HEALTH AND DISEASE 



the corpuscles, if capable of being distinguished, do not long persist in their 

 new host. 



The white or colourless corpuscles, named also leucocytes and lympho- 

 cytes, are present in the blood in much smaller number than the red. The 

 proportion that they have to each other is not, however, a constant one, 

 owing to the fact just stated, that a large accession to the numbers of the 

 white corpuscles occurs after every meal. In the fasting state there is 

 about one white corpuscle to every fifteen hundred red corpuscles, whilst 

 after food the proportion may rise to one white to three hundred red, or 

 even higher. Perhaps, taking the average, the proportion is about 1:500 

 or 1:1000. 



The white corpuscles are spheroidal in form, dotted or granular in aspect, 

 the granules they contain being in some instances coarse, in others fine, 



indicating in all pro- 

 bability a difference 

 in their place of ori- 

 gin. By the action 

 of various chemical 

 substances a nucleus 

 is brought into view, 

 and sometimes two 

 or three appear with 

 great distinctness. 

 Their diameter is 

 about -2T00 i"ch. Their most remarkable character is the power they 

 possess of undergoing changes of form and of moving from place to 

 place (fig. 186). They act, in fact, as if they were parasites, living in 

 the blood, but not necessarily confined to that medium. If a drop of 

 blood be received upon the warmed stage of a microscope, and evaporation 

 be prevented, they may be seen to exhibit perfectly independent move- 

 ments, thrusting out little processes in this or that direction and with- 

 drawing them again, exactly as an amoeba would do if placed under the 

 same conditions. By this means they are able to pass through the walls 

 of the smaller blood-vessels and then wander freely through the outlying 

 tissues, a process that is termed diapedesis. 



The small corpuscles known as platelets are flattened, disc-shaped, or 

 irregular particles, in regard to the nature of \\'hicli little has been ascer- 

 tained. 



The plasma of the blood, in which the corpuscles are suspended, is a 

 clear fluid having a specific gravity a little less than that of the corpuscles, 

 which, therefore, when the blood is at rest, have a tendency to fall to the 



Fig. 186. — Colourless Blood Corpuscles, showing successive changes of c 

 during a period of ten minutes 



