AQUATIC MAMMALS 



B. Wislocki (1929) found that the cartilaginous armature is such as to 

 give unusual strength and incompressibility. In the smaller bronchioles 

 possessing a diameter of less than 0.5 mm. there are muscular sphincter 

 valves. Moreover in the respiratory bronchioles there is a complete 

 lining of flattened respiratory epithelium instead of a partial one as in 

 other known mammals. There is a tremendous development of elastic 

 tissue, and double capillary beds of interalveolar septa instead of single 

 ones, as in terrestrial mammals. The respiratory bronchioles do not 

 possess sacculations and alveolar ducts are lacking. 



The above sphincters probably close at the end of inspiration and 

 ordinarily remain so until expiration begins, preventing the gradual 

 collapse of the air spaces as outside pressure increases, thus acting in 

 antagonism to the elastic tissue of the lungs. The individual sphincters 

 are weak, but there are probably several million of them, and each 

 imprisons but a minute amount of air, so that all of them working to- 

 gether are doubtless capable of preventing the escape of air even into 

 the trachea under any pressure to which the animal cares to subject it- 

 self. Thus external and internal pressure can be equalized, as it must 

 be in any animal which experiences an external pressure of a ton to the 

 square inch. No thorax could resrst this stress without collapsing: and 

 this is probably the chief physiological adjustment necessary for deep 

 diving. Others are necessary, to prevent such things as bleeding at the 

 eyes, and likely some alteration in the action of the heart, but it seems 

 that these should be of a more minor character. The epitheleal equip- 

 ment presents an increased surface area subserving the function of 

 respiration, and the elastic tissue forms a powerful mechanism for empty- 

 ing the lungs in a minimum of time. 



Pick (1907) has stated that the lungs of the dugong agree with those 

 of the Cetacea in having a cartilaginous armature (which, however, is 

 partially calcified) extending to the smallest bronchioles, a tremendous 

 quantity of elastic tissue, and in the size of the air-sacs and thickness of 

 the septa; but the distribution of muscular tissue differs. 



Schulze (1906) made an estimate of the number of air cells and their 

 respiratory surface in the sloth, cat, man and porpoise (Phoceana com- 

 munis, which reaches a length of about five and a half feet), as follows: 



Air cells Respiratory surface 



Sloth 6,250,000 5 square meters 



Cat 400,000,000 20 square meters 



Man 150,000,000 30 square meters 



Porpoise 437,000,000 43 square meters 



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