. The structure and life of birds . ense of hearing. It is fine to see an oldHeron, put on the alert, at the slightest sound of ahuman foot, by his wary ears, turn in the directionwhence the sound comes his equally wary eye. TheCurlew is all ears. The Thrush hears the worm movingbeneath the ground and waits for his appearance abovethe surface. The Organ of Voice. As I have said above, a birds upper larynx at thetop of the trachea or windpipe has no vocal chords,and is, therefore, incapable of producing soundthough tone may be raised or lowered by it. Thereis a lower larynx, to which the name o
. The structure and life of birds . ense of hearing. It is fine to see an oldHeron, put on the alert, at the slightest sound of ahuman foot, by his wary ears, turn in the directionwhence the sound comes his equally wary eye. TheCurlew is all ears. The Thrush hears the worm movingbeneath the ground and waits for his appearance abovethe surface. The Organ of Voice. As I have said above, a birds upper larynx at thetop of the trachea or windpipe has no vocal chords,and is, therefore, incapable of producing soundthough tone may be raised or lowered by it. Thereis a lower larynx, to which the name of syrinx iscommonly given, the mechanism of which is, in all VI FORM AND FUNCTION 37 important respects, the same as that of the humanlarynx. There are two membranes corresponding tothose called in man the vocal chords, which canbe stretched tight, and made parallel to one thus stretched, they are set vibrating by thepassage of the air between them, and a note is pro-duced. The syrinx is in principle a reed instrument,. Mu Fig. 34.—Syrinx. Raven, a with bronchi ; a, b, c, half-rings ; where the two bronchi face each otherthere is nothing but membrane, b, side view ; the outer part of the lower end of thetrachea and of the nearer bronchus being cut away; M, Membranous inner wall otbronchus; , Membrana semilunaris; p, Pessulus ; c, Muscles of Syrinx; mu,Muscles (after Owen). though, in the relative position of the vibrating mem-branes thus set edge to edge, it is, as far as I know,unlike every instrument commonly used. There arethree varieties of syrinx, distinguished by their differentpositions in the trachea or bronchi, but I shall describeonly the one which is by far the most common. Near to the point where the windpipe divides to form I3§ THE STRUCTURE AND LIFE OF BIRDS chap. the two bronchi leading to either lung, a bony enlarge-ment ■ is found, formed partly from the lower ringsof the windpipe, partly from the upper ones ofthe bronchi. The latter on the
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