THE RESPIRATORY SYSTEM 



261 



their walls, they function only slightly in gaseous exchange. They serve as 

 a reservoir of non- vitiated air which is used when the vitiated air is forced 



Fig. 242. — Air sacs and canals of pigeon, c'-c^, intertransverse canals; da^-da"^, 

 axillary sac and its ventral diverticulum; dc, canal for ribs; dot, infraclavicular canal; 

 ds, subscapular sac; dst, sternal canal; pc, preacetabular canal; sad, sas, right and left 

 abdominal sacs; sc, cervical sac; sia, sip, anterior and posterior intermediate sacs. 

 (From Kingsley's "Comparative Anatomy of Vertebrates," after Bruno Miiller.) 



from the lungs. Thus there is a double tide of fresh air through the 

 lungs. Wetmore suggests that they assist in the regulation of body tem- 

 perature, which is higher in birds than in mammals. 



