chics. When, in the history of the race, vertebrates came out of 

 the water and adapted themselves to a terrestrial element, they 



304 HISTORY OF THE HUMAN BODY 



in true vertebrates, so that even in the lowest of the fishes no 

 more than nine pairs are ever indicated, and this number 

 suffers a constant reduction in higher forms, the loss being 

 progressively from behind forwards. In the lower vertebrates 

 also the effectiveness of the system is increased by the forma- 

 tion, in the endoderm lining the pharynx, of soft structures, 

 richly supplied with blood vessels, which border the gill-slits 

 and form the true respiratory organs, the definite gills or bran- 



\ 



substituted for this branchial system a pulmonary one, em- 

 ploying as lungs a pair of sacs which open into the floor of the 

 pharynx a little behind the last gill-slits, and which were un- 

 doubtedly in existence at the time of the change, employed as 

 air bladders. In the gradual perfection of this second respira- 

 tory system many of the parts of the old one obtained employ- 

 ment, and were one after another selected and modified to add 

 to its efficiency. 



This history of the sudden replacement of one system by 

 another, and of the gradual perfection of the second by making 

 over to its own use the material of the first, forms one of the 

 most interesting although most complex bits of anatomical 

 history, and one of which the record has been especially well 

 preserved. As it involves, however, the entire region and in- 

 cludes skeletal parts, muscles, nerves and other elements aside 

 from those which may be strictly termed respiratory organs, 

 much of the history will be found in the chapters devoted to 

 those other parts. Here an attempt will be made to outline 

 the history of the parts as a whole, with special reference to 

 the function of respiration. 



The fish type of respiratory apparatus is presented in its 

 most primitive form in the sharks and dog-fish, since numerous 

 modifications which have been acquired in the more specialized 

 fish are absent. It is a type that looks both ways, and, while 

 in many respects similar to that of Amphioxus, from it may be 

 clearly derived the branchial respiratory system of higher 

 forms. Like all special respiratory organs of vertebrates, it 



