the developement and union of the last six or seven rings of the trachea. It is divided within into two unequal cavities by the os The Membrana tympaniformis.— This is a thin transparent membrane, extending from the os transversale to the extremities of the bronchial half rings g (figs. 912 and 914) : it forms the internal surface of the larynx and bronchi, and is a continuation of the semilunar membrane; so that the tympaniform and semilunar membranes are stretched simul taneously, and the latter is acted upon through the instrumentality of the former.
The Arytenoid Cartilage. — This is a small body situated at the head of the second bone : it is described by Savart as being of a lozenge shape in the starling, but very short in the nightingale. This form of larynx in the Cor vidm and Sylviadm is provided with six pairs of muscles.* (See 914.) Palnzipedes. — In the genera Anas and Mergus the inferior larynges of the males only are unsymmetrical, and composed of bony ca vities. In the Anas domestieus, the inferior larynx presents osseous cavities formed by transversale, of which the left is always the largest. The figure of the os transversale is very similar to that of the arytenoid cartilage in the human larynx,consisting of prominences, ridges, and base : the superior prominence of this bone projects high into the tube of the trachea, and completely divides the larynx at its base. The ridge on its left side is fur nished with a salient lamina, corresponding to a thin transparent semilunar lamina, situated opposite to it : the latter is placed at the en trance of a small osseous cavity at the infe rior boundary of the left larynx; a still larger osseous cavity lies obliquely above the semi lunar membrane. The air sets this membrane in motion as it circulates in the left chambers of the larynx, and contributes to the produc tion of the well-known peculiar character of voice in this bird. On the right side the cavity of the larynx is of smaller dimensions, of less irregular form, and destitute of those prominent ridges and salient lamina; within. There appears to be a small tympanic mem brane attached to the first few rings of the bronchus on the right side.
In the Mergus serrator the inferior larynx is partly osseous, and partly membranous : it consists of two irregular cavities. The os transversale, i (fig. 915), which has an exten sive union along its posterior edge and base with the bony boundaries of the larynx, divides this tube into two parts, except at its superior prominence, where it leaves a channel b for the air from the right bronchus f, to pe netrate the trachea. On the right lateral
surface of the larynx, a large oval membrane forms the boundary : the rest of this cavity is composed of bone. On the left side the larynx is furnished with four membranes, of unequal dimensions, which are inclined to each other at different angles ; of these membranes three are lateral, and the fourth forms the base. The internal lateral membrane b lies almost parallel to the superior promontory of the os transversale i, from which it is separated by a channel, leading from the left to the right larynx.* Through this channel the air from the left bronchus must pass to reach the trachea. The internal lateral membrane has a free salient edge inferiorly, over which the air brushes in its passage from the lungs to the trachea.f The posterior, anterior, and lateral mem branes are supported by bony rings, of which the anterior is the largest. The membrane forming the base is penetrated by the left bronchial tube, and is connected with several of the bronchial rings through the medium of the membrana tjmpaniformis. Thus the air from the lungs throws into vibration one large membrane in the right, and four in the left larynx, all of unequal dimensions, and con stituting a very complex piece of mechanism.
In the M. Merganser or Goosander the inferior larynx is most developed on the left side h (fig. 916), which is chiefly mem branous, while the other side d is composed principally of bone. On the left side there are four irregular membranes, a,b,c, supported by bones, and that on the external ring h. The os transversale is very small and does not reach the free edge of the internal mem brane b, which is similar to that of the Mergus serrator, thus leaving a communi cation open between the right and left side of the cavity of the larynx, through which the air from the left lung must pass to reach the tube of the trachea, which terminates on the superior part of the right side of the larynx. The right side is chiefly surrounded by bone externally, having a very small oval membrane anteriorly, and is furnished with a small tympanic membrane at its junction with the bronchus. The salient portions of these membranes, and the small membrana tym paniformis are best adapted for putting the air into a state of vibration, reinforced by the chambers of which the larynx is c posed.