222 Marsh. 



which the air is conveyed to various parts of the body, the 

 blood being in close connection with or surrounding these 

 tubes and chambers ; here the oxygen is obtained directly 

 from the air. But in the higher vertebrates (birds and 

 mammals), specially developed organs or lungs are found. 

 These contain hollow cavities or air-sacs, the lung substance 

 surrounding which is filled with blood capillaries, through 

 the thin walls of which oxygen passes from the air-sacs into 

 the blood, and the carbonic acid gas in the opposite direc- 

 tion. 



How does the blood convey the carbonic acid gas to the 

 lungs, and oxygen to the tissues ? There are no bubbles of 

 gas in the blood. No ! If you look at a drop of blood under 

 a microscope, you will notice that its red colour has dis- 

 appeared. The fluid is of a pale amber colour, and the red 

 colour is restricted to a number of little cells, called blood 

 corpuscles. These are composed of a basement membrane, 

 which encloses a substance that combines readily with 

 oxygen gas. In the lungs, this substance in the red cor- 

 puscles takes up a plentiful supply of oxygen gas, whilst 

 the carbonic acid gas, which has been brought from the 

 tissues in the ordinary plasma of the blood Y is set free. Then 

 the oxygen is carried by the corpuscles to the tissues, where 

 it is abstracted from them. As the robbing process goes on, 

 the lessened amount of oxygen changes the corpuscles to a 

 dark purplish colour, and the blood which contains them is 

 then known as venous blood. In this way the tissues in the 

 higher animals are constantly supplied with ox}^gen, and 

 thus they get rid, in part, of the deleterious gases produced 

 by their oxidation. 



The food in the blood is used up but slowly, and many 



