PARTS AND FEATURES 



to some degree. But it is probable that no mysticete can submerge as 

 deeply as some favored odontocetes, and it does not seem probable that 

 the majority of httoral porpoises are so phenomenally gifted in this 

 respect. 



Of particularly vital import in the function of respiration by marine 

 mammals is the diaphragm, and it is only to be expected that there 

 should occur some change in its details. In a pronograde mammal, as 

 discussed by Jones (1913) , the fixity of the fore limbs allows the muscles 

 passing therefrom to the thorax to assist in breathing by raising the ribs, 

 while in orthogrades this function has become obsolete. It should be 

 still further eliminated in those marine mammals which seldom or 

 never use the fore limbs for pressing against a hard surface. An increase 

 in the muscular character of the diaphragm of aquatic mammals indicates 

 that this is probably so. 



Evidently in all aquatic mammals that are very highly modified for 

 such a life the diaphragm is more sloping, or tending to assume a posi- 

 tion more nearly parallel with the body axis, than in terrestrial mammals, 

 and this is said to be so even in the otter. Certainly it is a character 

 of all pinnipeds, cetaceans and sirenians, in the latter order reaching 

 its greatest alteration. It is indicated that this character increases with 

 ontogenetic development, for Beddard (1900) has stated that in an 

 adult porpoise the ventral and dorsal extent of the thoracic cavity showed 

 a proportion of 1 to 2.25, while in a young individual this was as 1 to 

 1.75. I cannot state this proportion in the sirenians but Murie's de- 

 scriptions and figures show that in the manati the diaphragm extends 

 from the much reduced sternum below, quite to the last thoracic vertebra. 

 As this mammal has but two lumbar vertebrae this means that the dia- 

 phragm almost reaches the posterior end of the abdominal cavity and is 

 as nearly parallel with the body axis as it could well be. 



Muller (1898) believed that the Mysticeti breathe more with the 

 thorax and less with the diaphragm, for the latter is less muscular in 

 this group than in the Odontoceti, but cetacean conditions are difficult 

 to interpret. In the Sirenia it seems that the rigidity of the costal ar- 

 ticulations would largely inhibit much mobility of the thorax while 

 breathing. The reduction of the sternum and the costal cartilages, how- 

 ever, does point to the probability that these features do facilitate mobil- 

 ity of the diaphragm, and this must be of extraordinary efficiency. 



It seems that the pronounced slope of the diaphragm in marine mam- 

 mals may have been assisted by an increase in the lung capacity, and an 

 advantage thereby gained is that the levitation supplied by the inflated 



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