282 



COMPARATIVE ANATOMY 



the bronchi, trachea, and larynx, and this statement is supported by 

 a study of their comparative anatomy. 



PI) 



PD 



PD 



S- 



-,y' 



—^' 



Fig. 226. — A, B, C, Diagrams showing the Mode of Development of the 



Lungs. 



PD, primitive alimentary tube ; B, S^, the lung-sacs, which are at first unpaired ; 

 t, trachea ; i, bronchus. 



Fig. 227. — Diagram Illustrating the Phylogenbtic Development of the 

 Lungs ; A Gradual Increase of the Respiratory Surface is seen in 

 passing from a to D. 



Hollow outgrowths and buds arise from the endoderm of the 

 lungs and extend into the surrounding vascular mesoderm, which 



Fig. 228. — Diagram of the Embryonic Human Luni.:. (After W. His.) 



A%), pulmonary artery ; Ir, air-passage ; sp, ossophagus ; Ih, pulmonary vesicle 

 undergoing division ; O, right upper (anterior) lobe of the lung with its 

 eparterial bronchus ; M, F, middle and lower (posterior) lobes ; 0', left upper 

 lobe witli its hyparterial bronchus ; F^, left lower lobe. 



gives rise to muscular fibres and connective-tissue, and thus a 

 branched system of cavities communicating with the bronchi is 



