270 



HISTORY OF THE HUMAN BODY 



pand into large sacs or reservoirs and either retain or lose 

 their communication with the parent cavity. Such are the 

 various sorts of air-bladders found among fish ; these are gen- 

 erally situated dorsally with respect to the pharynx, but ven- 

 tral ones occur in a few ganoids. Occasionally these reser- 

 voirs possess an attenuated pneumatic duct, communicating 

 with the pharynx, and the supply of air is regulated by the 

 fish coming to the surface and making a snapping or swallow- 

 ing movement; but in the majority of cases the air-bladder 

 is a closed sac, filled by gases extracted from the blood. In 

 terrestrial vertebrates there appears in the embryo a mid- 

 ventral diverticulum which opens from the floor of the 

 pharynx, grows posteriorly and forks into two lateral branches. 

 This forms the pulmonary system, the lateral sacs becoming 

 lungs and bronchi, the median duct the trachea, and the open- 

 ing in the pharynx the glottis, which becomes regulated by a 

 series of cartilages and muscles derived from the visceral sys- 

 tem and forming the larynx. 



The idea naturally suggests itself that this ventral pulmon- 

 ary system is a direct inheritance from a similarly situated 

 air-bladder, such as actually occurs in some ganoids, and al- 

 though there is no direct proof of this, it seems very probable. 

 It has also been noted that the diverticulum which produces 

 it lies immediately back of the converging line of pharyngeal 

 pockets and it has thus been interpreted by some as a continua- 

 tion of the system, the forking into the two lungs being taken 

 as proof of the formation of the median diverticulum from the 

 confluence of two lateral pockets. The first of these theories 

 seems by far the more probable, especially since the air-blad- 

 der of certain ganoids is richly supplied with respiratory 

 blood-vessels, and thus forms a better lung physiologically 

 than that of the more primitive amphibians, which are often 

 simple sacs, yet a direct continuity from one to the other can- 

 not be traced, since the lungs are always paired and an air- 

 bladder is always single. 



The nasal cavities are separated from the pharynx by an 

 approximately flat plate of bone that forms the roof of the I 



