48 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



based on morphological characteristics is the following: 1. Lymphocytes, 

 which arc small corpuscles with a round vesicular nucleus and very scanty 

 cytoplasm ; they are not capable of amoeboid movements. These corpuscles 

 are so called because they resemble the leucocytes found in the lymph-glands, 

 and are supposed in fact to be brought into the blood through the lymph. 

 According to Ehrlich, they form from 22 to 25 per cent, of the total number 

 of leucocytes. 2. Mononuclear leucocytes, which are large corpuscles with a 

 vesicular nucleus and abundant cytoplasm : they have the power of making 

 amoeboid movements and arc present in only small numbers, 1 per cent. 

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

 the nucleus divided into lobes that are either entirely separated or are con- 

 nected by line protoplasmic threads. This form shows active amoeboid move- 

 ments and constitutes the largest proportion of the blood leucocytes, 70 to 72 

 per cent. 4. The eosinophile cells, similar in general to the last, except that 

 the cytoplasm contains numerous coarse granules that take acid stains (eosin) 

 readily. They are present in small numbers, 2 to 4 per cent. 



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

 distinct histological units that 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 pow r er of making amoeboid 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, 

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

 are spoken of as "phagocytes" {ipaystv, to eat; xvrot;, cell). This theory of 

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

 kotf ;" it is founded upon the fact that the amoeboid 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 inte-tine. It maybe noticed here that these theories apply 

 to the leucocytes found SO abundantly in the lymphoid tissue of the aliment- 

 ary canal, rather than to those contained in the blood itself. (4) They take 

 pari in the process of blood-coagulation. A complete statement with refer- 

 ence to this function must be reserved until the phenomenon of coagulation is 



