CHAPTER III. THE CIRCULATION. 



1. The Vascular System. The next thing to con- 

 sider is the distribution of the food -material absorbed 

 through the walls of the alimentary canal to the living 

 and active parts of the body. This is one of the functions 

 of the series of structures heart and blood-vessels called 

 the circulatory or vascular system. It is not the only 

 function. The blood also carries the oxygen from the 

 lungs to all parts where katabolism occurs, and it carries 

 away the katastases to the points where they are excreted 

 the carbon dioxide to the lungs, urea to the kidneys, 

 and so on. 



2. The Blood (fig. 3) is not homogeneous ; under 

 the low power of the microscope it 

 may be seen to consist of 



(i.) a clear fluid, the plasma, in 

 which float 



(ii.) a few transparent colourless 

 bodies of indefinite and changing shape, 

 the white corpuscles, and 



(iii.) flat round discs, which appear 

 of a yellowish colour as seen under 

 the microscope, but in bulk give the 

 characteristic colour to blood, and hence are called the red 

 corpuscles. They are vastly more numerous than the white. 

 The two gases oxygen and carbon dioxide are not 

 carried in exactly the same way by the blood. The student 

 will know from his chemical reading that neither of these 

 gases is very soluble, but carbon dioxide is sufficiently so 

 in an alkaline fluid to be conveyed by the liquid plasma, 

 as all the other materials are. The oxygen, however, is 

 carried by the colouring matter of the red corpuscles, the 

 haemoglobin, with which it combines to form a bright red 

 compound, oxy -haemoglobin. This decomposes again easily 



90 



Fig * 



