580 



ANATOMY OF VERTEBRATES. 



lages of the bronchial tubes arc continued spirally into one another, 

 fig. 4,') 1 : the pulmonary artery lies to the outer side of the bronchus 

 and is deeper seated, the pulmonary vein to the inner side and 

 superficially. The principal branch of the bronchus, //, fig. 452, 

 runs down near the inner margin of the lung, and continues 

 distinct to within four inches of the end ; it then divides into 

 smaller branches ; the larger ramifications are given off from 

 its outer side, c, c. In all the branches the cartilaginous rings 

 continue distinct and strong till their diameter is contracted to one 



o 



452 



rt 



Section of terminal part of lung, Dugong. 



or two lines ; the rings passing irregularly into each other as in 

 the main trunks. The lining membrane of the .air-tubes is thrown 



o 



into longitudinal ruga;, indicating their dilatability. The super- 

 ficial air-cells, ib. a, are six times larger than in the Porpoise. The 

 * pleura costalis ' is dense in both Cctacea and Sirenia, as is the 

 subserous tissue of the c pleura pulmonalis.' 



In the Elephant the right lung sends a lobular process, behind 

 the thoracic postcava, into the space between the heart and dia- 

 phragm, but both this and the left lung are undivided. The 

 trachea has thirty rings, many of which are partially cleft. 



In the Rhinoceros (Rh. indicus) the trachea has thirty-one 

 rings: they are close-set, cleft behind, the ends meeting: the 

 lining membrane is longitudinally rufous, as is that of the bron- 



r? / o 



chial ramifications for some way into the lung. Each lung is 

 divided into a small upper and a large lower lobe ; the right lung 

 also gives oft' a transversely elongated narrow azygous lobule : 

 the upper lobe has numerous deep marginal notches. In the 

 Horse the trachea has fifty-two posteriorly incomplete cartilagi- 

 nous rings, the ends of which are flattened, expanded, and over- 



