THE RESPIRATORY SYSTEM 337 



in the group, and some respire by means of a highly vascular pharynx. 

 Nevertheless, even in those adult forms which are devoid of functional 

 gills, gill pouches occur in the embryo, and the embryos of gymnophiona 

 may have as many as six such pouches, suggesting a corresponding number 

 of functional gills in their ancestors. Most amphibian larvae have 

 functional gills. 



Functional gills are lacking in amniotes, but rudiments of gills are 

 represented by transient embryonic gill pouches and their intermediate 

 visceral arches. In the embryos of reptiles, some of the gill slits usually 

 become perforate and later close. The perforation of gill slits in mam- 

 mals is aijnormal. Pharyngeal pouches are, however, always formed in 

 the human embryo, and when these become perforate listulae in the throat, 

 they may persist and require surgical treatment. The presence of five 

 pharyngeal pouches and six visceral arches alternating with them receives 

 its only reasonable interpretation in the evolution theory. 



As has already been explained, the disappearance of the visceral 

 arches in man and mammals is incomplete. The skeletal elements are 

 converted into ear bones, attachment for the tongue, and support of the 

 larynx. Three of the aortic arches also persist, as will be shown in the 

 next chapter. Moreover, in addition to these rudiments th^re are 

 certain derivatives of the gill pouches which require special discussion. 



Pharyngeal Derivatives. From the epithelial lining of the embryonic 

 pharyngeal i)ouches arise some of the important endocrinal glands, 

 thyroid, parathyroid, thymus, and the ultimobranchial bodies. In 

 addition to these which occur in man, some vertebrates have also epithelial 

 bodies and suprapericardial bodies. From the second pair of {pharyngeal 

 pouches come the palatine tonsils. The history and development of these 

 pharyngeal derivatives will be taken up in the chapter on endocrinal 

 organs. 



B. The Puhnonary System. The respiratory system of man and 

 mammals includes lungs, larynx, trachea, bronchial tubes, narial passages, 

 and diaphragm. 



Lungs. Lungs are the essential respiratory organs of land vertebrates. 

 Man, like virtually all land animals except snakes, has two, the left 

 having two lobes and the right three. The absence of a middle lobe on 

 the left side appears to be an adaptation to the presence of a left-sided 

 aorta. The lungs lie within the rib basket, and when expanded obliterate 

 the potential pleural cavities. They are separated from one another 

 by the mediastinum or interpleural space, which contains the heart, 

 esophagus, and the great blood vessels which leave the heart. In child- 

 hood, the color of the lungs is pinkish, but becomes slaty grey in the adult 

 as the result of the accumulation of soot. Each lung is conical with an 

 apex and base. However, two surfaces, costal and mediastinal, and three 



