STRUCTURE OF THE HUMAN BODY 45 



minute coiled tubes, each opening by a very small pore on the 

 surface. The coiled part of each such gland is sunk deeply in the 

 dermis, and is surrounded by capillary blood-vessels, from the blood 

 contained in which the materials making up sweat are separated. 

 2. The Lungs (fig. 21) are two spongy bodies contained in the 

 thorax, and their function as breathing or respiratory organs is 

 twofold — (i) to excrete or get rid of the waste carbonic acid gas, 

 together with a large quantity 

 of water, and (2) to supply the 

 blood with free oxygen, which 

 is necessary as the agent by 

 which the waste constantly 

 going on in the body is ren- 

 dered possible. This waste is 

 in fact a process of oxidation, 

 i.e. a conversion of protoplasm, 

 &c., into simpler bodies con- 

 taining a larger proportion of 

 oxygen. The lungs are spongy 

 in character because they are 

 made up of innumerable little 

 air -tubes, ending blindly in 

 groups of minute air-sacs, with 

 walls invested by a close net- 

 work of capillaries, the blood 

 contained in which is so near to 



the air in the air-cells that an exchange of materials is possible. 

 On the one hand, a large amount of the carbonic acid gas with 

 which the impure blood is highly charged is able to diffuse into 

 the air-sacs, from which, on the other hand, the oxygen of the 

 air is able to diffuse into the blood. The hcsmoglobin of the red 

 corpuscles plays an important part as an oxygen-carrier, taking 

 it up from the air in the lungs and readily parting with it to the 

 substance of the body. A large amount of water in the form 

 of vapour also diffuses out of the blood into the air-sacs. As 

 a result of this, the air breathed out contains far more carbonic 

 acid gas and water-vapour, but much less oxygen, than the air 

 breathed in. This exchange of gases constitutes the essential part 

 of breathing; but the mechanical part has also to be considered, 

 i.e. the arrangements by which the air -passages of the lungs 



Fig. 21. — Lungs and Air-passages 

 A, Windpipe; BC, bronchi; dd, smaller air-passages. 



