THE AMERICAN NATURALIST [Vol. XLII 



membranes and "adrenals" have been essentially rewritten. As 

 a result, the book is improved by many small increments as well 

 as by the new treatment of a few subjects. 



Among these is the interesting physiological difference be- 

 tween the lungs of birds and of mammals, now properly noted 

 for the first time. In mammals during inspiration the lung 

 expands and draws into its blind terminal respiratory chambers 

 a mixture of fresh and residual air. The residual air greatly 

 dilutes the oxygen which comes in contact with the absorbing 

 blood vessels, and also prevents the carbon dioxide from being 

 expired directly. In birds the long is a network of anastomos- 

 ing tubes which are not expansile. Fresh air is drawn through 

 these tubes by the expansion of the air sacs, which are non- 

 respiratory terminal prolongations of the lung. The lung of 

 the bird may, in a sense, be compared with the respiratory 

 bronchioles of mammals, the mammalian alveoli corresponding 

 with the avian air sacs. Thus the lung of birds is peculiarly 

 adapted for the rapid oxidation correlated with the. require- 

 ments of flight and with a high body temperature. Other ad- 

 ditions of this sort contribute to the value of this edition. 



L. W. Williams. 



