230 MANUAL OF PHYSIOLOGY. 



No doubt they absorb fluid nutriment continually from the 

 surrounding medium, as is shown by the effect of poisons on 

 them ; and, by the repeated contractions and relaxations of parts 

 of their substance in the form of pseudopodia, they appear to take 

 into the inner parts of the protoplasm solid particles, which after 

 some time are ejected after the manner of the small unicellular 

 animals known as amoebae (p. 94). 



While in motion in the circulation none of these amoeboid 

 movements appear to take place, but when an arrest of the flow 

 of blood in the capillaries occurs they not only change their form, 

 but also their position ; and if there be no onward flow of blood 

 for some little time, they creep out of the capillaries, passing 

 through the delicate vessel walls. This emigration of the blood 

 cells is possibly a common event when a tissue is in need of tex- 

 tural repair. When excessive, it forms one of the most striking 

 items of the series of events occurring in inflammation. 



The cells differ much in size ; generally they are somewhat 

 larger than the red disks. Nothing like a cell wall can be seen 

 to surround them, and from the movements above described it 

 would appear certain that they are free masses of active proto- 

 plasm. 



The number of white cells that can be collected is too small to 

 allow of accurate chemical analysis, but there is no reason to 

 suppose that they differ from other forms of protoplasm. 



ORIGIN OF THE COLORLESS BLOOD CELLS. 



Since such an unimportant circumstance as a hearty meal can 

 materially influence the numbers of the white corpuscles, it would 

 appear that they must be usually undergoing rapid variations in 

 their number probably by their being constantly used up and 

 periodically replaced by new ones. The places in which they 

 occur in greatest number are the lymphatic glands, the spleen, 

 and the lymph follicular tissue in the intestinal tract. 



There is no doubt that the lymph contains a much larger pro- 

 portion of these cells after it has passed through the lymph 

 glands, and the blood coming from the spleen contains an exces- 

 sive proportion of them. 



