144 PHYSIOLOGY FOR DKNTAL STUDENTS. 



questionable if the inorganic forms of iron can be utilized by the 

 body, the iron in our blood being probably derived from a con- 

 jugated protein known as hsematogen, found in small quantities 

 in the food. 



The White Blood Cells. In normal human blood there are 

 about ten thousand cells in a cubic millimetre of blood, or about 

 one to every five hundred red cells. In many ways they resemble 

 the unicellular amoeba, for like it they have the power of making 

 independent movement by extending tiny processes called psi-u- 

 dopodia in one direction and by retracting them in another. In 

 virtue of this peculiar movement they are able to flow, as it 

 were, between the endothelial cells of the capillaries and find 

 their way into the tissue spaces. There are a number of forms 

 of white cells differing from each other in size, in character of 

 their nucleus, and in the granules they contain. In general, they 

 are classified in two main groups on morphological grounds, viz., 

 leucocytes and lymphocytes, 



The Leucocytes are the most numerous and compose about (>."> 

 per cent of the total white cells. They are characterized by a 

 lobed nucleus, the parts of which are connected by strands of 

 chromatin material. To this class belong several sub-groups. 

 The most important of these are the cells known as polymorpho- 

 nuclear leucocytes. They comprise about 96 per cent of the 

 leucocytes. Another type are known as eosinophyles, since they 

 have granules with a marked affinity for acid stains. 



The Lymphocytes. The second variety are so-called, since 

 they are supposed to be formed in the lymph glands of the body. 

 They possess a single large round nucleus surrounded by a clear 

 layer of protoplasm. There are two sub-groups in this class; 

 the large mononuclcar lymphocytes, which contain a rather abun- 

 dant cytoplasm about the nucleus, and the small motion n< !(> 

 lymphocytes, in which the amount of cytoplasm is very small. 

 The former comprise about 4 per cent, and the latter about 30 

 per cent, of the white cells. 



ESTIMATION OF THE WHITE CELLS. The number of white cells 

 found in the blood is estimated by the same principle that is em- 

 ployed in the counting of the red cells (see p. 142). In certain 



