260 ANIMAL BIOLOGY 



a soluble substance called fibrinogen by a process of fer- 

 mentation. Clotting is therefore analogous to the forma- 

 tion of cheese (casein) from a protein which was previously 

 dissolved in the milk. Exposure to air and especially 

 contact with solid objects causes the blood to clot, and 

 the process may be hastened by adding fine powder to the 

 blood or beating it with a stick. The clotting of blood 

 performs the very useful function of checking bleeding; 

 otherwise bleeding would be very difficult to control and 

 even slight wounds might produce fatal results. The yel- 

 lowish fluid which remains after the 

 fibrin has been removed by clotting is 

 called serum. 



The corpuscles of the blood which are 

 true cells are of two kinds, the red and 

 the white corpuscles or leucocytes. The 

 red corpuscles are round biconcave disks; 

 they do not contain a nucleus, although 

 cies. /, leucocytes a nucleus occurs in the early stages of 



or white corpuscles; ,-,.,, mi 



r, red corpuscles; s, their development. Their most import- 

 red corpuscles seen t ingredient is hemoglobin, the sub- 



on edge. 



stance which gives the blood its red color. 

 Haemoglobin is a protein containing iron and having the 

 property of combining readily with oxygen, and also of 

 giving up its oxygen again with equal readiness. This 

 curious property enables haemoglobin to perform its 

 important function of a carrier of oxygen, since when 

 oxygen is abundantly supplied to the blood as it circulates 

 through the lungs the haemoglobin becomes oxidized; 

 whereas when the blood passes into a region where the 

 cells of the body use all the oxygen that is available the 

 haemoglobin gives up its oxygen or, as the chemists w r ould 

 say, becomes reduced. Haemoglobin combined with oxy- 

 gen is red in color, while the reduced haemoglobin is bluish ; 



