1878.] TRACHEA OF RHYNCH^.A CAPENSIS. 749 



the trachea, and the remarkable modification of the intrathoracic 

 rings, to be presently described, had not proceeded so far. 



3. Immature females, indistinguishable in point of plumage, though 

 already larger than males. In all of these, without exception, the 

 windpipe was found quite straight and simple throughout, though 

 even at this stage females are infallibly to be distinguished from males 

 by their stouter trachea, by the more powerful musculature of this, 

 and by the more inflated condition of the delicate membranes 

 connecting the bronchial half-rings with one another and with the 

 three-way piece. 



4. Young, adult, and apparently old males, all agreeing together 

 in plumage and in the straight and simple condition of the trachea. 



We shall see that the superficial loop which is invariably to be found 

 in birds belonging to group 1 {old females) is the outward expres- 

 sion, so to speak, of a modification of the intrathoracic tracheal rings 

 that takes place pari passu with those external changes which, when 

 they are completed, mark the adult. But in order to make my 

 description of this curious modification more intelligible, a few 

 words, by way of preface, about the unmodified trachea of the male, 

 or, better, of an immature female. If such a trachea be drawn 

 through the fingers from end to end, a broad and shallow constric- 

 tion will be felt near its posterior end, twenty rings or so from the 

 compound three-way piece ; the rings composing it are cylindrical 

 instead of flattened, more than thrice as numerous as they are in an 

 equal length of any other part of the tube, and so closely packed 

 and firmly bound together as to possess little or none of that power 

 of expansion and contraction which, by reason of their peculiarly 

 bevelled ends, so eminently distinguishes the rest ; it occupies a 

 position as much within as without the thorax ; and the great extrinsic 

 muscles which pass between the sternum and the trachea, serving 

 amongst other purposes as "guys" to keep the latter in place, 

 expand and unite sheath-like over it, being inserted into it at nume- 

 rous points, but especially at its anterior extremity ; it is, in fact, the 

 part of the trachea upon which the sterno-tracheal muscles directly 

 pull when they contract, and thereby approximate the rings of the 

 extensible intrathoracic portion of the windpipe in the adult female, 

 to which we may now return. 



On more closely examining the extrathoracic portion of the traehea 

 in a bird in which that part is in the condition represented in fig. 1, 

 p. 748, it can be seen that the loop is almost wholly composed of the 

 constricted portion above described, and that the 2 or 3 rings that 

 immediately succeed in order from before backwards (those situated 

 at the point where the tube disappears within the cavity of the chest) 

 suddenly get coarser and more prominent, and at the same time 

 separated from one another by perceptible membranous intervals. 

 On cutting away the sternum so as not to sever the sterno-tracheal 

 muscles from their attachments, and so as to leave the furcula together 

 with the membrane included between its two arms in place, the rest 

 of the tube is displayed in a completely extended condition ; it is 

 then seen that the 12 or 13 rings immediately succeeding the coarser 



