32 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



the humerus, femur, sternum, and pelvis), so that the whole body 

 is penetrated by these cavities. While this great extension of air 

 sacs is especially characteristic of birds, it has its forerunners in 



the reptiles, where, as in the chame- 

 leons, similar air sacs invade the ab- 

 dominal region, while in the fossil 

 dinosaurs some of the bones contain 

 cavities which are regarded as having 

 been occupied by similar air sacs. 1 

 The function of these air sacs in the 

 birds is not certainly known. The 

 chief suggestions made are that they 

 largely increase the respiratory sur- 

 face, and by introducing air into close 

 connection with the tissues, they les- 

 sen the demands upon the circulatory 

 system ; they also to a slight extent 

 lessen the specific gravity of the ani- 

 mal ; and it may be that compression 

 of them in one region or another 



^ causes a shifting of the position of 



FIG. 35. Diagram of air-sacs the centre of gravity, a matter of no 

 of bird, the lungs shaded. A liu]e importance i n flight. Another 



axillary sac ; AB, abdominal ... 



sac; AI, anterior intermediate vi ew IS that they allow the air to flow 



sac ; B, bronchus ; PI, posterior twice over the respiratory surface, 



intermediate sac; PB, prebron- thus a ll ow i ng a more complete CX- 



chial sac ; SB, subbronchial sac ; 



T, trachea. chan g e f S aseS ' 



Thyroid Gland. The thyroid 



gland arises from the floor of the pharynx in the neighborhood 

 of the anterior gill slits. In the typical condition there is a 

 median or unpaired invagination of the oral epithelium, which 

 later becomes cut off as a hollow vesicle or a solid body. 

 Farther back, in most if not in all vertebrates, a pair of sec- 

 ondary invaginations are formed. 2 Like the anterior invagina- 



1 A somewhat similar pnettmaticity of certain bones is found in mammals, and espe- 

 cially in monotremes, where air cavities occur in certain bones, especially those of the skull. 

 These cavities, however, are not connected with the lungs. 



2 In reptiles but one of these paired structures comes to full development, the other 

 being rudimentary. 



