Chap. VI.] 



CETACEA ; SIREWA. 



243 



Here, then, milk 



B 



age too early to allow them to actively suck the 

 mother, hang on to a long nipple, and have the 

 milk injected by the mother (by the contraction of 

 the cremaster muscle, and the consequent compres- 

 sion of the mammary glands), 

 flows on either side of the 

 air tube, and the latter is, 

 as in the whale, a direct 

 continuation of the air 

 passages in the head. It 

 is important to observe that 

 there is no prolongation of 

 the air tube in the Sirenia, 

 but that their epiglottis is 

 large, and capable of com- 

 pletely closing the entrance 

 into the trachea; at the 

 same time it will be remem- 

 bered that the dugong and 

 manatee are herbivorous. 



So, again, the Sirenia 

 differ from the Cetacea in 

 the manner in which they 

 obtain a large supply of air. 

 In the former the dia- 

 phragm, in place of forming 

 a more or less vertical par- 

 tition between the thoracic 

 and abdominal cavities, slopes backwards and upwards, 

 so as to largely increase the area of the thoracic cavity, 

 the extension of which is occupied by the large lungs. 

 In the dolphins and porpoises the nasal passages open 

 into lateral sacs with elastic walls ; the possession of.' 

 these sacs must, in addition to their air-containing 

 function, diminish to a certain extent the specific 

 gravity of the skull. The commonly received story 

 that a whale " blows " water is due to the fact that a 



Fig. 102 B. Bronchial Tree of a 

 Mammal (Horse). 



A, Eparterial : B, hyj>arterial ventral 

 (1; (<f) hyparterial dorsal bronchi; 

 pa, pv, pulmonary artery and 

 vein. (After Aeby.) 



