524 Comparative Morphology of Chordates 



The coelenterate hydra and the common gastrula stage of the metazoan 

 embryo alike consist of an ectoderm and an alimentary cavity whose 

 wall is the endoderm (Fig. 285). In adult stages of large, complex, 

 three-layered metazoans, the two primary layers become widely 

 separated by development of systems of bulky mesodermal structures, 

 but in birds the endoderm invades the entire body, even to the deeper 

 level of the skin, and in insects the ectodermal skin penetrates even 

 into the territory of the digestive tube. 



The bird, in breathing, alternately contracts and expands the 

 body-wall. In contraction the thoracic ribs (Fig. 401) become more 

 sharply bent at the joints between their vertebral and sternal parts, 

 and the sternum is raised. The volume of the body-cavity is thereby 

 decreased and air is expelled from the respiratory cavities. Inhalation 

 accompanies the reverse movements, the straightening of the ribs and 

 lowering of the sternum increasing the dorsoventral diameter of the 

 body. The lungs are so closely confined by adjacent structures that 

 they can undergo only very slight change of volume during these 

 respiratory movements. The structures which are free to expand and 

 contract are the large primary air-sacs. During inhalation the air is 

 not merely drawn into the lungs but sweeps entirely through them, in a 

 volume many times greater than that of the lungs themselves, and 

 fills the capacious air-sacs. At exhalation a large part of the contents 

 of the air-sacs, on its outward course, again sweeps through the lungs. 

 The arrangement of the bronchial tubes connecting lungs and air-sacs 

 indicates that the incoming air enters a sac mainly via the single large 

 bronchial connection with the lung, whereas the sac empties itself 

 mainly through the several smaller recurrent bronchi (Fig. 410). It 

 seems likely that there is some muscular control of the diameters of 

 these tubes. The extent to which the air is changed in the small out- 

 lying sacs must depend on their location. In those which are very 

 small blind pockets remote from the lungs, and in those situated in 

 the interior of bones, there can be little or no change of air except by 

 slow diffusion. The important respiratory surface is that of the air- 

 capillaries in the lungs. The walls of the air-sacs lack the close network 

 of capillary blood-vessels necessary in an effective respiratory mem- 

 brane. The great importance of the large air-sacs is the effecting of 

 complete ventilation of the lungs during both inhalation and exhala- 

 tion. The volume of air inhaled is so great and it rushes so swiftly 

 through the lungs that it cannot become much vitiated. Therefore the 

 air returned through the lungs from the air-sacs still has high respira- 

 tory value. In mammals the minute air-spaces in the lungs are "dead 

 ends." Exhalation leaves a residuum of vitiated air. Therefore the air 

 in the lungs merely alternates between better and worse; it is never 

 pure. In birds it is pure during intake and nearly so during outgo. 



