226 



RESPIEATIOtf. 



ditional space. When the dia- 

 phragm is again relaxed, and the 

 ribs are allowed to subside, the 

 cavity is again diminished, and 

 the air expelled. These move- 

 ments are termeclinspiration and 

 expiration. The spongy pulmo- 

 nary substance being thus dis- 

 tended with air, the blood sent 

 fromthe heart is brought into such 

 contact with it as to allow the re- 

 quisite interchange to take place. 

 [ 387. The minute anatomy 

 of the lungs, in vertebrate ani- 

 mals, exhibits many interesting 

 varieties. The structure is sim- 

 plest -in the naked amphibia, 

 where it is but little more com- 

 plex than in the snails.* In the 

 water-newt, for instance, the 

 lungs present themselves as a 



> -r; "? f N he T^f ' P air of sim P le elongated sacs 

 newt (Triton crtstatus) : A. the 



natural size; B, magnified: a 



O o\ 



236 )> attached to an ex- 



, , 



pulnionary artery ; b, pulmonary tremely short rudimentary la- 



rynx, and internally exhibiting 

 no projection ; the air distends 

 the entire hollow internal sac, or 

 cavity. In the frogs the mem- 

 branous surface of the lungs is 

 increased by the development of 

 cells upon their internal aspects 

 (figs. 237 and 238), upon the 

 bottoms of which cells other secon- 

 dary and smaller ones can be per- 

 ceived ; all these pulmouic cells, 



* The lung presents itself in its 

 ^ sim P le st form in the snails and 



of the Triton cristatus. The ves- slu & s ' The contra ctile respiratory ori- 

 sels are injected with fine size and fice here leads to a sim l )le smooth in- 

 vermilion, and form so dense a net- ternal cavit >' hned Wlth a debate 

 work that minute islets only of mucous membrane, upon which the 

 parenchyma remain visible. ' pulmonary vessels are distributed. 



Fig. 237. Portion of the lung 



