IV.
Of the absorption of the materials thus prepared it is not necessary to say much in a work of this compass, but the absorption of oxygen is too important to be passed over.
Oxygen is required by the body pure, and, as it is uncombined with anything in the air, it needs no digestion to free it. A special organ, however, is necessary to absorb it. This is the lung. The lungs originate, just like a gland, by a pouching of the alimentary canal near its origin, but differ from a gland in their cells being very much flattened, to offer a large surface to the air on one side and to the bloodvessels on the other. Incessantly during life air is being drawn into the lungs; that the cells to which it is there exposed may transfer its oxygen to the blood; and then, after the cells have also transferred the carbonic acid gas from the blood to the air, driven out again to be replaced by fresh.
(The mechanical means by which the lungs are filled and emptied come under another heading.)