Chap. VIII.] THE VASCULAR SYSTEM. 99 



movement, and are capable of changing their form and place. 

 While, when in a state of rest, they assume in general the 

 spheroidal form, we find that when they become active they 

 send out variously shaped processes, some fine and delicate, 

 others broad, and of very irregular shape. We often see, after 

 a process has been thrown out, that it becomes larger and 

 larger, the cell-body becoming correspondingly smaller, until 

 finally the whole cell passes over into the process, thus moving 

 forward. These amoeboid movements are always very slow, 

 and are greatly influenced by the temperature, density, and 

 amount of oxygen in the fluid in which the cells lie. By virtue 

 of this locomotive power the white blood cells perform certain 

 evolutions within the blood-vessels ; they also escape through 

 their walls, and sometimes singly, sometimes in vast numbers, 

 move through the lymph spaces in the surrounding tissues. 

 This is spoken of as the "migration of the white corpuscles." 

 In an "inflamed area" large numbers of white corpuscles are 

 thus drained away from the blood. These migrating corpuscles, 

 or wandering cells, may, by following the devious tracks of the 

 lymph, find their way back into the blood ; some of them, how- 

 ever, may remain and undergo various changes. Thus in in- 

 flamed areas, when suppuration follows inflammation, the white 

 corpuscles which have migrated may become "pus corpuscles." 



Again, by virtue of their amoeboid movements, the white 

 corpuscles can creep around objects, enveloping them with their 

 own substance, and so putting them inside themselves. As an 

 illustration of this action of the white corpuscle, we may state 

 that, according to some observers in certain diseases in which 

 micro-organisms make their appearance in the blood, the white 

 corpuscles take up these micro-organisms into their substance, 

 and probably exert an influence over them, which modifies the 

 course of the disease of which these micro-organisms are the 

 essential cause. 



Furthermore, the white corpuscles are not only capable of 

 taking up particles in the blood, but are also capable of giving 

 up products which they have changed or modified, to the blood, 

 and it follows that these metabolic changes must necessarily 

 affect the composition of the fluid plasma in which they lie. 



The plasma of the blood. — The plasma is a clear, slightly 

 yellowish coloured fluid, consisting for the most part of water, 



