most birds the epiglottis is situated on the internal surface of the thyroid ; it is rudimentary, and is termed the processes epiglotticus it is generally osseous, but ac cording to Henle it is leaf-like in the stork and heron ; and in some of the Gallinacetu, as in Sterna,Rallns, and Lams, it is thin, flat, and flexible, as in Mammalia.
Rima glottidis.— The form of this chink in a state of repose is triangular, the apex being directed backwards : it is bounded an teriorly by the thyroid, laterally by the ary tenoids, and posteriorly by the cricoid carti lage, but is destitute of salient membranous laminx.* The superior larynx is raised by the hyo-thyroideus and the thyro-tra chealis muscles, and depressed by the hypsilo and sterno-trachealis. The glottis is opened by the thyro-arytenoideus posticus, and closed by the thyro-arytenoideus lateralis, as in the higher order of reptiles.
The superior larynx is supplied by the superior laryngeal nerve alone, the inferior laryngeal terminating in the inferior larynx and trachea.
The inferior This organ is pecu liar to birds. It is exceedingly diversified in form and structure. It is always found except in the condor, and other vultures. If we make a section of the lower larynx of birds in the 'Desist plane, its lateral segments, if viewed separately, present a double organ of sound ; the exceptions to this rule are, the parrot, perroquet, and cockatoo. The inferior larynx of birds is often a very complex structure, and may be considered a double reed furnished with a pipe. It is symmetrical in most orders of birds except the Palmipedes, and is situated between the last ring of the trachea and first of the bronchi; it lies upon the cesopliagus pos teriorly, where there is generally a triangular space for the passage of that tube.
The frame-work of the inferior larynx is formed by the developement of several lower rings of the trachea, which take diversified forms in different orders of birds, and some times in the sub-genera of the same order, as in Mergus and Anas among the Palmipedes. In those birds which have a pure quality of voice, and whose instinct excites them to pro duce a continuous succession of tones, con stituting some defined melody, we find the inferior larynx not only better adapted to pro duce a purer quality of tone, but it is pro vided with a greater number of muscles for modulating the voice.
Among the Ins essores, the Corvide and Sylviadm have the external walls of the infe rior larynx composed on each side of three semilunar bones which are developed from the inferior portion of the trachea. The larynx is bounded internally by a bone, which tra verses the lower end of the trachea, the verti cal longitudinal section of which coincides with the plane of the menial section i (fig. 912): this os transversale, after stretching across the tube, divides at each end into two laminae, which diverge laterally from the axis of the bone to meet the two first external bones of the larynx, thus strengthening and complet ing its solid framework. The superior part of the os transversale is concave, and fur nished with a very thin delicate membrane, rising vertically from the bone h (fig. 912): i this is called by Savart the membrana semi lunaris.
This membrane is most developed in sing ing birds, and is considered by Savart to be one of the most essential organs of voice in the inferior larynges of the nightingales, thrushes, linnets, finches, and other singing birds, as well as in speaking birds, such as the pies, jays, Sze. The inferior edge of the os trans versale gives attachment to the membrana tym paniformis.
Bones of the inferior larynx. — The first bone a (fig. 913) is flattened and curved, being convex externally, and concave inter nally: it forms the boundary of the internal lateral face of the larynx.
The second bone b (fig. 913) presents nearly the same form as the first, but it possesses greater mobility, the muscles which are in serted into it drawing it upwards and out wards perpendicularly to the axis of the bonchus of the same side, by which means the area of the latter is varied.
The third bone is hut very slightly curved (e). It is separated from the second bone by a triangular membrane ; having its extremities articulated to the second bone by ligaments which permit of an extensive freedom of mo tion, particularly of a rotatory movement on its axis, and it is an important agent, accord ing to Savart, in modulating the voice. The internal surface of this bone is lined with a fibrous cord which forms the external lip of the glottis. The posterior extremities of these three bones are not united, but have a triangular space between them for the passage of the oesophagus.