Importance of the Air-Spaces in Flying Animals. 151 



breathing- apparatus. There are no such arrangements, 

 however. In insects, by means of wide sections of tracheal 

 tubes, the great air-vesicles are in direct, immediate, and free 

 communication with the stigmata, so that the air which 

 enters them from without and that which is given off from 

 them to the exterior cannot pass through the respiratory 

 terminal branches of the ramifying tracheal trunk. In birds 

 the cephalic air-sacs, as well as some of the subcutaneous 

 ones in certain species, are connected with the nasal cavity ; 

 consequently the currents of air passing to and fro between 

 these sacs and the exterior cannot enter into the lungs at all. 

 The air-sacs of the body and limbs communicate with the 

 bronchi, with which they are openly connected by means of 

 wide canals. It is true that from the walls of these spacious 

 tubes, which traverse the lungs, there arise narrow ducts 

 which lead into the actual lung-parenchyma ; but in spite of 

 this the greater part of the air, which passes to and fro 

 between the exterior and these sacs, will take its course 

 through those wide tubes, and it is only quite insignificant 

 driblets from the air-stream that will pass through the paren- 

 chyma of the lungs. 



The cephalic air-sacs and the subcutaneous air-sacs 

 belonging thereto in birds, as well as the majority of the air- 

 sacs in insects, consequently do not contribute at all, while 

 the remaining air-sacs contribute only in a very slight degree, 

 by no means in proportion to their size, to the ventilation of 

 the parts of the body which absorb oxygen and excrete 

 carbonic acid. As an accessory respiratory apparatus they 

 can therefore at best function only incidentally : their chief 

 function must be a different one. 



We thus arrive at the second and third of the alternatives 

 stated above : we have to ask in what manner the air-sacs 

 can mechanically support the faculty of flight. In any case 

 they reduce the specific gravity of the body very considerably, 

 and I presume that this might increase the power of flight, 

 and especially facilitate materially the maintenance of equili- 

 brium in the air, although the enlargement of the body 

 connected therewith would considerably increase the resist- 

 ance of the air, particularly in rapid flight, and so under 

 certain circumstances would also bring disadvantages in its 

 train. 



Since the air contained in the pneumatic spaces in warm- 

 blooded birds has a higher temperature than the outer air, the 

 air-sacs in them certainly have a direct lifting effect — they 

 operate as balloons — but this is so inconsiderable that prac- 

 tically it cannot come into account at all. 



