238 PECULIAR MECHANISM IN THE TRACHEA OF BIRDS. 



state is the most curious part of their history — a state that belongs more strictly 

 to insect transformations than to their labours. I may, however, just mention 

 that many of them, previous to this change, spin silk, of which only the cocoon 

 of the Common Silk-worm is available for useful purposes to man. The value 

 of this insect is much enhanced by domestication, and a careful attention to their 

 wants. The universal application made of the material derived from it places 

 it at the head of the direct contributors to the wants of man among insect 

 architects. 



In every branch of the works of insect labourers we perceive an infinite variety 

 of beautiful contrivances, both offensive and defensive, all tending towards the 

 preservation of the insect on which they are bestowed or the continuance of their 

 species. They are guided by an unerring instinct, which instructs them in every 

 thing necessary for fulfilling their part on this earth. " They learn nothing, they 

 forget nothing." Such arrangements cannot but fail to strike the careful observer 

 with the highest reverence for the Being who has implanted them in creatures 

 individually so feeble ; and in this corner of science, as in every other branch 

 of it, we are impelled to rise with admiration to the contemplation of Nature's 

 Author. 



EXPLANATION OF A PECULIAR MECHANISM IN THE TRACHEA 



OF BIRDS. 



By William MacGillivray, A.M., F.R.S.E., M.W.S., &c. &c. 



Having lately submitted to a rather minute examination a great variety of 

 tracheae, with reference to a second volume of my History of British Birds, I 

 have made some observations which, I trust, will be found of considerable interest, 

 and among them the following. The trachea of birds is formed of rings con- 

 nected by elastic membranes. These rings, which are generally bony, but 

 sometimes, as in the Golden Eagle, Ostrich, and Emu, cartilaginous, differ from 

 those of the Mammalia in being complete (excepting from two to five at the 

 upper, and sometimes one or two at the lower extremity), although they are 

 seldom of uniform breadth in their whole extent, the back part being usually 

 narrower. The bronchial rings, on the other hand, are generally incomplete, and 

 frequently cartilaginous, although in many cases they are osseous, as in Swans, 

 and in a few complete and rigid, as in the Grebes. It is not, however, my inten- 

 tion here to describe the trachea in detail ; but to point out a circumstance 

 relative to its structure which I have not seen noticed in any book, although, 

 having for same time back studied from Nature alone, I may be describing what 



