33^ 



COMPARATIVE ANATOMY 



borders are distinguished. The heart and pericardial cavity produce a 

 deep concavity on the mediastinal surface of each lung, while the costal 

 surface is convex in conformity with the inner surface of the chest. The 

 base of each lung is concave to correspond with the diaphragm with 

 which it is in contact. 



The structure of the lungs is admirably adapted to the need of exposing 

 to the air a large amount of surface, estimated to equal that of a balloon 



HYPOPHYSIS 



■MOUTH 



-,POUCH 1 , 



i (EUSTACHIAN TUBE) 



-THYROID GLAND 



-PALATINE TONSIL 



- POUCH 2 



ijfc-PARATHYROID 1 

 K"~~POUCH 3 

 m-THYMUS 1 

 JBF-PARATHYROID 2 



a ^"THYMUS 2 



POSTBRANCHIAL BODY 



LUNG LOBE 



ESOPHAGUS 



Fig. 282. — Ventral view of pharyngeal region of a human embryo showing the pharyngeal 

 pouches and their glandular derivatives — semidiagrammatic. 



ten feet in diameter, and a section of the lungs shows that the volume of 

 air space greatly exceeds that of soUd tissue. The required moisture is 

 supplied by mucous glands. 



The trachea or wind-pipe subdivides into bronchi, both structures 

 having cartilaginous supports. The bronchi divide into bronchioU, 

 the bronchioU into alveolar ducts, the alveolar ducts into atria, alveolar 

 sacs, and alveoli, which form the ultimate subdivisions. Exchange of 

 gases occurs chiefly in the alveoU, although the thin respiratory epithelium 

 is found also in the atria and alveolar sacs and mav extend even into the 



POUCHY- 

 POUCH 5""^ 

 TRACHEA- 



