Encyclopaedia Britannica > Volume 3, Athens-BOI

726
tions above the intestine; the latter is divisible into small
and large, and the last always terminates in a cloaca. It
is invested by a peritoneal coat, which follows the curva¬
tures of the intestine. In most Birds, as in the Croco¬
diles, the pyloric and cardiac apertures are approximated.
In many Crocodilia and Aves (e.g., Ardeidce) there is a
pyloric dilatation before the commencement of the duo
denum. In the Aledoromorphce, in Eagles and Hawks
amongst the Aetomorphce, and in Pigeons, the oesophagus
is enlarged into a “crop.” In the latter it is bilobate
and symmetrical (Macg., op. dt., vol. i. plate 7).
In the Crocodilia and in Aves the walls of the stomach
are very muscular, and the muscular fibres of each side
radiate from a central tendon or aponeurosis. The thick¬
ening of the muscular tissue of the stomach attains its
maximum in the Graminivorous Birds; and it is accom¬
panied by the development of the epithelium into a dense
and hard coat, adapted for crushing the food of these ani¬
mals. Birds commonly aid the triturating power of this
gastric mill by swallowing stones; but the habit is not
confined to them, Crocodiles having been observed to do
the same thing.1 Birds are further remarkable for the
development of a broad zone of glands in the lower part
of the oesophagus, which is usually dilated, and forms a
proventriculus, connected by a narrow neck with the gizzard
(gigerium). In Sula alba and Phalacrocorax carbo, the
writer, long ago, saw this zone to be imperfect. In these
birds the gullet is very capacious from the pharynx down¬
wards, but the proventriculus is still more so,—it is a
large “ paunch.”
Some Ophidia have a caecum at the junction of the small
intestine with the large; and two such caeca, which some¬
times attain a large size, are generally developed in Aves.
In this class, also, the small intestine not unfrequently
presents a caeca! appendage, the remains of the vitelline
duct. The writer’s drawings show this in Gallinula chlo-
ropus, Ardea cinerea, and Colymbus septentrionalis. The
duodenum of Birds constantly makes a loop, within which
the pancreas lies, as in the Mammalia.
The liver in the Sauropsida almost always possesses a
gall bladder, which is usually attached to the under surface
of the right lobe, but in Ophidia is removed to some dis¬
tance from it.
A peculiar glandular sac, the bursa Fabricii, opens into
the anterior and dorsal region of the cloaca in Birds.2
The Heart.
In Birds, the venous and arterial blood currents com¬
municate only in the pulmonary and systemic capillaries.
The auricular and ventricular septa are complete (see Owen,
‘•Aves,” p 330), as in the Crocodilia; but the right ven¬
tricle only gives off the pulmonary artery, the left aortic
arch has disappeared, and the right arch (the 4th of that
side in the embryo) becomes the most important of all the
arches. The septum of the cavum pulmonale becomes a
great muscular fold, and takes on the function of an auri-
culo-ventricular valve. At the origin of the pulmonary
artery, and at that of the aortic arch, three semilunar
valves are developed
1 See Sir S. Baker’s Ismailia, vol. i. p. 295 “ The stomach con¬
tained about five pounds' weight of pebbles (in a Crocodile 12 feet 3
inches long in its entire length), as though it had fed upon flesh resting
upon a gravel bank, and had swallowed the pebbles that adhered.’
This intrepid traveller seems to be unaware that the Crocodile has a
strong gizzard.
2 Besides copious unpublished materials on this subject from his
own dissections, the writer is largely indebted to Macgillivray’s most
valuable work, so full of illustrations of the digestive organs of Birds ;
also to Prof. Owen’s article “Aves” {op. cit.) ; tc Prof. Huxley he
owes form and order. For the development of these parts the reader
is still directed to Foster and Balfour’s work, as also of the parts yet
to be described.
[anatomy.
In Aves there is no renal portal system, and the anterior
abdominal vein opens into the inferior vena cava, close to
the heart. Nevertheless, a median trunk, which is given
off from the caudal vein, carries a considerable proportion
of its blood directly into the hepatic portal system. The
pericardium of the Bird is thin, but of a firm texture, and
adheres by its external surface to the surrounding air-cells.
(Owen, “ Aves,” p. 330.)
Respiratory and Vocal Organs.3
“In Birds there are distinct thyroid, cricoid, and arytenoid car¬
tilages, which may be more or less completely ossified. Sometimes
an epiglottis is added.4 * The voice of Birds, however, is not formed
in the larynx, but in the syrinx or lower larynx, which may be de¬
veloped in three positions 1. At the bottom of the trachea, from
the trachea alone; 2. At the junction of the trachea and bronchi, and
out of both ; 3. In the bronchi alone. The syrinx may be altogether
absent, as in the Ratitce and the Cathartidoe or American Vultures.
The commonest form of syrinx is the second mentioned above, or the
bronchi-tracheal syrinx. It is to be met with in all our common
Song Birds, but is also completely developed in many Birds, such as
the Crows, which have no song. In its commonest condition this
form of syrinx presents the following characters : 'The hindermost
rings of the trachea coalesce, and form a peculiarly formed chamber,
the tympanum. Immediately beyond this the bronchi diverge, and
from their posterior wall, where one bronchus passes into the other,
a vertical fold of the lining membrane rises in the middle line
towards the tympanum, and forms a vertical septum between the
anterior apertures of the two bronchi. The anterior edge of this
septum is a free and thin membrana semilunaris, but in its interior
a cartilaginous or osseous frame is developed, and becomes united
with the tympanum. The base of the frame is broad, and sends
out two cornua, one along the ventral, and the other along the
dorsal edge of the inner wall of the bronchus of its side, which in
this part of its extent is membranous and elastic, and receives the
name of the membrana tympaniformis interna.
“ The bronchial ‘ rings ’ opposite this are necessarily incomplete
internally, and have the form of arches embracing the outer moiety
of the bronchus. The second and third of these bronchial arcs are
freely movable, and elastic tissue accumulated upon their inner
surfaces gives rise to a fold of the mucous membrane, which forms
the outer boundary of a cleft, bounded on the inner side by the
membrana semilunaris.
“Ihe air forced through these two clefts from the lungs sets
these elastic margins vibrating, and thus gives rise to a musical
note, the character of which is chiefly determined by the tension of
the elastic margins and the length of the tracheal column of air.
The muscles, by the contraction of which these two factors of the
voice are modified, are extrinsic and intrinsic. The former are
possessed by Birds in general, and are usually two pairs, passing from
the trachea to the furcula and to the sternum (Macg., vol. ii plate
^2, fig- 8, d.d., e.e.; and vol. iii. plate 15, m.m., n.n.) Some Birds
possessing a broncho-tracheal syrinx such as has been described, as
the A lector omorphce (see Macg., vol. ii. plate 12, fig. 8,/.), Chcno-
morphce, and I)ysporom.orphce, have no intrinsic muscles. Most
others have one pair, attached on one side to the rings of the trachea
above, and to the tympanum, or the proximal bronchial arcs below
(Macg., vol. ii. plate 12, figs. 1, 2; and vol. iii. plate 19). The
majority of the Coracomorphce (Macg., vol. ii. plates 10, 11) have
five or six pairs of intrinsic syringeal muscles, which pass from the
trachea and its tympanum to the movable bronchial arcs.6 The
Parrots have no septum, and only three pairs of intrinsic muscles.
“ 1 he tracheal syrinx only occurs in some American Coraco¬
morphce. The hinder end of the trachea is flattened, and six or
seven of its rings above the last are interrupted at the sides, and
held together by a longitudinal ligamentous band. These rings are
excessively delicate, so that the part of the trachea is in great part
membranous. The bronchial syrinx occurs only in Steatornis and
Crotophaga.
3 We shall here give Professor Huxley’s excellent abstract of what
is known upon this subject up to this time ; but the reader is referred
to Joh. Muller s work, “ Researches on the Comparative Anatomy of
the Vocal Organs of Birds,” Berlin Acad., June 1845, and Ann. and
Mag.N. H., vol. xvii. p. 499. Macgillivray has many excellent illus¬
trations and descriptions of these parts ; and the writer followed him
step by step many years ago.
4 For a clear description of the exquisite structure of the tracheal
rings in Birds, see Macg., vol. ii. p. 34. They are often thoroughly
ossified, and are notched above and below, both before and behind ; and
alternate ridges allow a marvellous amount of overlapping, the edges
being well bevelled ; each ring is an ellipse.
6 Macgillivray {op. cit., vol. ii. pp. 26,28) was afraid to bethought
overstating the number of these intrinsic muscles. He understated
them, not thoroughly making out their divisional lines.
BIRDS