566 PROF. HUXLEY ON THE RESPIRATORY [June 20, 
each side, a layer of unstriped muscular fibres. I have not been able 
to make out any such fibres in Apteryz. 
6. The most remarkable difference, however, lies in the development 
of the air-sacs in the two birds. 
In Apteryz, 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 the 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 Apteryz, 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 Apferya. 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 axillee 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 Apterya 
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 thus becomes greatly increased. 
But, as the capacity of the posterior air-sac is also vastly greater 
than in Apterya, its posterior wall has been, apparently, driven out, 
like a hernial sac, between the peritoneum and the parietes, and pro- 
jects into the abdominal cavity. It would be incorrect, therefore, 
to say that the abdominal air-sac is absent in Apleryz : 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 preebroncbial 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 
