THE BLOOD AND THE LYMPH 269 



as agents for the destruction of any foreign bodies, dead or alive, to which 

 they have access. Some of these they devour as if taking them as food, 

 while others are chemically destroyed. It is possible at least that they 

 have much to do with determining the opsonic index of the blood, this 

 being in a word the power of the latter's resistance to invading bacteria 

 and poisons through the presence in the plasma of substances of unknown 

 nature termed opsonins. 



A second function of the leukocytes is undoubtedly to aid in the 

 absorption of fat and of proteid substances from the intestine. They 

 help also to carry nutrition from the tissue-capillary to the tissue-cell. 

 They may aid in the coagulation of the blood. Perhaps their proteids 

 take part in maintaining the normal composition of the blood, and it is 

 possible that they contribute carbohydrate and fat as well. In local 

 inflammation, when tissue-cells are in danger of destruction, they crowd to 

 the region in enormous numbers and do 

 their best to destroy the invading bacteria, FIG. 141 



to restore to the endangered cells their 

 normal constituents, and in general to 

 help in the repair of the injured proto- 

 plasm. Another possible function which **^ 

 we may note is that of carrying katabolic jj$t 

 products from the tissue-cells into the r *4M|V; 

 circulation on their way out of the body. 



The life-history of the leukocytes can- 1: // 



not be written with any degree of com- 

 pleteness, but we know more about their 

 places of origin than as to the way in 

 which they disappear. A phagocyt e in the intestinal epithe- 



THE THROMBOCYTES OR PLATELETS. Hum of a frog. (Heidenhain.) 

 The physiological status of these minute 



particles of the blood is at present somewhat uncertain. They were 

 discovered by Bizozero. They are colorless, jagged, or irregular 

 masses of protoplasm, very variable in size, but averaging not 

 far from three microns in diameter. Early in the summer of 1906 

 Wright published a report of observations which seem to prove beyond 

 much doubt that the platelets are not complete cells, but that they may 

 have ameboid movements. According to this observer, they come from 

 the giant-cells (called by Howell megakaryocytes) common in the bone- 

 marrow and in the spleen. These giant-cells are of spherical form mostly, 

 but some of them "are of varied and irregular shape by reason of the 

 distortion of .their cytoplasm into processes and pseudopod-like pro- 

 longations of varying length, form, and width, so that they present all 

 the varieties of form and outline shown by a motile ameba." (See 

 Chapter I.) In some giant-cells nearly all the cytoplasm is extended 

 thus as pseudopods, and the thrombocytes are these pseudopodia de- 

 tached in the circulation. It is known that the giant-cells do lose their 

 cytoplasm and the actual movements of their cytoplasm as well as that of 



