STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 267 



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 283). 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 over 8.5 per cent con- 

 sists of the various food stuffs which the blood carries for the 

 nourishment of the body-cells. Nearly all of this is protein, serum 

 albumin, serum 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 

 into the blood from the tissues, and which are to be gotten rid of 

 in the excretory organs. The most important of these are urea, 

 creatinine, uric acid, and similar bodies. The plasma contains 

 also the various hormones, mentioned in a previous paragraph, 

 and a group of substances, known as biological reagents, which are 

 part of the disease-resisting mechanism. These will be considered 

 in detail in later paragraphs. 



The plasma carries in solution a certain amount of oxygen, 

 nitrogen, and carbon dioxid, but no more than a similar amount 

 of pure water would dissolve at the same temperature. Most of 

 the oxygen carried by the blood is in combination with the hemo- 

 globin of the red corpuscles; most of the carbon dioxid is in com- 

 bination with sodium, forming sodium carbonate or bicarbonate, 

 to which the alkilinity of the blood is due. 



Summary. Practically the composition of the blood may be 

 thus stated: It consists of (1) plasma, consisting of watery solu- 

 tions of serum-albumin, paraglobulin, fibrinogen, sodium and 



