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.
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.
357. In man there are two pulmonary bags (fig. [CXXXIV]. 2, 3), of nearly equal size, which, together with the heart, completely fill the large cavity of the thorax (fig. [CXXXIV].), their external surface being everywhere in immediate contact with the thoracic walls. One of these bags is placed on the right side of the body, constituting the right lung (fig. [CXXXIV]. 2), and the other on the left, constituting the left lung (fig. [CXXXIV]. 3). Each lung is divided by deep fissures, into large portions called lobes (figs. [CXXXIV]. 4, and [CXXXV]. 6), of which there are three belonging to the right, and two to the left lung. Each lobe is subdivided into innumerable smaller parts termed lobules (figs. [CXXXIV]. 6, and [CXXXV]. 6), while the lobules successively diminish in size until they terminate in minute vesicles that constitute the great bulk of the organ (fig. [CXXXV]. 8).
358. The complete centralization of the respiratory function which thus takes place in man, renders the apparatus exceedingly complex both on account of the expedients which are necessary to obtain the requisite extent of surface, in the small allotted space, and to bring into contact within that space the fluids that are to act on each other.
Fig. CXXXV.—View of the Air Tubes and Lung.
1. The larynx. 2. Trachea. 3. Right bronchus. 4. Left bronchus. 5. Left lung; the fissures denoted by the two lines which meet at 6, dividing it into three lobes, and the smaller lines on its surface marking the division of the lobes into lobules. 7. Large bronchial tubes. 8. Minute bronchial tubes terminating in the air cells or vesicles.