616 Comparative Morphology of Chordates 



the trachea divides into right and left bronchi. In some whales and 

 some ungulates the trachea divides into three bronchi before entering 

 the lungs. 



Lungs 



Within the lungs each bronchus divides repeatedly into tubes of 

 successively smaller diameter, becoming reduced ultimately to the 

 bronchioles (Fig. 472), each of which divides into several short 

 alveolar ducts which differ from ordinary bronchioles in that the 

 walls of the former are beset with numerous small outwardly bulging 

 pockets, the alveoli. Each alveolar duct expands distally (the ex- 

 panded region being sometimes distinguished as the atrium) and gives 

 rise to one or more rounded saccules (or infundibula) whose walls 

 contain close-set hemispherical alveoli like those of the alveolar duct. 

 The essential wall of an alveolus consists of an inner layer which is an 

 extremely thin epithelium. This is the primary endodermal pulmonary 

 tissue. Externally, this epithelium is closely invested by a dense net- 

 work of blood-capillaries. The capillary wall consists merely of an 

 endothelium which is just about the thinnest possible layer of cells. 

 Thus blood and air are brought so close together that the respiratory 

 interchange of oxygen and carbon dioxide readily takes place by diffu- 

 sion through the two apposed and exceedingly thin membranes. The 

 alveoli provide the chief respiratory surfaces, but the characteristic 

 thin respiratory epithelium and the accompanying capillary net may 

 extend throughout the walls of the alveolar saccules and ducts and 

 even into some bronchioles. But the larger bronchial passages within 

 the lungs are lined by a relatively thick cuboidal and nonrespiratory 

 epithelium. The walls of these larger passages are stiffened by incom- 

 plete cartilaginous rings similar to those of the trachea. The narrower 

 bronchi have small irregularly placed plates of cartilage in their walls. 

 Usually no cartilages occur in the walls of bronchioles, but in sea cows 

 {Sirenia) and the whalelike mammals, even the bronchioles are sup- 

 ported by cartilages which are probably important in serving to pre- 

 vent collapse of the small tubes under the tremendous pressure to 

 which the animal is subjected when deeply submerged. 



The many-branched bronchial system within a mammalian lung 

 is often called the "bronchial tree." The term "lobule" is applied 

 to the cluster of bronchioles, together with the terminal respiratory 

 structures into which they lead, arising from one of the smallest 

 bronchi (Fig. 472). The lobules and their constituent parts are all 

 bound together and supported by fibrous connective tissue. In this 

 tissue is a considerable proportion of elastic fibers. In the walls of the 

 respiratory spaces the elastic fibers are especially abundant and are 

 closely associated with a delicate network of smooth muscle-fibers. 



