254 LECTURES TO SCIENCE TEACHERS. 



and so arises the necessity for another arrangement, by 

 which the oxygen-carrying power of the blood may be 

 increased above that of other liquids. This is effected by 

 means of the haemoglobin or colouring matter of the blood. 

 This is a very complicated body : it contains iron, and it 

 may be split up into two bodies called hematine and 

 globuline. It is one of the few animal substances of com- 

 plicated chemical structure which are crystalline. It 

 crystallises in the forms shown on this diagram. Its 



FIG. 1. A. Hsemoglobin from 'guinea-pig's blood. B. Haemoglobin from horse's 

 blood. C. Haemoglobin from squirrel's blood. 



essential property is to take up oxygen readily from the 

 external air, and give it off readily to any substance which 

 is greedy of oxygen. 



In this diagram of the circulation you see that the blood in 

 the right side of the heart, and, in fact, the whole venous 

 system, is coloured blue, while on the left side it is coloured 

 red. These colours in the picture indicate corresponding 

 differences in the colour of the blood itself. Although 

 the blood in the veins is not quite blue, it is very dark red, 

 whereas that in the arteries is light red. Now there are 

 two points in the circulation at which this change of colour 

 takes place, viz., at the heart in the centre of the body, 

 , and at the capillaries in the periphery. There is as marked 

 a difference between the blood in the veins and that in the 

 arteries at the periphery as there is at the heart, and this is 

 due to the fact that the blood containing haemoglobin has had 

 its oxygen removed in the capillaries, whereas at the lungs 

 it has had the oxygen again restored. This great distinction 

 between venous and arterial blood has been long known, 

 but it was not until the spectroscope was introduced as an 

 instrument of research that we were able to ascertain with 



