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 



