vii CHARACTERS OF BLOOD 107 



with yellow. Thus the colouring matter is evidently a 

 distinct substance from the protoplasm, and is called 

 hczmoglobin. It is characterised, among other things, by 

 a strong attraction for oxygen : in combination with 

 that gas it assumes a bright scarlet colour ; when 

 deprived of oxygen, it becomes purple. This affinity for 

 oxygen accounts for the change undergone by the blood 

 when exposed to the air, as described on p. 79. 



Coagulated blood, as seen under the microscope, is 

 characterised by the plasma being traversed by ex- 

 tremely delicate threads, forming a sort of network in 

 which the corpuscles are entangled. These threads are 

 formed of a substance called fibrin, which is separated 

 from the plasma during coagulation, the remaining or 

 fluid portion of the plasma constituting the serum. We 

 may therefore express the coagulation of the blood in a 

 diagrammatic form as follows : 



Fresh Blood. Coagulated Blood. 



( Serum 



Plasma j Fibrin 1 n . 



Corpuscles }^ 



Having observed the microscopic characters of a drop 

 of blood, let us examine once more the circulation in 

 the web, this time under the high power (Fig. 27). The 

 red corpuscles (F) can be seen streaming through the 

 vessels, those in the capillaries in single file, those in the 

 small arteries and veins, two or more abreast : as they 

 pass through narrow capillaries or round corners, they 

 become bent or squeezed (G, H). The leucocytes (/) 

 travel more slowly and often stick to the sides of the 

 vessels. 



Columnar Epithelium. By carefully teasing out a 

 small piece of the inner surface of the mucous membrane 



