In the Falcons, the larynx being provided with but one pair of muscles, viz. the sterno tracheal, its structure is much more simple than in the singing birds. The membrana tympani is however large, but does not appear to give off any semilunar membrane. The bronchial rings are distant from each other, and bound together by thin membranes ; the rings of the larynx are almost in juxtaposition, leaving no vibratile membranous space be them ; consequently the membrana tympani is the chief vibrating tissue. The order also has one pair of vocal muscles.
Scansores. —In the parrots the inferior larynx differs in structure from that of any other order of birds. They have no os transversale dividing the vocal organs, which is conse quently a single larynx. The segments of the rings in which the lower end of the trachea terminates are consolidated together on each side. The last two of these segments are of a crescent-like or semilunar form, with its axis nearly perpendicular to the axes of the ring of the trachea ; they are concave to each other, and their extremities articulated together, forming an elliptical space which is furnished with a thin vibratile membrane, as seen in fig. 917. At their junction is the joint or axis on which they revolve upon each other, and by which the tension of the mem brane of the glottis is capable of being varied.
voluted trachea are found in the Gal linacem ; as in the Rhynchcca Australis, and Phasianus Purraqua. In the singing birds the trachea is a cylindrical tube ; the rings which are at first cartilaginous become ossi fied with age. The tracheal rings, which are entire in the adult state, being connected by elastic membranes, are at a sufficient distance from each other to allow of all the movements of the neck.
The glottis is formed on each side by the elastic membranes that fill the space between the semilunar bones:; these membranes leave a narrow chink between them through which the air from the lungs passes, and puts them in a state of vibration whenever the bird draws them sufficiently tense ; for this pur pose they are provided with three pairs of muscles, of which two a, b, are tensors, and one c laxator of the glottis.
The trachea of birds comprehends that portion of the vocal tube which lies between the superior and inferior larynx its diameter and length generally depend on the length of the neck of the bird. In the common crane, the trachea after making its exit from the thorax, penetrates the sterno-tracheal space, and then making four turns upon itself in the same plane, it leaves the breast bone to follow its course into the neck : by this ar rangement the vocal tube is greatly lengthened for the purposes of voice, and the surface of the sternum increased for the attachment of the great pectoral muscles without adding materially to the weight of the bird. The voice of the crane is very sonorous, and may be heard at a great distance.
In Bewick's swan (fig. 918), the manner in which a considerable additional length is given to the trachea by its convolutions, will be evident upon an inspection of the figure.
The spoonbill presents another remark able instance of convoluted trachea, doubling twice upon itself. Other examples of con The diameter of the trachea in singing birds is nearly uniform and proportional to its length ; but among the Patmipedes, the Mergansers, and some species of Anas, as the A. fusca and A. rufina, there are tracheal en largements with additional muscles, to control their dimensions. The number of rings in the trachea of some birds is very great, sur passing that of any other class of animals. In the Phcenicopterus, according to Dr. Grant, there are at least 350 rings.