THE RESPIRATORY SYSTEM 



253 



Pharyngeal Derivatives. From the epithelial lining of the embryonic 

 pharyngeal pouches 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 



m. HYPOPHYSIS 



-MOUTH 



-,POUCH 1 . 



iTeustachian tube) 

 1 -thyroid gland 

 -palatine tonsil 



-POUCH 2 



PARATHYROID 1 

 ^ -POUCH 3 

 _j%-THYMUS 1 

 gp-PARATHYROID 2 



^"THYMUS 2 

 POSTBRANCHIAL BODY 



POUCH 4: 

 POUCH 5--^ 

 TRACHEA" 



^.-LUNG LOBE 



ESOPHAGUS 





Fig. 235. — Ventral view of pharyngeal region of a human embryo showing the pharyn- 

 geal pouches and their glandular derivatives; semidiagrammatic. 



pharyngeal derivatives will be taken up in the chapter on endocrinal 

 organs. 



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

 mammals includes lungs, larynx, trachea, bronchial tubes, nasal 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 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, 



