16 BLOOD 



" lymphoid " tissue, which is widely distributed throughout 

 the body — particularly in relation to the ahmentary canal — 

 the tonsils, adenoids, and Peyer's patches. The thymus, 

 the Malpighian corpuscles of the spleen, and the lymphatic 

 glands are tissues of the same nature. In all these organs 

 lymphocyte-formation by mitosis can be seen. 



Functions of the Leucocytes 



Besides circulating in the blood, leucocytes wander 

 through the intercellular spaces of the tissues. Their 

 function is the destruction and digestion of foreign bodies, 

 such as bacteria, and the absorption of tissues which are 

 undergoing degeneration. This process is known as -phago- 

 cytosis. In acute inflammatory conditions, there is. a 

 mobihsation of leucocytes, particularly of the polymorpho- 

 nuclear variety, at the site of infection, and an increase 

 in the number circulating in the blood. 



The ingestion of foreign bodies is carried out by the 

 polymorphonuclear and mononuclear cells. The part 

 played by the lymphocytes is unknown. They increase 

 in number in chronic affections such as tuberculosis. It 

 is beheved that from the granules of the coarsely granular 

 cells, both oxyphile and basophile, are excreted substances 

 which are toxic to bacteria. 



BLOOD-PLATELETS 



These are small bodies 1-5 /t in diameter. In form, 

 size and number they vary according to the way in which 

 the blood has been collected. They are usually circular 

 discs, containing fine granules. They number from 100,000- 

 500,000 per c.mm. In the circulating blood they are only 

 seen when the vessel-wall is injured. When blood is care- 

 fully collected, and kept at body temperature, no platelets 

 can be found. It is therefore beheved that they are not 

 present in normal circulating blood. How they are pro- 

 duced is uncertain, some observers beheving that they 

 arise from the disintegration of red-cells and leucocytes. 



