vii CHARACTERS OF BLOOD 107 



Thus the colouring matter is evidently a distinct substance 

 from the protoplasm, and is called haemoglobin. 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 extremely 

 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 . 

 Corpuscles 



^ib"" ' Clot. 



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 arteries and veins two 

 or more abreast : as they pass through narrow capillaries or 

 round corners, they become bent or squeezed (G^, ZT). 

 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 of the 



