410 DR. J. MURIE ON THE EMU. [Apr, 1 1, 



If the sac were distended, these, from their position, would natu- 

 rally pull outwards, and thus retain the orifice of the tracheal rings 

 in an open state. The trachea itself, no doubt, is kept tense or re- 

 laxed according to the degree of contraction exerted by the sterno- 

 tracheal muscles (fig. 2, S. t. m'.). 



The width of the trachea itself is about 2| inches ; and it has a 

 rather flattened or low arched curve from side to side. 



In proceeding to note the diiferences exhibited in the young male 

 I may say that this bird appeared as if little more than a third of 

 the size of the adult female ; but its exact age was not known. In 

 this instance I managed to inflate the sac and raise the tissues from 

 above it, so that the front and distended view was perfect. This is 

 shown in the woodcut (fig. 2), where its diminutive size, as com- 

 pared with the older female, is very marked. 



In the first place its shape was decidedly oval, and it barely reached 

 the borders of the trachea when fully blown out. Its greatest length 

 then was 1*3 inch, and its greatest diameter I'l inch. During in- 

 flation the rings composing the borders of the opening rose up and 

 became stretched outwards, giving an oval contour to the slit itself. 

 There were five rings on either side of the opening, and above this 

 fifty-eight rings, four rings lower therefore than in the adult female. 

 The walls of the sac in the young bird under consideration were very 

 thin and transparent, and seemed almost a continuation of the areolar 

 tissue covering the trachea, but more directly springing from the 

 cartilaginous edges of the longitudinal slit. When uninflated the 

 sac could not be distinguished from the tissues of the trachea, and 

 the slit in the windpipe shone distinctly through. The distance be- 

 tween the upper larynx and the anterior end of the opening was 

 15 inches. The opening itself was l"l inch long and 0*2 broad, 

 and from its lower end to the sternum 3 inches. 



This observation goes to prove that in the young Emu the tra- 

 cheid sac is insignificant in proportion compared with the size that 

 it afterwards attains. Its parietes then are also of extreme tenuity, 

 and do not show any trace of vascularity. Moreover the difi'erences 

 existing in the two specimens here described, as also in the two birds 

 examined by Fremery and Knox, are evidence of the opening occur- 

 ring at no regular fixed ring, counting from above, the limits being 

 in these cases between the fifty-second and fifty-eighth rings. 



Lastly, it exists both in the male and female sex, as found by 

 Fremery and myself. But Fremery has not noted whether the males 

 or females have the largest sacs, and my own observations do not 

 permit me to state positively regarding this point. In the adult male 

 the tracheal opening was certainly as large as in the female ; but, not 

 being aware of the nature of the sac before the parts were cut 

 through, I did not make sure of this point. It certainly was very 

 much smaller in the young male bird ; but imperfect development 

 must be taken into account. 



Function. — I have so far explained the appearances and diff'er- 

 ences in the stages of develoj)ment of this remarkable appendage in 

 the young and adult Emu. When, however, the function which it 



