THE THORAX AND LXJNGS 135 



No conditions could be more favourable to a ready 

 exchange between the gaseous contents of the blood and 

 those of the air in the alveoli than the arrangements 

 which obtain in the pulmonary capillaries ; and, thus far, 

 the structure of the lung fully enables us to understand 

 how it is that the large quantity of blood poured through 

 the pulmonary circulation becomes exposed in very thin 

 streams, over a large surface, to the air. But the only 

 result of this arrangement would be, that the pulmonary 

 air would very speedily lose all its oxygen, and become 

 completely saturated with carbonic acid, if special luo- 

 vision were not made for its being incessantly renewed. 

 The renewal is brought about by the working of certain 

 structural and mechanical arrangements which must now 

 be described in detail. 



4. The Thorax and Lungs.— The lungs (and heart) 

 are enclosed in what is practically an air-tight box, whose 

 walls are movable. This box is the thorax. In shape it 

 is conical, with the small end turned upwards, the back 

 of the box being formed by the spinal column, the sides 

 by the ribs, the front by the sternum or breast-bone, the 

 bottom by the diaphragm, and the top by the root of the 

 neck (Fig. 38). 



The two lungs occupy almost all the cavity of this box 

 which is not taken up by the heart (Fig. 41). Each 

 is enclosed in its serous membrane, the pleura, 

 a double bag (very similar to the pericardium, the chief 

 difference being that the outer bag of each pleura is, over 

 the greater part of its extent, quite firmly adherent to 

 the walls of the chest and the diaphragm, while the outer 

 bag of the pericardium is for the most part loose), the 

 inner bag closely covering the lung and the outer forming 

 a lining to the cavity of the chest ' (Fig. 42, pi.). So long 

 as the walls of the thorax are entire, the cavity of each 



1 There is a small amount of fluid between the two surface* of the 

 pleura, to facilitate their rubbing easily ajrainst one another. This 

 serous fluid is m reality, as is pericardial fluid, a form of lymph 



