TRANSPORTATION SYSTEM 127 



human red cell has no nucleus when in the blood, but in its develop- 

 mental stages in the red marrow of the bones, by which it is produced, 

 it has a nucleus. Red cells contain an important protein substance 

 known as hemoglobin, which gives the cells their red color. From the 

 rate at which hemoglobin is disintegrated in the liver, -it is estimated that 

 at least 5 per cent of the red corpuscles are destroyed every day. In 

 other words, more than 10 million of them disappear every second. Hence 

 there must be a rapid replacement of them by the marrow. 



The white cells are of half a dozen kinds. Two-thirds of them belong 

 to one type having an irregularly lobed or even divided nucleus (Fig. 

 lOOB), the power of movement like Amoeba, and the ability to engulf 

 bacteria. These cells may creep out of the capillaries, through small 

 crevices between the cells of the capillary walls (Fig. 101). They emerge 

 from the capillaries in great numbers at the site of an infection, to 

 engulf the infecting organisms. In their battle with the bacteria many 

 of the white cells are killed, and their bodies make up a large part of the 



(CD 



Fig. 101. — Successive stages in the emergence of a white blood cell from a capillary. 



pus which collects in an abscess. White cells of this kind originate in bone 

 marrow. The next most numerous kind, about one-fourth of the total, 

 originate in lymphoid tissue (Ij^mph glands, spleen). The remaining 

 types are recognized by different staining reactions as well as by their 

 size and nuclear structure; some of these devour bacteria, others do not, 

 but their functions are not well understood. All kinds of white cells 

 together number about 30 to 40 billions in an average human being. 



The platelets are not cells, but pieces of cells. They come from 

 certain large cells in the bone marrow by fragmentation. They dis- 

 integrate so rapidly when the blood leaves the capillaries that it is 

 difficult to count them. By special techniques it has been estimated 

 that there must be from one to three trillion of them in a human being. 

 Only the mammals are certainly known to have them. Their disinte- 

 gration on leaving the blood vessels yields a substance which is important 

 in the clotting of the blood. 



Regulation of Heart Beat. — Because the heart is histologically practi- 

 cally a unit, it beats also as a unit. It is one of the best organs with 

 which to demonstrate the all-or-none principle, because of this unity 



