302 THE HUMAN BODY 



ing into themselves foreign particles present in the blood; this 

 they do in a manner similar to that in which an amoeba feeds: 

 the process is known as phagocytosis and the cells exhibiting it as 

 phagocytes. Among the substances observed to be taken up by 

 white corpuscles are the minute organisms known as Bacteria, 

 certain species of which have been proved to be the causes of some 

 diseases. The white corpuscles doubtless in this way play an 

 important part in the cure of such diseases, or in their preven- 

 tion in persons exposed to infection. The accumulation of white 

 corpuscles in inflamed or injured parts is probably primarily as- 

 sociated with the removal of dead and broken-down tissues, 

 though it may be carried to excess as in the case of purulent ac- 

 cumulations. 



The Blood-Plates. These are a third. kind of corpuscle which 

 remained undiscovered for a long time after the others were known 

 because they break up and disappear- very soon after the blood 

 is shed unless special precautions are taken to preserve them. 

 They are smaller than the red corpuscles; in structure and com- 

 position they appear to resemble somewhat the colorless corpus- 

 cles, although they do not possess a well-marked nucleus. They 

 are said to exhibit amoeboid movements under certain conditions. 

 The only function that is known for them is in connection with 

 the process of blood-clotting (see page 317). They are fairly 

 numerous, the blood containing perhaps one-tenth as many plate- 

 lets as red corpuscles. The promptness with which they disin- 

 tegrate when exposed to a foreign environment is their most 

 marked characteristic. 



The Blood-Plasma. This is a very complex liquid, containing 

 as it does all the varied substances which are associated in the 

 carrying out of the blood's many functions. The plasma is 90 

 per cent water; of the remaining 10 per cent the chief portion con- 

 sists of organic compounds, mostly protein, serum albumin, para- 

 globulin, and fibrinogen; the remainder is sugar, about 0.15 

 per cent, and fat; the latter constituent varies greatly, being 

 considerable after a meal rich in fats, and slight at other 

 times. About 0.8 of 1 per cent of the plasma consists of sodium, 

 potassium, and calcium salts, the sodium salts constituting by 

 far the greatest part of the inorganic content. The small residue 

 is made up mostly of the waste materials which have been received 



