THE BLOOD AND CIRCULATION 261 



a fact which accounts for the difference between the red 

 color of blood fresh from the lungs, and the bluish color 

 of blood in veins from other parts of the body. Blue 

 blood is therefore blood with little oxygen. 



While the red corpuscles are specialized for the function 

 of carrying oxygen, the white cells or leucocytes perform 

 very different functions. These leucocytes are very active 

 cells with irregular, changing form. They have the prop- 

 erty of creeping about much like Amcebas which indeed 

 they closely resemble in form and general behavior. They 

 engulf and digest many foreign materials and they are 

 known to devour bacteria and other minute organisms 

 that gain access to the blood. By means of this property 

 they guard the body against many disease germs that 

 might otherwise have an opportunity for unrestricted 

 multiplication. This appetite of the leucocytes for bac- 

 teria renders the body more or less immune to various 

 diseases. Leucocytes tend to congregate around centers 

 of bacterial infection, and they may even pass through 

 the walls of delicate blood vessels and creep about in the 

 tissues, especially in regions of injury or bacterial invasions. 

 If small tubes containing cultures of certain bacteria are 

 introduced under the skin of a rabbit it is found that 

 leucocytes creep into the tubes, while other tubes similarly 

 prepared, but containing no bacteria are not entered. Ap- 

 parently, therefore, the leucocytes are drawn into the 

 tubes with bacteria on account of the fact that the bacteria 

 produce some substances that attract these wandering 

 cells. In regions where swelling occurs there are generally 

 large numbers of leucocytes. Pus, which is a common 

 product of inflammation, is composed largely of leu- 

 cocytes together with broken down cells of other kinds. 

 Leucocytes wander through the walls of the alimentary 

 canal and they may also be found in the mouth. The 



