310 HISTORY OF THE HUMAN BODY 



pharynx and oesophagus are dilated and contracted in asso- 

 ciation with the usual respiratory movements of nostrils and 

 floor of the mouth, and the anterior part of the alimentary 

 canal thus becomes a functional lung with the power of in- 

 spiration and .expiration, forming doubtless a more efficient 

 organ than the simple air-sacs which these salamanders allowed 

 to atrophy. 



Above the amphibians, which, with their numerous methods 

 of respiration, suggest the experimentation of our early an- 

 cestors in their attempts to occupy what must have been at 

 first an unnatural environment, the pulmonary system becomes 

 supreme, and its further development is shown principally in 

 the increased efficiency of its two main organs, the lungs and 

 the larynx. The later history of this system is quite well 

 known, especially that of its development in terrestrial verte- 

 brates, but the origin of the system is still in part obscure, 

 and rests upon surmises rather than upon actual proof. 



The history begins with the period represented by fishes, 

 during which the pharynx exhibits a tendency to throw off 

 median diverticula, sometimes dorsal and sometimes ventral, 

 for the purpose of forming pneumatic cysts or air-bladders 

 to add to the buoyancy and thus aid in swimming. In many 

 cases these become closed and depend upon the adjacent blood 

 vessels for the gases with which they become distended, but 

 in others the original connection with the pharyngeal cavity 

 is retained and the two are kept in communication through a 

 small duct. In this latter case the cyst is filled with air, which 

 is expelled and renewed through the mouth when the fish is 

 at the surface of the water, a proceeding that demands some 

 sort of regulator at the orifice of the duct, an opening to which, 

 by an extension of meaning, the term glottis may be applied. 

 Such an apparatus, which consists of muscles and fibro-carti- 

 lage, is a functional larynx, of which there must be two distinct 

 organs, a larynx dorsalis, and a larynx ventralis, in accordance 

 with the position of the pneumatic cyst. That cysts in these 

 two positions cannot be homologous is evident; indeed, those 

 in the same position in fish not closely related are not neces- 



