346 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



are supposed in fact to be brought into the blood through the lymph. 2. Mono- 

 nuclear leucocytes, which are large corpuscles with a vesicular nucleus and 

 abundant cytoplasm : they have the power of making amosboid movements. 

 3. Polymorphous or polynucleated leucocytes, which are large corpuscles with 

 the nucleus divided into lobes that are either entirely separated or are connected 

 by fine protoplasmic threads. This form shows active amoeboid movements. 



It is impossible to say whether these varieties of blood-leucocytes are 

 distinct histological units which have independent origins and more or less 

 dissimilar functions, or whether, as seems more probable to the writer, they 

 represent different stages in the development of a single type of cell, the 

 lymphocytes forming the youngest and the polymorphic or polynucleated 

 leucocytes the oldest stage. Perhaps the most striking property of the leuco- 

 cytes as a class is their power of making ameboid movements a charac- 

 teristic which has gained for them the sobriquet of " wandering " cells. By 

 virtue of this property some of them are able to migrate through the walls 

 of blood-capillaries into the surrounding tissues. This process of migration 

 takes place normally, but is vastly accelerated under pathological conditions. 

 As to the function or functions fulfilled by the leucocytes, numerous sugges- 

 tions have been made, some of which may be stated in brief form as follows: 

 (1) They protect the body from pathogenic bacteria. In explanation of this 

 action it has been suggested that they may either ingest the bacteria, and thus 

 destroy them directly, or they may form certain substances, defensive proteids, 

 which destroy the bacteria. Leucocytes that act by ingesting the bacteria 

 are spoken of as "phagocytes" (<pdytv, to eat; x#ro<r, cell). This theory of 

 their function is usually designated as the " phagocytosis theory of Metschni- 

 koff ;" it is founded upon the fact that the amosboid leucocytes are known to 

 ingest foreign particles with which they come in contact. The theory of the 

 protective action of leucocytes has been used largely in pathology to explain 

 immunity from infectious diseases, and for details of experiments in support 

 of it reference must be made to pathological text-books. (2) They aid in 

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

 peptones from the intestine. These latter two theories will be spoken of 

 more in detail in describing the process of absorption. (See the section upon 

 Digestion.) 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 pro- 

 cess of blood-coagulation. A complete statement with reference to this 

 function must be reserved until the phenomenon of coagulation is de- 

 scribed. (5) They help to maintain the normal composition of the blood- 

 plasma as to proteids. It may be said for this view that there is considerable 

 evidence that the leucocytes normally undergo disintegration and dissolu- 

 tion in the circulating blood, to some extent at least. The blood-proteids are 

 peculiar, and they are not obtained 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. None of the theories mentioned 



