and convey the food to the gullet, or oesophagus, and hinder
its return so as to enter the windpipe. This orifice is
bounded on either side by the arytsenoid cartilages, seen
more plainly in Fig. 2 (6, 6), where the greater part of the
cricoid cartilage (a, a, a) has been removed, together with
the investments of the windpipe (c), that the bony rings of
which this is composed may appear more clearly. Figs. 3
and 4 illustrate the muscles which control the size of the
orifice, and constitute one of the accessory means by which
the sound of the voice is regulated. Of these there are
two pairs. The first, of which a portion is shewn in (Fig. 8
(a) and the whole displayed in Fig. 4 (a, a), extend from
the upper portion of the cricoid cartilage (Fig. 2, a) along
the two branches of the arytaenoid cartilages (Fig. 2, b) in
the outer edge of each of which they are respectively
inserted, and serve to close the orifice. The second,
sufficiently visible in Fig. 3 (6, 6), are those which open the
orifice, and arise from the lateral and posterior portion of
the cricoid cartilage (Fig. 2, a), and their fibres, passing
over the closing muscles just described, are inserted on the
inner edge of each arytaenoid cartilage (Fig. 2, 6).
The tube of the windpipe, or trachea, is composed of two
membranes enclosing numerous rings forming a cylinder
from end to end. At first cartilaginous, they become bony
as the bird grows older, and their ossification begins in
front and gradually extends backward towards the gullet*.
So far then there is no essential difference between the
Raven and other birds in the parts described.
The inferior larynx or syrinx, which is the real seat of
^ In certain birds ossification of all the tracheal rings is not completed.
Various inequalities of diameter and convolutions of the tube (some of which
will be hereafter described and figured) also occur, producing, as might be
expected particular effects on the voice. Generally the proportionate length of
the trachea deserves consideration, for shrill notes are produced by short tubes
and mce versâ. On the structure of the tube, too, certain effects depend. As a
general rule, though not without exceptions, birds which possess strong and
broad cartilages or bony rings have a monotonous and loud voice, while slenderer
rings with wider interspaces allow a freedom of motion producing a corresponding
variety in the scale of tone. ' P
the vocal organ in Birds, is at the bottom of the tube, and
is formed by the more or less firm union of several of the
lower bony rings of the trachea, as shewn in Fig. 5 (b),
where the parts are divested of their attachments, and again
in Fig. 6 (6) where one bronchus has also been removed to
exhibit the inner side (c) of the other, together with a
median cross-bone, as represented in Fig. 7 (a, a), extending
from behind to the front, and dividing the tube into
two equal parts, from the outer side of each of which the
Pig. 5. Fig. 6,
bronchi spring and diverge to the lungs*. From the upper
edge of this cross-bone a crescent-shaped membrane, concave
above, ascends for a short distance inside the main tube, and
thus forms a “ three-way piece ” .
The bronchi are, like the trachea, perfectly flexible, but the
rings of which they are formed, though similarly connected
with one another by membrane, as shewn in Fig. 5 (c), are
incomplete on the inner side, which is composed of a
delicate membrane, known as the membrana tympaniformis,
seen in Fig. 6 (c), on the change of form and length of
which some of the varieties of intonation depend. These
tubes diminish in size as they approach the lungs, and they
are slightly attached to each other and to the oesophagus.
The muscles of the glottis consist, so far as is known,
uniformly of the two pairs already described, but those of
* In one group of Birds—the American Vultures (Cathartidce)—and therein
only, it is believed, there is no special modification of the trachea into a syrinx.