282 



COMPARATIVE ANATOMY 



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

 a study of their comparative anatomy. 



PD 



PD 



FIG. 226. A, B, C, DIAGRAMS SHOWING THE MODE OF DEVELOPMENT OF THE 



LUNGS. 



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

 t, trachea ; b, bronchus. 



FIG. 227. DIAGRAM ILLUSTRATING THE PHYLOGENETIC 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 LUNG. (After W. His.) 



Ap, pulmonary artery ; lr, air-passage ; sp, oesophagus ; lb, pulmonary vesicle 

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

 eparterial bronchus ; M, V, middle and lower (posterior) lobes ; O 1 , left upper 

 lobe with its hyparterial bronchus ; F 1 , left lower lobe. 



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

 branched system of cavities communicating with the bronchi is 



