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it is easily understood that some control of diaphragmatic action 

 will be advisable. In this, it may be, lies the reason for the absence 

 of pleural cavity. The diaphragm being directly attached to the 

 lung substance by means of the connective-tissue previously described, 

 it contracts at the rate at which air can fill the large flattened lungs by 

 passing along the narrow channels of the proboscis. Were the dia- 

 phragm free to contract at its own rate, it might act faster than the 

 lungs could expand with air. The consequence of this would be sudden 

 and pronounced diminution of pressure in the pleural cavity (ii that 

 were present) with consequent haemorrhage or other pathological 

 change. 



Obliteration of the pleural cavity; its replacement by simple 

 connective-tissue attaching the lungs to surrounding structures, yet 

 permitting free respiratory movement, and especially the attachment 

 of lung to diaphragm, provide against such a contingency-. In this 

 way the rate which the air can pass along the proboscis and fill the 

 lungs, controls the rate of contraction of the diaphragm. 



That some such explanation is true is apparent from the fact that 

 the pleural cavity is absent in elephants and tapirs, two proboscideans, 

 and not, so far as I am aware, in other mammals. To this view it 

 has been objected that the edentata likewise possess no pleural cavity 

 but this is certainly an error of observation. 



The diaphragmatic arrangement in the tapir I have had no 

 opportunity of studying. 



It is true that the visceral and parietal pleural coverings are, 

 adherent in places in Cetacea (9). But in these animals definite lung 

 tissue is also absent in the neighborhood of the adhesions and all 

 the conditions of breathing in Cetacea are widely different from 

 those in elephants. 



From Goldschmidt's dissections and from those of other ob- 

 servers it would appear that the connective tissue obhterating the 

 pleural cavity is at birth very delicate, but that as age increases the 

 tissue becomes firmer and more resistant. It is probable, as Gold- 

 scHMiDT suggests (8), that in the embryonic life of the elephant 

 pleural cavities exist. These are obliterated later, so that at birth 

 no cavity exists. In this connection it is interesting to observe that 

 Bertelli has shown that in birds (chick, duck) a pleural cavity exists 

 in embryonic life but is partially obliterated later so that at hatching, 



