264 



COMPARATIVE ANATOMY. 



is the case, for instance, in Man. It leads to the following 

 conclusions. 



As the anterior lobe of the right lung belongs to the eparterial 

 and that of the left lung to the first hyparterial bronchus, these lobes 



FIG. 211. DIAGRAM OF THE ARRANGEMENT OF THE BRONCHI IN MAMMALS. 

 (From the ventral side. ) . 



a, a, eparterial bronchus of either side ; b, series of ventral, and c, of dorsal 

 hyparterial bronchi ; A and V, pulmonary artery and vein. 



are evidently not homologous, the middle right lobe corresponding 

 much more nearly to the anterior lobe of the left side. Thus 

 there is a want of symmetry between the right and left 

 sides, the right lung retaining one element more than 

 the left. 



The formation of lobes, which always begins at the anterior 

 end of the lung, is of less fundamental importance as regards the 

 structure of the lung than is the arrangement of the bronchial 

 system, as a single lateral bronchus is present to each lobe. Thus it 

 follows that what has hitherto been known in human anatomy as 

 the inferior (posterior) lobe does not correspond to a true lobe, but 

 represents the mainaxisof the lung, enclosing the main bronchus. 



In the description of the peritoneum (p. 208), attention has 

 already been directed to the fact that the thoracic cavity is 

 lined by a serous membrane, the pleura. As in the case of the 

 peritoneum, a parietal and a visceral layer may be distinguished 



