566 PROF. HUXLEY ON THE RESPIRATORY [June 20, 



each side, a layer of uustriped muscular fibres. I have not been able 

 to make out any such fibres in Apteryx. 



6. The most remarkable difference, however, Hes in the development 

 of the air-sacs in the two birds. 



In Jptetyx, as in the Duck, the attachment of the dorsal end of 

 the dissepiment between the intermediate loculi corresponds with 

 the exit of the cceliac artery from ihe dorsal median septum; and 

 the relation of the air-sacs in front of and behind this dissepiment to 

 the bronchia which open into them is such, that there can be no 

 question of their homology with the anterior and the posterior 

 intermediate air-sacs in the Duck, notwithstanding the vastly greater 

 size of the latter. Hence the air-sac in front of the anterior inter- 

 mediate in Apteryx must be the homologue of the subbronchial in 

 the Duck ; and the position of this sac and its relation to the sub- 

 bronchial ostium leave no doubt that such is the case. 



But while, in Apteryx, the subbronchial air-sac does not extend 

 beyond the front margin of the sternum, and is floored by that part 

 of the oblique septum which lies at the sides of the fore part of the 

 pericardium and is attached to the front edges of the sternum, in 

 the Duck only a small part of the sac is thus related to the oblique 

 septum, and even this extends much further backwards and more 

 towards the median line than in Apteryx. In front, each of these 

 sacs has enlarged forwards to the space between the furcula and the 

 sternum, and there has opened into its fellow in the middle line. 

 Thus the two subbronchial air-sacs are fused into one great air- 

 chamber, and their mesial walls are so closely applied to the trachea 

 and great vessels as to invest them like a serous coat. Moreover 

 the common sac sends prolongations into the axillse and elsewhere, 

 and communicates with the pneumatic cavities of the adjacent bones. 

 A similar modification has taken place in the posterior air-sacs of the 

 Duck, but has been carried to a still greater extent. In Apteryx 

 the whole of this sac is enclosed between the oblique septum and 

 the pulmonary aponeurosis, the dissepiment between its loculus and 

 that of the posterior intermediate sac being situated almost midway 

 between the second dissepiment and the posterior extremity of the 

 pneumatic chamber. In the Duck, on the contrary, the dorsal end 

 of this dissepiment is attached close to the posterior extremity of 

 the lung, and thence slopes very obliquely backwards. The capacity 

 of the posterior intermediate air-sac tiius becomes greatly increased. 

 But, as the capacity of the posterior air-sac is also vastly greater 

 than in Apteryx, its posterior wall has been, apparently, driven out, 

 like a hernial sac, between the peritoneum and tlie parietcs, and pro- 

 jects into the abdominal cavity. It would be incorrect, therefore, 

 to say that the abdominal air sac is absent in Apteryx : it is just as 

 much present as in any other bird : but its small size and the small 

 relative development of the posterior intermediate sac permit it to 

 occupy a different position. 



7. The first or prsebronchial air- sac has hitherto been overlooked 

 in Apteryx. It is of a long ovate or spindle-shape, 21 millim. long 

 by 8 millim. wide in the middle, and lies between the longus colli 



