580 



CETACEA. 



Fig. 268. 



Structure of the lung of the Dugong. 



the pulmonary artery is proportionally smaller. 

 From the difference that exists in the locomo- 

 tive habits of the two animals arising from the 

 difference in the nature of the food, may be 

 deduced the circumstances which relate to the 

 difference in the respiratory organ. The Por- 

 poise, ever bounding and gambolling on the 

 surface, breathes as it were at will ; whilst the 

 Dugong is compelled to prolonged submersion 

 in order to acquire its food, which from its 

 fixed attachment, and comparatively innutri- 

 tious nature, necessarily demands much time 

 in collecting.] 



It is said that, in the Dolphins, each lung 

 is surrounded by muscular fibres, which take 

 part also in the acts of inspiration and expi- 

 ration, and that the lobes communicate with 

 each other in such a manner that, air being 

 introduced through one of the bronchi alone, 

 they are all filled with it. 



Fig. 



But though the diaphragm, the lungs, the 

 bronchi, and the trachea are only found 

 with modifications of a secondary order, the 

 nostrils, which serve intermediately for the 

 passage of the air, between the atmosphere 

 and the respiratory organ, present very im- 

 portant ones. It is especially upon these mo- 

 difications that the exterior distinction between 

 the Herbivorous and the Spouting Whales de- 

 pends. In the structure of the nostrils, the 

 mechanism by which the phenomenon of the 

 spouting is produced has necessarily caused 

 some changes, which, on the one hand, appear 

 to have necessitated the exclusion of the organ 

 of smell, and, on the other, to have led to the 

 formation of a new organ entirely peculiar to 

 this order of Mammalia. 



We may be allowed to believe that this 

 organ is essentially the same in the Dolphins, 

 the Cachalots, and the Whales ; it has only, 



269. 



Vertical section, shewing the tongue, larynx, and nostrils of the Porpesse. 



