muscular cord 3 to 6 centimeters in diameter. The caudal end is thicker,
becoming gradually thinner toward the anterior portion. The bundles of
muscular fibers extend toward the oviduct blending with the circular
fibers of that viscus. The ligaments terminate anteriorly in such a
manner that they aid in forming the serous ovarian pocket, which guides
the yolk into the fimbriated portion, or funnel, of the oviduct. The
walls of the ovarian pocket are formed by the left abdominal air-sac, a
part of the intestine, and the mesentery. The dorsal portion is formed
by the roof of the abdominal cavity, and the ventral portion is formed
by the dorsal wall of the air-sac. The medial, the anterior, and the
lateral limit of the pocket are formed by a fusion of the wall of the
air-sac to the mesentery and to the body wall. Posteriorly, the wall
consists of the transverse part of the small intestine and the caudal
portion of the left cæcum with their attached mesentery.
THE DUCTLESS GLANDS
These glands do not possess excretory ducts. They furnish materials
which are added to the blood or lymph as it passes through them. The
material from each gland is known as an internal secretion, or hormone.
Some of these secretions are powerful materials and influence profoundly
the body nutrition. The ductless glands are usually given as follows:
the spleen, the lymph glands, the pineal gland, the pituitary body, the
thyroid gland, the thymus gland, the adrenal glands, and the
parathyroids. The spleen, the pituitary, the pineal, and the lymph
glands are described in other sections.
=The Thyroid Gland= (Fig. 21, No. 16). _Location._—The thyroid gland
lies on the ventral side of the carotis communis at a point where the
carotis communis touches the jugular vein, which is about the point of
origin of the vertebral artery.
_Shape._—The thyroid gland is small, oval or somewhat roundish, and red
or rose-colored.
_Structure._—The thyroid gland has a fibrous capsule, which sends into
the interior septa which divide it into acini. These acini are closed
and contain a fluid. The thyroid is a ductless gland. Short arteries
from the carotis enter this gland, and some large veins connect it with
the jugular vein. The lymph vessels which lie along the neck are closely
connected with it and receive twigs from the gland. The minute lymphatic
capillary endings are found in its septa and in its capsule. The acini
are lined with a single or a double row of cuboidal secreting cells.
There are two kinds of cells, namely, secreting and resting cells. The
actively secreting cells secrete colloid.
The =parathyroids= consist of two small bodies attached to the lower
pole of the thyroid.
=The Thymus Gland= (Fig. 47, No. 10).—The thymus gland is an organ of
fetal and early baby chickhood. It soon undergoes retrogressive changes
into fat and connective tissue. It is of epithelial origin, being formed
in fetal life from the entodermal cells of the dorsal end of the throat
fissure.
_Location._—The thymus gland lies anterior to the thyroid, the latter
lying the deeper.
_Shape._—The thyroid consists of two lobes, which are united by
connective tissue, and appears as a loop-like acinous gland lying along
the neck and near the region of the bronchi and the jugular veins with
fibrous extensions toward the head.
_Structure._—The gland lobes are divided into lobules, which consist of
a cortical and a medullary portion. The cortex consists of nodules of
compact lymphatic tissue similar to those found in the lymph glands.
These occupy the chambers formed by the septa of connective tissue. In
the medulla there are a number of spherical, or oval bodies composed of
concentrically arranged epithelial cells. These are known as Hassall’s
corpuscles, and represent only the remains of the original glandular
epithelium. They are characteristic of the thymus gland. The thymus
appears to be a type of lymph organ. Lymph vessels are rare; a few
blood-vessels on the upper side form capillary nets.
=The Adrenal Gland= (Fig. 54, _F_). _Location._—The adrenal gland, often
called the suprarenal capsule, lies just anterior to the front part of
the anterior lobe of the kidney, adjacent to the testicles in the male
and to the ovary in the female. It is loosely attached by connective
tissue to the posterior aorta and to the vena cava.
_Shape._—It is yellowish-brown or reddish-pink in color, small, and of
irregular formation.
_Structure._—The adrenal gland consists of a cortical and a medullary
portion, although these two parts are not distinctly marked. The
cortical portion has columns which extend deeply into the gland, and the
medullary portion sends columns into the cortical portion. Therefore,
the two substances, lying side by side, form a cord-like structure.
It is probable that the cortical portion is derived from the ingrowths
of the peritoneum, and the medullary cords from the sympathetic
ganglion.
The cells are cylindrical or polygonal in shape, with an eccentric
substance between the columns. The cords, or columns, form between them,
elongated channels which extend into the interior of the gland and end
as blind or cæcal extremities. Large ganglionic nerve cells belonging to
the sympathetic system occur near the surface of the gland. The
blood-vessels are not well developed in the interior of the gland but
are numerous and of good size in the outer parts. Lymph vessels are also
present.
_Function._—The adrenals are ductless glands. They secrete an internal
secretion, or hormone, which influences the tonus of the blood-vessels.
An extract from these glands is called adrenalin.
THE RESPIRATORY APPARATUS (Figs. 50 and 61, _A_)
Owen says: “Notwithstanding the extent and activities of the respiratory
function in birds, the organs subservient thereto manifest more of a
reptilian than of the mammalian type of formation.”
By the action of the respiratory organs certain chemical and physical
changes take place in the blood. The chief of these consists in
absorption of oxygen from, and giving off carbon dioxide to, the
atmospheric air, the former changes being necessary for the elaboration
of the fluid, the latter for the elimination of a substance which, if
retained, would prove injurious. The organs of respiration are
invariably adapted to the wants of the animal and the medium in which it
lives.
In the bird, which breathes through its nose, the organs of respiration
are nostrils, nasal chambers, pharynx, superior larynx, trachea,
inferior larynx, bronchi, bronchial tubes, lungs, and air-sacs.
=The Nostrils and the Nasal Chambers.=—The nostrils of the bird open
externally by two small elliptical openings, which pierce the upper
mandible. Within each nasal chamber (Fig. 26, _A_) are three turbinated
laminæ, or _turbinated bones_. The inferior one is a simple fold
adhering to the lower and anterior part of the nasal septum. The middle
turbinated bone is the largest. It is of infundibular form, and adheres
by its base to the septum and externally to the side wall of the nose.
It is convoluted with two and a half turns. The superior bone, of bell
shape, adheres superiorly to the frontal bone. The internal turbinated
bone extends toward the orbit; the external terminates in a cul-de-sac
behind the middle turbinal (Fig. 26, No. _A_, 1 and 2, and _G_).
The nostrils are separated by a partition which is partly bony and
partly cartilaginous. The posterior nares is represented by a long slit
in the hard palate.
=The Pharynx and the Superior Larynx.=—A transverse row of horny,
filiform papillæ marks the anterior border of the pharynx, where in
other animals the soft palate, or velum, is located. (For further
description of the pharynx see special chapter.)
The supero-posterior border of the larynx, at the juncture of the larynx
and the esophageal margin, is marked by a second transverse row of
horny, filiform papillæ, which point backward. There is no epiglottis.
The superior part of the larynx is pierced by an oval, slit-like
opening, the _glottis_, which is provided with two lips. These when
brought together, tightly close the glottis so that nothing can fall
through into the larynx in the act of deglutition. The margin, or rim,
of this opening is called the _rami glottis_. The glottis is controlled
by two pair of muscles.
The superior surface of the larynx is somewhat triangular with the apex
directed forward. A few delicate, filiform papillæ are upon its surface.
The bird has, as already indicated, two larynxes, the _superior larynx_
located at the upper end of the trachea, and the _inferior larynx_ at
the bifurcation of the trachea. The inner surface of the superior larynx
is smooth and does not contain vocal cords; it is in these animals
simply a passage for air. It is joined to the trachea inferiorly by a
ligament, the _crico-trachealis_, and lies at the base of the tongue
supported by two cornua of the os hyoideum.
The cartilages forming the principal support of the superior larynx,
consist of four pieces, as follows: one unequal ventral piece, two side
pieces, and one unequal dorsal piece. The cartilaginous, flat, ventral
_cricoid_, early in the bird’s life, often becomes bony. The side pieces
are separated from it, only exceptionally fusing with it. The dorsal
cricoid piece also often becomes bony. The two _arytenoid cartilages_,
joined with the cricoid superiorly, are three-sided, and are united to
each other in a sharp angle. They form the superior opening of the
superior larynx.
=The Trachea.=—The trachea is cylindrical and varies in length in
different kinds of birds in accordance with the length of the neck. It
consists of from 90 to 120 cartilaginous rings, complete with the
exception of the two uppermost, which rings are held together by
intercartilaginous ligaments. The tracheal rings are constructed of
hyaline cartilage and the ligaments of fibrous tissue. It is lined with
a mucous membrane covered by columnar epithelium. The trachea is a
passage for air alone and terminates in the inferior larynx.
=The Inferior Larynx.=—The inferior larynx, called the _true larynx_
because it is the organ of voice, is located at the inferior end of the
trachea and the superior ends of the bronchi. By some anatomists this
organ has been called the _larynx broncho-trachealis_. The larynx is
flattened laterally in fowls. It contains two membranous folds, which in
the production of sound are caused to vibrate. These folds are half-moon
shaped elastic structures, located in the bony, arrowlike way,
intero-inferiorly. These structures are called the _membrana tympana
interna_. In the duck this inferior larynx is represented by a drum-like
cartilaginous and bony structure, called the _bulla tympaniformis_. This
bulla is a resonant apparatus which serves to strengthen the voice.
In song birds there is a double glottis, usually produced by a bony bar,
called the _pessulus_, or _os transversale_, which traverses the lower
end of the trachea from front to rear. It supports a thin membrane which
ascends into the tracheal area, and, terminating there by a free concave
margin, is called the _membrana semi-lunaris_. This is most developed in
singing birds, and being vibratile, forms an important part of their
trilling vocal apparatus. The air passes on each side of the membrana
semi-lunaris and its sustaining bone to and from the bronchi and lungs.
The last ring of the trachea usually expands as it descends, with its
fore and posterior parts produced, and the lower lateral borders
concave; the extremities of the pessulus, butts against the angle thus
formed and expands to be attached, also with the fore and posterior
terminations of the first half ring of the bronchus, strengthening and
clamping together the upper part of the vocal framework. The second
bronchial half ring is flattened and curved with the convexity outward,
like the first, but is more movable. The third half ring is less curved
and further separated from the second, to the extremities of which its
own are connected by a ligament, and, for the intervening extent, by a
membrane; its inner surface supports the fibrous cord, or fold, which
forms the outer lip of the glottis of that side; it is capable of rotary
movements on its axis, and is an important agent in the modulation of
the voice. All these parts just described are bony.
=The Bronchi and the Lungs.=—The bronchi, two in number, are provided
with only incomplete cartilaginous rings. They enter the inferior face
of the lungs, toward their anterior and middle thirds and break up into
primary bronchi, which give off at right angles, secondary bronchi, and
these latter in turn give off tertiary branches.
[Illustration:
FIG. 61.—The lung.
_A._ The outer surface of one lung. Note the flattened oval shape. It
is not divided into lobes. 1, The bronchus. 2, Primary tubules
showing openings leading from the primary tubules to the secondary
tubules. 3, Openings of two of the large tubes into the
diaphragmatic and abdominal air-sacs.
_B._ Sectioned surface of lung. 1, Secondary tubules. 2, Tertiary
tubules. 3, Interlacing capillaries and air cells.
_C._ 1, Cavity of tubule. 2, Its lining membrane supporting
blood-vessels with large areolæ. 3, Perforations in the membrane at
the orifices of the lobular passage. 4, Interlobular space
containing the terminal branches of the pulmonary vessels supplying
the capillary plexus, 5, to the meshes of which air gets access by
the lobular passage.
]
The lungs occupy only about one-seventh of the thoracic space. They are
long, flattened, and oval, extending along each side of the spine from
the second dorsal vertebra to the anterior end of the kidneys, and
laterally to the juncture of the vertebral with the sternal portions of
the ribs. They present two faces, a _superior convex_ and an _inferior
concave_; two borders, an _external_ and an _internal_; and two
extremities, an _anterior_ and a _posterior_ (Fig. 61). The convex
surface is also called the dorsal, costal, or superior face. It is
moulded on the walls of the thorax and occupies a part of the
intercostal space, pushing the intercostal muscles outward. When the
surface of the lung is examined it is seen to be furrowed where the ribs
pressed during life. These furrows are as deep as the ribs are thick.
The sides of the lungs are covered with connective tissue which attaches
them to the costal walls.
The concave, or inferior, face also called the _diaphragmatic_ or
_visceral face_ is directed downward. The diaphragm separates it from
the abdominal viscera. The surface is covered by connective tissue which
closely attaches it to the diaphragm. It is perforated by the five
tubules which bring the posterior air-sac into communication with the
lungs.
The borders of the lungs extend parallel to the long axis of the body.
The internal border is rectilinear, thick, and rounded. The external
border is convex, thin, and sharp.
The anterior extremity terminates in a sharp point which occupies a
space formed by the ribs externally and the inferior spines internally.
The posterior extremity is somewhat rounded and extends as far back as
the anterior border of the kidneys.
As soon as the bronchi enter the lungs they become broadened, the
cartilaginous rings disappear, and they continue as membranous channels
whose diameters gradually decrease, as they extend backward, to the
point where they terminate in the _ostium caudale_, at which point they
are surrounded by a cartilaginous ring. The ostium caudale brings the
tubules into communication with the ventral air-sacs.
Twelve _air tubules_ have their origin from each common bronchus, or
trunk. Four are given off from the internal wall of the main bronchus by
a series of openings arranged in a row. Seven are given off from the
external wall by a second series similar to that of the first. The
twelfth extends from the inferior wall, and immediately takes a course
downward and outward and communicates with the posterior diaphragmatic
air-sac. This may be considered as the terminal branch of the trunk.
All of these secondary canals, except the last, pass toward the
periphery of the lung. They divide and subdivide at the periphery,
covering it with their ramifications. The canals extending from the
inner wall are distributed to the inferior face of the lung. Those
extending from the outer wall are distributed to the outer face of the
lung. The first constitute the _diaphragmatic_ and the second the
_costal bronchial tubes_.
The four _diaphragmatic bronchial tubes_ are numbered in the order in
which they are given off. The first is carried forward horizontally, the
second transversely inward, the third obliquely inward and backward, and
the fourth directly backward. They have, by some anatomists, been called
the anterior, the internal, and the posterior diaphragmatic bronchial
tubes. There are two posterior diaphragmatic bronchial tubes; the larger
called the great posterior, and the smaller, which passes directly
backward, the small posterior.
The _costal bronchial tubes_, seven in number, are numbered from the
front backward in the order they are given off. Parallel at their
origin, and side by side, like pipes of an organ, they soon spread out
in fan-shape like the preceding. They extend from their central origin
to the periphery. The first extends obliquely upward and inward to the
anterior extremity of the lung. All branches from this bronchus extend
from its anterior wall. The first branches are inflected to reach the
external border of the lung. The succeeding branches are directed
forward and the last forward and inward. They all meet those from the
anterior diaphragmatic bronchus, but do not anastomose with them.
The second, the third, and the fourth costal bronchi extend in a
transverse manner and ramify on the inner border of the lung.
The fifth and the sixth are directed toward the posterior extremity of
the lung. The seventh, very small, reaches this extremity, where it
disappears.
The first costal bronchus is the largest; those following it gradually
become smaller. At their points of origin they adhere closely to the
ribs. They are all imperforate, which is a distinguishing feature from
those occupying the opposite face.
The _canaliculi_ or _tertiary tubules_ given off by these secondary
bronchial tubes do not differ greatly in caliber in the various parts.
They are given off at right angles from the pulmonary wall of each
bronchus, and extend perpendicularly into the lung substance. Thus we
find three kinds of conduits, the primary, the secondary, and the
peripheral, or tertiary. The first are like the barbs of a feather on
its shaft; and the second and parenchymatous are implanted on the
pulmonary walls of the first, like the hairs of a brush on their common
base. Thus instead of the branching of the bronchi being dichrotomous,
as in mammals, it is piniform.
The canaliculi, or finer tubules, communicate with one another. The
inner microscopic appearance of the canaliculi indicate that they are
divided into areola, which gives them a cellular aspect. These tertiary
bronchi open on a dense labyrinth of blood capillaries (Fig. 61, _C_).
At this point the ciliated epithelial cells give way to simple squamous
epithelium.
Thus we find three kinds of bronchi, or their ramifications, as follows:
the primary, the secondary, and the tertiary.
=The Air-sacs= (Fig. 61, _A_).—The air-sacs are bladder-like structures
consisting of a delicate cellulo-serous membrane, an extension from the
bronchial tubes, in some places strengthened by an external envelope of
elastic fibrous tissue. Long thin blood-vessels are distributed in the
substance of these walls. They are branches from vessels of the general
circulation and not extensions from those of the lungs. No lymphatics
have been found in the air-sacs.
[Illustration:
FIG. 61, _A_.—Diagram of air-sacs and their location. 1, The proximal
end of the humerus. 2, The proximal end of the right clavicle. 3,
The cervical air cell. 4, The right coracoid bone. 5, The anterior
thoracic air cell. 6, The right side of the sternum. 7, The right
side of the liver. 8, The peritoneum. 9, The right abdominal air
cell. 10, The coccyx. 11, The proximal end of the right femur. 12,
The right supero-posterior air-sac. 13, The right infero-posterior
air-sac. 14, The right lung. 15, The axillary extension of the
air-sac. 16, The obturator foramen. 17. The pelvis.
]
These sacs do not communicate with each other and normally they are not
fully inflated. In some locations they extend into the bones and are in
communication with the extensions of the bronchial tubes. In fact, by
some anatomists they have been called “bladder-like, extra-pulmonary
expansions of the bronchial tubes, free from cartilage.” The air-sacs
make the bird’s body lighter, thus making long-continued flight
possible. They are best developed in those birds which fly most. There
are four pairs of cells and one single cell from which all other
expansions and extensions are made. These sacs are as follows: a single
anterior thoracic, and, in pairs, cervical, anterior diaphragmatic,
posterior diaphragmatic, and abdominal.
_The Anterior Thoracic Air-sac._—The anterior thoracic air-sac is
located above the clavicles and the interclavicular space, in the cavity
of the thorax. It is related superiorly with the trachea and the
esophagus; laterally with the lungs and the cervical air-sacs;
inferiorly with the sternum, the clavicle, and the interclavicular
aponeurosis; posteriorly with the heart and the anterior diaphragmatic
reservoir; and anteriorly with the integuments of the neck. It contains
the inferior larynx and the two primary bronchi, and large vascular
trunks from which are given off vessels supplying the neck and the
wings.
Three prolongations, called subpectoral, subscapular, and middle, or
humeral, arise from the lateral walls of this air-sac. These
prolongations cross the walls of the thorax and pass around the
articulation of the shoulder.
The _subpectoral prolongation_ extends from the thoracic reservoir by an
orifice situated behind the coracoid, and passes beneath the tendon of
the great pectoral muscle. When the pectoralis major contracts, this
contraction dilates the subjacent cell and draws into it a greater
quantity of air.
The _subscapular_ and the _humeral prolongations_ communicate with the
thoracic air cell by a common opening situated behind the small adductor
muscle of the humerus. The subscapular prolongation, after leaving this
point, spreads under the scapular and the subscapular muscle, which it
separates from the ribs and corresponding intercostal muscles, and
extends in a longitudinal direction.
The humeral prolongation, smaller than the subscapular, occupies the
axilla, and is in shape triangular. It has from its summit into an
infundibular fossa, an extension which enters the canal of the humerus.
The walls of this cell form the lining of the air space in the humerus.
The thoracic air-sac thus possesses numerous membranous folds which
divide its cavity. The contiguous structures which it overlies, as the
trachea, the esophagus, the muscles of the inferior larynx, as well as
the arteries and veins, make its outer walls irregular. This orifice is
dilated during inspiration, by the contraction of the two first
fasciculi of the diaphragm.