CIRCULATION 21 



which probably explains the increase in their numbers 

 following a meal. Cold baths and pregnancy are other 

 ''physiologic" causes of increased numbers of leuco- 

 cytes. A very important duty of the white cells is to 

 combat bacterial invasion, and it is found that many in- 

 fectious diseases are accompanied by an increase in the 

 number of the leucocytes., this leucocytosis often being of 

 value in the diagnosis. In pneumonia, for instance, the 

 number not infrequently is increased to forty or fifty 

 thousand to the cubic millimeter. In the leucemias, 

 there is often a still larger increase ; while in some other 

 diseases, such as typhoid fever, there is a reduction 

 in the number, this being known as a leucopenia. 



There are about three hundred thousand platelets to 

 the cubic millimeter. These are small disc-shaped bodies 

 when examined with proper care, but usually disinte- 

 grate and appear simply as detritus in the ordinary 

 stained specimens of blood. Their functions are imper- 

 fectly understood; they appear to be of importance in 

 the coagulation of the blood, and it is claimed that their 

 number is reduced in the hemorrhagic diseases. 



Lymph. It must be understood that the tissue cells 

 are not bathed direct in blood. The blood, carried in its 

 system of vessels, has been likened to a wholesaler, and 

 the pnrt of the retailer, coming in intimate contact with 

 the consuming tissue cells, is taken by the lymph. The 

 lymph is derived from the blood by the processes of fil- 

 tration and osmosis, and is poured out into the spaces 

 surrounding the cells. Containing the nutritive sub- 

 stances derived from the blood, it turns these over to the 

 cells and receives waste products from the latter. It is 

 forced into special vessels, knoAvn as lymphatic chan- 

 nels, and finally is carried back into the blood stream 

 through the right and left lymphatic ducts. Along the 



