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Show 410 DR. J. MURIE ON THE EMU. [Apr. 1 1, If the sac were distended, these, from their position, would naturally pull outwards, and thus retain the orifice of the tracheal rings in an open state. The trachea itself, no doubt, is kept tense or relaxed according to the degree of contraction exerted by the sternotracheal 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 differences 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 compared 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 1*1 inch. During inflation 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 tbe 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 between the upper larynx and the anterior end of the opening was •15 inches. The opening itself was 1*1 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 E m u the tracheal 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 differences existing in the two specimens here described, as also in the two birds examined by Fremery and Knox, are evidence of the opening occurring 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 m y 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 differences in the stages of development of this remarkable appendage in the young and adult Emu. When, however, the function which it |