355. In birds, the next order of vertebrata (fig. CXXXIII.), as in insects, the class of invertebrated animals which are formed for flight (352), the respiratory organs extend through the greater part of the body (fig. CXXXIII. 4). The lungs (fig. CXXXIII. 2), which still consist of a single pulmonic sac on each side (fig. CXXXIII. 2), are divided into cells, minute compared with those of the reptile, yet large compared with those of the quadruped; at the same time numerous air sacs, similar in structure to those of the lungs, but of larger size, are distributed over different parts of the body (fig. CXXXIII. 4), which communicate with the air cells of the lungs (fig. CXXXIII. 3); while of these larger sacs, several communicate also with the bones (fig. CXXXIII. 6), so as to fill with air those cavities which in other animals are occupied with marrow. 356. In the mammalia, the highest order of the vertebrata, respiration is less extended through the system, and is concentrated in a single organ, the lung, which, though comparatively smaller in bulk than in some of the lower classes, is far more developed in structure. The lung in this class consists of a membranous bag, divided into an immense number of distinct vesicles or cells, in the closest possible proximity with each other, yet not communicating, and presenting, from their minuteness, a vast extent of internal surface. This bag is confined to a distinct cavity of the trunk, the thorax (fig. CXXXIV.), completely separated from the abdomen by the muscular partition, the diaphragm (fig. CXXXIV. 10). This organ no longer sends down cells into the abdomen, nor membranous tubes into the bones; but is concentrated within the thorax along with the heart (fig. CXXXIV. 2, 3, 8). In all the orders of this class, the development and concentration of the organ are in strict proportion to the perfection of the general organization. [Illustration: Fig. CXXXIV.—_View of the Respiratory Apparatus in Man._ 1. The Trachea. 2. The right lung. 3. The left lung. 4. Fissures, dividing each lung into, 5. Large portions termed lobes. 6. Smaller divisions termed lobules. 7. Pericardium. 8. Heart. 9. Aorta. 10. Diaphragm separating the cavity of the thorax from that of the abdomen.]