LUNGS OF MAMMALIA. 



579 



450 



Elastic tissue of air-cells, uiagn. Lalainoptera. cclxviu. 



451 



fibres of the elastic tissue are abundant and conspicuous on 

 the walls of the pulmonary air-cells in the Whales, as shown 

 by V. der Kolk, in Balce- 

 noptera, fig. 450, a, b ; 

 in which figure a portion 

 of the injected capillary 

 web is represented at c. 

 The elasticity of the 

 lungs with the pressure 

 on the surface of the 

 body, makes expiration 

 very easy, and the cur- 

 rent strong when force 

 is exerted, as e. g., to 

 clear the naso-palatine 

 breathing passages, fig. 

 297,/, d: the pulmonary 

 vapour so expelled mainly forms the ( spout ' of the Whale. 



In Sirenia the lungs resemble in shape and position those of 

 Clielone, but are loosely suspended at the 

 back part of an elongated thorax, defined 

 by an oblique diaphragm from the abdo- 

 men. This resemblance is further exem- 

 plified in the shortness of the trachea, 

 the completeness of its cartilaginous 

 rings, the length of the bronchi, and the 

 extent to which their cartilages are con- 

 tinued into the substance of the lungs. 

 These are convex on the dorsal aspect, 

 flattened on the opposite surface along 

 which the principal branches of the 

 bronchi can be seen through the pleura 

 pulmonalis. The fore end of each lung 

 is thick and obtuse but narrow : they 

 soon become flattened as they recede and 

 broaden. In the Manatee their anterior 

 or outer margin is crenately notched. 



There are but three rings in the tra- 

 chea of the Dugong, the first being the 

 largest. The tube is somewhat flattened 

 from before backward : I found it, in a specimen 8 feet long, 5 

 inches in circumference and 1 inch in antero-posterior diameter. 

 In older specimens the rings have been found bony. The carti- 



p r 2 



Bronchial cartilages of the Dugong. 



