MICROSCOPIC STRUCTURE OF THE BLOOD. 291 



given to account for this immunity are many and various. One 

 of these is the " phagocytosis theory of Metschnikoff " : That 

 " immunity against infection is essentially a matter between the 

 invading bacteria on the one hand and the leukocytes of the tissues 

 on the other ; that during the first attack of the disease the white 

 blood-corpuscles gain a tolerance to the poisons of the bacteria, 

 and so are able to resist the next incursion of the enemy." At the 

 present time, however, the theory of immunity which is most gen- 

 erally accepted is that known as " the lateral-chain theory " of 

 Ehrlich, for the explanation of which the reader is referred to text- 

 books on Pathology. The phagocytotic power of the leukocytes is 

 also manifested in their destruction of the products of inflammation. 



It is believed that the colorless corpuscles are concerned in the 

 process of coagulation of the blood (p. 294). 



For the consideration of other properties of the leukocytes, the 

 reader is referred to pages 24, 25, 293, and 300. 



Development of Colorless Corpuscles. The first leukocytes are 

 formed from the embryonic cells of the mesoblast, and afterward 

 from lymphatic glands and lymphatic tissues generally. They 

 pass into the lymphatics and thence into the blood-vessels. 



Plaques. These are also known as blood-plates, blood-platelets, 

 and hematoblasts. They are circular or elliptical in shape, and 

 smaller than the red corpuscles, but vary very much in size, from 

 0.5 /J. to 5.5 /Jt, and are colorless. Their number is said to vary 

 from 180,000 to more than 600,000. 



Various theories have been propounded to explain their occur- 

 rence in blood. One of these is that they are not formed struct- 

 ures, but simply precipitates of nucleoproteid from the plasma. 

 This theory is, we think, no longer held. There is little doubt 

 that they are formed elements existing normally in the blood, 

 although the theory of Hayem, who discovered them, that red 

 corpuscles are formed from them, is no longer maintained. 



Lilienfeld has obtained from them a nucleo-albumin, called by 

 him nucleohiston, which is also found in the nuclei of the leuko- 

 cytes, and it is believed by many that the blood-plates are nothing 

 more than the nuclei of the polynucleated colorless corpuscles, 

 which are set free when these corpuscles disintegrate, and that 

 these plates also disintegrate, and are dissolved in the plasma. 

 Their possible relation to the coagulation of the blood is discussed 

 in connection with that subject (p. 294). 



Blood-plasma. The plasma or liquor sanguinis is yellowish in 

 color and alkaline in reaction, having a specific gravity of 1027 

 to 1031. It contains the following ingredients : AYater, inorganic 

 salts, extractives, enzymes, proteids, and gases. 



Water and Inorganic Salts. Water exists in plasma to the 

 approximate amount of 90 per cent., so that 10 per cent, consists 

 of solids. Of these solids, about 0.85 per cent, are inorganic 



