138 Dr Knox on the Anatomical Structure of the 
(Anas the goiden-ejed duck (Anas clangula,) the 
Cassowary of India, and that of New Holland ; of some of which 
I have caused accurate drawings to be made. In these, though 
diminished to about a sixth, the proportions have been strictly 
observed. It will be necessary, therefore, only to recall to the 
recollection a few elementary ideas, in order to place the subject 
in its true light. 
In birds the trachea is, generally speaking, proportioned to 
the length of the neck, but to this there are some remarkable 
exceptions, the chief of which, as far as I have observed, is found 
in the Anas Cygnus^ or wild swan. Moreover, in birds general- 
ly, the cartilaginous rings or circles of the trachea or bronchia 
(which in the tracliea are complete), are found of equal diameter 
nearly throughout ; but to this, also, there are some exceptions. 
Several present one or more dilatations in the course of one or 
other of these canals, and the actual diameter of the cartilaginous 
circles varies either gradually or suddenly. These peculiarities in 
structure are found chiefly in swimming birds, and one of the 
most remarkable is that which I now present to the Society. It 
would be difficult to decide on the precise use of these dilatations 
of the trachea in the swnmming birds ; but as they seem to be 
generally found in the male only, it is not improbable that they 
are connected with the organs of voice. But in the Cassowary 
of New Holland, the structure of the appendage of the trachea 
is altogether dissimilar to those described. It may be recol- 
lected, that in this bird the rings of the windpipe are com- 
plete, from their commencement at the upper larynx to about 
the fifty-second, v/hcn the next rings suddenly open by a wide aper- 
ture into a strong muscular bag, as large as the human head, 
closely attached to the sides of the trachea, and expanded rings. 
This bag is situated in the neck, immediately above the bone 
called Merry-thought : it was seen by me in the female, though 
it is probable that the male also possesses it. It is quite peculiar 
to the bird, no such appendage having been ever seen attached 
to the trachea of any of the feathered creation ; nor do I know 
of any thing analogous to it in any other animal, excepting in the 
cameleon, to the upper portion of w^hose trachea there is append- 
ed a comparatively large membranous bag. 
I stated in my former observations, that I believed it to per- 
