72 STRUCTURE AND CLASSIFICATION OF BIRDS 



When the lung is thus bared it is seen to be provided 

 with a number of conspicuous orifices where the covering 

 aponeurosis is deficient ; these are termed the ostia, and - 

 they lead into the air sacs. Their number is variable in 

 correspondence with the variability in the number of air 

 sacs. The free surface of the lung is supplied with bands 

 of muscle, which have been termed ' diaphragm,' but which 

 are called by IIuxley' costo-pulmonary muscles. These 

 muscles arise from the ribs and spread out over the aponeu- 

 rosis covering the lung ; they are, as a rule, extensive, 

 extending but a short distance from their origin. The 

 number of these costo-pulmonary muscles varies i much 

 among birds ; but little attention has been hitherto paid to 

 them. For instance, among the storks the muscles in 

 question are reduced to a minimum ; there are only two 

 pairs at the anterior end of the lung, which arise not from 

 ribs, but from the end of the windpipe. In herons, on the 

 other hand, and in the emu, all the ribs bordering upon 

 the lungs give off fasciculi of fibres, which in the emu are 

 of considerable thickness. Each bronchus enters the lung 

 at a little distance from its anterior end, and sometimes, as 

 in the condor, the cartilaginous rings cease some little way 

 before it enters. The bronchus dilates somewhat when it 

 has entered the lung, and from the posterior end of this 

 dilatation a tube is continued backwards, which opens into 

 the posterior or abdominal air sac. This trunk is termed 

 by Huxley the mesobronchium. Further on in its course 

 the mesobronchium gives off another branch, which opens 

 into the posterior intermediate air sacs (a description of the 

 air sacs will be found a little further on). From the 

 dilatation of the mesobronchium, the vestibule, arise four 

 other tubes, which are called the entobronchia by Huxley - 

 (whose nomenclature is adopted throughout in the present 

 section). The first curves forward and gives off several 

 branches, one of which opens into the preebronchial air sac, 

 while the main trunk is continued into the subbronchial 

 air sac. The second entobronchium passes dorsally and 

 ' ' On the Respiratory Organs of A'gterya,' P. Z.S. 1882. 



