296 



COMPARATIVE ANATOMY 



of surface for muscular attachment without any considerable in- 

 crease in weight. Everything, in fact, combines to establish an 

 organ of flight with a large wing-surface and an increased strength 

 of the muscles. 



Mammals. As in Reptiles, the blood-vessels are of funda- 

 mental importance in determining the structure of the bronchial 

 system. The pulmonary artery crosses the main bronchus formed 



by the bifurcation of the trachea at 

 its anterior end, and this point may 

 be taken as dividing the lateral 

 bronchi into two systems an an- 

 terior eparterial and a posterior Jiy- 

 parterial. 



The hyparterial series is always 

 well developed, and consists of a 

 double row of lateral bronchi, be- 

 tween the roots of which the pul- 

 monary artery passes backwards 

 dorsally, while the corresponding vein 

 runs along the median side of the 

 main bronchus (Fig. 239). The epar- 

 terial system, on the other hand, 

 gradually becomes of much less im- 

 portance and in certain cases is re- 

 presented only by a single external 

 lateral bronchus on either side (Fig. 

 239) ; and, as a rule, even the left of 

 these disappears, only the right re- 



fl|P: JMJJ\ maining, and even this is not always 



retained. The eparterial bronchus, 

 whether developed on one or on both 



instead of from the main bronchus. 



In by far the greater number of 

 Mammals, then, the left eparterial 

 bronchus has disappeared, while the 



FIG. 239. DIAGRAM OF THE AR- 

 RANGEMENT OF THE BRONCHI 

 IN MAMMALS. (From the ven- 

 tral side.) 



a, a, eparterial bronchus of either 

 side ; b, series of ventral, and 



, 

 monary ven the anterior lobe of the right lung 



belongs to the eparterial and 1hat of 



the left lung to the first hyparterial bronchus, these lobes are 

 evidently not homologous, the middle right lobe corresponding 

 much more nearly to the anterior lobe of the left side. There is 

 thus a want of symmetry between the right and left sides, the 

 right lung usually retaining one more element than the left 

 (Fig. 240A). The so-called accessory fourth lobe does not correspond 

 to a true lobe, but represents the main axis of the lung enclosing 

 the main bronchus. 



