and thus probably, by exerting an 

 influence on them, modify the course of the disease of which these 

 bacteria are the essential cause. 



If the white corpuscles are thus engaged during their life 

 in carrying on important labours, we may expect them to differ 

 in appearance according to their condition. Some of the corpuscles 

 are spoken of as 'faintly' or 'finely ' granular. Other corpuscles are 

 spoken of as ' coarsely ' granular, their cell substance being loaded 

 with conspicuously discrete granules. It may be of course that 

 there are two distinct kinds of corpuscles, having different functions 

 and possibly different origins and histories ; but since intermediate 

 forms are met with containing a few coarse granules only, it is more 

 probable that the one form is a phase of the other, that a faintly 

 granular corpuscle by taking in granules from without or by pro- 

 ducing granules within itself as products of its metabolism, may 

 become a coarsely granular corpuscle. 



Whether however the white corpuscles are really all of one 

 kind, or whether they are different kinds performing different 

 functions, must at present be left an open question. 



Blood Platelets. 



33. In a drop of blood examined with care immediately after 

 removal, may be seen a number of exceedingly small bodies (2 ^ 

 to 3 /A in diameter) frequently disc-shaped but sometimes of a 

 rounded or irregular form, homogeneous in appearance when quite 

 fresh but apt to assume a faintly granular aspect. They are 

 called blood platelets. They have been supposed by some to become 

 developed into and indeed to be early stages of the red corpuscles, 

 and hence have been called hsematoblasts ; but this view has not 

 been confirmed, indeed, as we have seen ( 27), the real haemato- 

 blasts or developing red corpuscles are of quite a different nature. 



They speedily undergo change after removal from the body, 

 apparently dissolving in the plasma ; they break up, part of their 

 substance disappearing, while the rest becomes granular. Their 

 granular remains are apt to run together forming in the plasma the 

 shapeless masses which have long been known and described as 

 "lumps of protoplasm." By appropriate reagents, however, these 

 platelets may be fixed and stained in the condition in which they 

 appear after leaving the body. 



The substance composing them is peculiar, and though we 

 may perhaps speak of them as consisting of living material, their 

 nature is at present obscure. They may be seen within the living 

 blood vessels, and therefore must be regarded as real parts of the 

 blood and not as products of the changes taking place in blood 

 after it has been shed. 



