580 



ANATOMY OF VERTEBRATES. 



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

 fig. 451 : 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, b, 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 



Section of terminal part of lung, Dugoug. 



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

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

 into longitudinal rugae, 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 Cetacea and Sirenia, as is the 

 subserous tissue of the i 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 rugous, as is that of the bron- 

 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 off 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- 



