THE BLOOD PLATELETS 207 



clearly shows that the leukocytes constitute a most important safe- 

 guard against bacterial invasion. They are therefore directly con- 

 cerned with the production of immunity. 



In this connection mention should also be made of the fact that the 

 mammalian body contains other types of phagocytes to which differ- 

 ent names have been given. Contrary to the white corpuscles, 

 which are migratory phagocytic entities, the cells now referred to 

 remain "stationary.^' They are found, for example, in bony tissue 

 where they have to do with the absorption and removal of all super- 

 fluous material, or in the spleen and liver where they take up the 

 worn out red corpuscles and destroy them. To the first type of cells 

 belong the myeloplaxes of the bone-marrow, while the second group is 

 represented by the so-called giant cells and the third, by the endothelial 

 lining cells of the hepatic capillaries, generally known as the ''Stern- 

 zellen" of Kupfer. Since the aforesaid cells are so closely related in 

 function, it is quite probable that they are also allied to the leuko- 

 cytes in structure as well as embryologically. 



Allied Functions. — Certain other functions have been ascribed to 

 the white corpuscles, the most important of which is their power of 

 taking up nutritive material and of carrying it to different parts of the 

 body. Thus, the lymphocytes are said to absorb fat globules and to 

 convey them into the lymph channels. They are also supposed to 

 aid in the absorption of the peptones and to help in maintaining a 

 proper protein content of the blood. Both functions are in keeping 

 with their phagocytic properties. Sufficient evidence is also at hand 

 to show that the leukocytes contain a substance which, when liberated, 

 plays an important part in the coagulation of the blood. 



CHAPTER XVIII 



THE BLOOD PLATELETS 



Physical Characteristics. — While the blood platelets are usually 

 described as rounded biconvex discs, it must be granted that their 

 shape varies considerably from almost globular to flat. They have 

 also been observed to assume a spindle shape; in fact, it is stated that 

 they possess this form normally in the horse. They give no particu- 

 lar color impression. Their granular centers refract very strongly, 

 and stain deeply with basic dyes. For this reason, they are said to 

 contain a real nucleus, and may therefore be regarded as true cells. 

 They display ameboid movements, and if collected in a favorable 

 medium, present a number of variegated processes. Their specific grav- 

 ity is less than that of the other formed elements of the blood, which 

 fact accounts for their occupying the outermost layers of the blood 

 stream. As their diameter naeasures as a rule no more than 3/x, they 



1^- 



