308 



HISTORY OF THE HUMAN BODY 



specialized, and is of considerable importance in the produc- 

 tion of sexual calls. The slight respiratory importance of the 

 lungs in amphibians is further emphasized by the fact that in 

 a large number of species of salamanders, both lungs, trachea 

 and larynx are entirely wanting, although in a few cases rudi- 

 ments of these parts attest the former presence of these organs. 



The question will naturally occur at this point: what are 

 the means of respiration in adult amphibians if they have lost 

 their branchiae and yet possess either no lungs at all or those 

 of slight functional importance? The answer to this lies in 

 the fact that amphibians have developed two other systems, 

 neither branchial nor pulmonary, the assumption of which 

 shows how great may be the systematic response to a physi- 

 ological need, and suggests also the trying period of transition 

 when vertebrates first essayed the terrestrial environment, and 

 when attempts were made in all possible directions to adapt 

 themselves to the new respiratory medium. These two sys- 

 tems are the mtegumental and the pharyngo-cesophageal, and 

 as both of these demand for their highest efficiency a moist 

 environment with an occasional submersion in water, they are 

 successful in amphibians with their semi-aquatic mode of life, 

 but in higher forms have been discarded in favor of the pul- 

 monary system, which enables its possessor to leave the 

 marshes and inhabit the dry land. 



The origin of the assumption of a respiratory function by 

 the amphibian skin may be traced to the abundance of integu- 

 mental glands, inherited from the fishes and used to protect 

 the surface from the action of the water. The presence of 

 these glands necessitates the formation of a superficial net- 

 work of capillaries to supply them with nourishment, and the 

 integument becomes thus transformed into an organ that pos- 

 sesses the qualities necessary for a respiratory organ, that is, 

 a moist surface bathed by the respiratory medium and sup- 

 plied with a rich capillary net-work. Thus apparently by ac- 

 cident, as in all morphological changes, an organ which be- 

 comes modified for a certain function shows a capability of 

 assuming a second one, not intended in the original plan, and 



