406 BLOOD AND LYMPH. 



the "phagocytosis theory of Metchnikoff " ; it is founded upon the 

 fact that the ameboid leucocytes are known to ingest foreign par- 

 ticles with which they come in contact. The theory of the protec- 

 tive action of leucocytes has been used largely in pathology to 

 explain immunity from infectious diseases, and for details of experi- 

 ments in support of it reference must be made to text-books of 

 pathology. (2) They aid in the absorption of fats from the intestine. 

 (3) They aid in the absorption of peptones from the intestine. It 

 may be noticed here that these theories apply to the leucocytes found 

 so abundantly in the lymphoid tissue of the alimentary canal, rather 

 than to those contained in the blood itself. (4) They take part in 

 the process of blood coagulation. A complete statement with 

 reference to this function must be reserved until the phenomenon of 

 coagulation is described. (5) They help to maintain the normal 

 composition of the blood-plasma in proteids. It may be said for 

 this view that there is considerable evidence to show that the 

 leucocytes normally undergo disintegration and dissolution in the 

 circulating blood, to some extent at least. The blood proteids are 

 peculiar, and they are not formed directly from the digested food. 

 It is possible that the leucocytes, which are the only typical cells in 

 the blood, aid in keeping up the normal supply of proteids. From 

 this standpoint they might be regarded in fact as unicellular glands, 

 the products of their metabolism serving to maintain the normal 

 composition of the blood-plasma. The formation of granules within 

 the substance of the eosinophiles offers a suggestive analogy to the 

 accumulation of zymogen granules in glandular cells. 



Physiology of the Blood Plates. The blood plates are small, 

 circular or elliptical bodies, nearly homogeneous in structure and 

 variable in size (0.5 to 5.5 /*), but they are always smaller than the 

 red corpuscles. Less is known of their origin, fate, and functions 

 than in the case of the leucocytes. When removed from the circu- 

 lating blood they are known to disintegrate very rapidly. This 

 peculiarity, in fact, prevented them from being discovered for a long 

 time after the blood had been studied microscopically. It has been 

 shown that they are formed elements, and not simply precipitates 

 from the plasma, as was suggested at one time. The theory of 

 Hay em, their real discoverer, that they develop into red corpuscles 

 may also be considered as erroneous. There is considerable evi- 

 dence to show that in shed blood they take part in the process of 

 coagulation. The nature of this evidence will be described later. 



On account of their small size and somewhat indefinite form the 



structure of the blood plates is not satisfactorily known. Deetjen* 



has demonstrated that they are capable of ameboid movements. 



When removed from the blood vessels to a glass slide they usually 



* "Virchow's Archiv f. path. Anat. u. Physiol.," 164, 239, 1901. 



