246 ZOOTOMY. 



have thin membranous walls, and in uninjected specimens are not 

 readily recognised. 



285. The apertures in the lung which communicate with the air-sacs, 

 also seen readily only in injected specimens : the aperture of the 

 posterior sac (/) is at the postero-external angle of the lung : that of the 

 posterior-intermediate {p.in) just anterior and external to the last : that 

 of the anterior- intermediate {a.in) at the end of a prominent secondary 

 bronchus on the ventral surface, just posterior to the entrance of the 

 principal bronchus: that of the sub-bronchial {sb.b) is also on the 

 ventral surface, just anterior to the entrance of the chief bronchus : and 

 that of the prebronchial (sp.b) at the anterior end of the lung. 



287. By tracing the principal bronchus through the lung (if un- 

 injected, this can be done by passing in a probe and cutting down upon 

 it), it is seen to pass backwards and outwards to the postero-external 

 corner of the lung, and then to divide into two short tubes, the 

 anterior and external of which is connected with the posterior-inter- 

 mediate air-sac {p.in), the posterior with the posterior sac (ab) : the tubes 

 which communicate with the other air-sacs are given off from a dilata- 

 tion of the principal bronchus, termed the vestibule, just after it 

 enters the lung. Soon after entering the lung the bronchus loses its 

 cartilaginous half-rings. 



2S8. Both from the principal bronchus and the secondary bronchi, 

 tertiary bronchi are given off in a pinnate manner : these in their 

 turn give rise to tubes of the fourth order, and so on. 



XLV. Sever the bronchi at their entrance into the 

 lungs, and open the syrinx, under water, by the 

 removal of its ventral wall ; note : 



289. The cushion-like lateral thickenings of the walls of 

 the tympanum, projecting into and narrowing its cavity : the 

 mucous membrane covering them is thicker than elsewhere. 



290. The membrana semilunaris, a delicate, in- 

 conspicuous, vertical fold of mucous membrane, projecting 

 forwards from the posterior wall of the tympanum at the 

 point of bifurcation of the trachea : it is supported by a 

 delicate bar of cartilage — the pessulus — which extends 

 vertically between the adjacent dorsal and ventral extremities 

 of the first pair of bronchial half-rings. 



