174 



ELEMENTS OF BIOLOGY 



LEUCOCYTE 



tion with oxygen serves to increase the oxygen-carrying property of 

 the Hquid. In the annehd worms the system o£ tubes that plumb the 

 circulatory liquid about the body is fairly complex (Fig. ii6). The 

 blood contains a red coloring compound containing iron in solution 

 in the liquid portion of the system. This red substance is h^mo- 

 olobin; its chemical reaction with oxygen increases the oxygen- 

 carrying power. This same substance, in solution in the annelid 

 blood, is the substance that colors vertebrate blood, but in verte- 

 brates is entirely confined in cer- 

 tain blood corpuscles. 



Human Blood. In Man and 

 in other vertebrates the circula- 

 tory system includes two types of 

 liquid media, the blood and the 

 LYMPH. The human blood is com- 

 posed of a liquid plasma and two 

 types of blood cells, one a highly 

 modified cell, lacking a nucleus in 

 the mature stage and containing 

 the red substance, haemoglobin. 

 The red blood cells are known as 



ERYTHROCYTES (Fig. II7). The 



erythrocytes number about 4,500,000 per cubic millimeter of blood 

 in women, and about 5,000,000 per cubic millimeter in men. The 

 white blood cells, known as leucocytes (Fig. 117), are not so 

 numerous, the number being normally between 5,000 and 7,000 per 

 cubic millimeter in both sexes. There are several varieties of leuco- 

 cytes, one of which, the phagocytes, ingests bacteria and other for- 

 eign particles in a manner similar to the feeding process of amoeba; 

 hence phagocytes are highly important in the defense of the organ- 

 ism against infection (p. 83). 



The liquid plasma of the blood is a complex colloidal system. It 

 is slightly alkaline and the degree of alkalinity does not alter ma- 



ERYTHROCYTE 



Fig. 117. — Human blood cells. 



