Ixxvii 
to admit of more easy dilatation ; perhaps, also, the posterior 
transverse muscular fibres of the trachea may, by increased 
contraction, tend to divaricate the anterior cornua of the car- 
tilages, whilst the extension of the neck and elongation of 
the trachea have separated the cartilages above and below the 
opening; once the bag has been filled by this expiratory 
effort, the glottis being closed, it may be retained in that 
state, even should this opening become relaxed, provided the 
walls of the sac do not contract, and respiration may continue 
without the reservoir being affected, further than that the in- 
cluded air may be more or less changed by the admixture of 
fresh air from each inspiratory current: the closure of the 
glottis, and an expiratory effort, then appear to be the simple 
agencies whereby the distension of the sac is effected. 
The sac can be emptied by the contraction of its muscular 
covering, the distending force having ceased, and the air may 
be expelled by expiration, or it may be drawn into the lungs 
by inspiration; the elasticity of the cartilages, and the com- 
pression of the surrounding parts, will then approximate the 
edges of the opening, which will be supported by the internal 
vertical projection on the back part of the tube, and thus the 
orifice will become so perfectly closed, that inspiration can 
have no effect in drawing within it the superincumbent soft 
parts. 
When, from surveying this curious and elaborate struc- 
ture, we turn our attention to its use, and endeavour to explain 
the design of this anomalous arrangement, we are at once met 
by the fact, that although this bird is in all respects so simi- 
lar to the ostrich, and to the Indian cassowary, yet in it alone 
is this bag developed. Were a similar structure found in all 
the struthiones, we should have had little hesitation in con- 
necting it—though, no doubt, erroneously—with some of the 
peculiar habits and endowments of this class generally ; but 
such not being the case, we naturally ask, is there any pecu- 
liarity of climate, or any other circumstance in the locality in 
