above the thoracic air-sac at the inferior part of the neck and in front
of the lungs. They are cone-shaped with the base directed forward and
the apex backward. They are related superiorly with the cervical
muscles, and inferiorly with the thoracic air-sac from which they are
separated by the trachea, the esophagus, the pneumogastric nerve, and
the jugular veins. The walls touch each other internally, and form a
median septum which includes in its substance the two common carotid
arteries. Externally they are related to the origin of the cervical
nerves, to each of which they contribute a small sheath. They surround
the vertebral artery, and are connected with the subcutaneous muscles
and the skin. The summits communicate with the anterior diaphragmatic
bronchus. Prolongations extend from their bases which conduct the air
into all the vertebræ of the neck and the back, into all the vertebral
ribs, and into the spinal canal. Parallel with and adjacent to the
vertebral arteries, and lodged in the canals excavated in the transverse
processes of the cervical vertebræ, are two cervical prolongations, one
on each side, which extend to the cranium from the base of the cervical
reservoirs. From their sides, at the last six cervical vertebræ, are six
extensions in the form of diverticuli, which, lying against each other,
pass from each side into the muscles of the neck. They are surrounded by
a thin fibrous envelope, a continuation of the mucous lining of the sac,
and apparently form a canal in the inferior part of this region. These
prolongations are better developed in palmipedes than in chickens. On
the internal side of these prolongations, one or more foramina penetrate
the vertebral segment, which allow the extensions of the prolongations
into the spinal canal. Chauveau states that “as the medullary tissue is
replaced by air in the bones of birds, so is the subarachnoid fluid
replaced by air around the spinal cord.”
The prolongations extending from the cervical air-sacs, having entered
the thorax, terminate by passing into the first dorsal vertebra. After
permeating every part of this vertebra, it escapes by a lateral opening
and forms a small sac located between the first two ribs, near the
origin of the first dorsal nerve. From this sac an extension is given
off, which enters the second vertebral segment at the antero-lateral
part; from this point it passes back, forming a new air-sac between the
second and third ribs. It now passes in the same manner into the third
vertebra and extends through the third intercostal sac, and so on till
the last dorsal vertebra has been served. At the same time that these
sacs receive the air from the vertebræ preceding them, and transmit it
to those which follow, they communicate it to all the vertebral ribs.
The aerial currents which leave the cervical air-sacs do not communicate
with those of the cranium. Experiments show that the cranial bones have
apparently no communication with the respiratory apparatus.
_The Anterior Diaphragmatic Air-sac._—The two anterior diaphragmatic or
supero-posterior air-sacs are related with the lungs anteriorly, and
with the abdominal viscera posteriorly. Anteriorly also is the thoracic
air-sac, posteriorly are the posterior diaphragmatic air-sacs, and
laterally the ribs and the intercostal muscles and internally is the
esophagus. The lungs communicate with these air-sacs through circular
openings from the great posterior diaphragmatic bronchus and frequently
by a second opening from this same tube. These are the only sacs which
receive air from the lungs through two openings.
_The Posterior Diaphragmatic Air-sac._—The two posterior diaphragmatic,
or infero-posterior air-sacs are oval in shape and located between the
thoracic and the abdominal cavity. They are related anteriorly with the
anterior diaphragmatic air-sacs. These two sacs form a vertical
transverse partition. The posterior diaphragmatic air-sacs are related
posteriorly with the abdominal air-sacs from which they are separated by
the diaphragm. They are related below with the lateral parts of the
sternum and the sternal ribs, and externally with the ribs and the
intercostal muscles. These air-sacs communicate with the lungs through
openings located in the middle part of the external border of the lung,
into the extremity of voluminous bronchial tubes which follow the
direction of the largest air tubes.
_The Abdominal Air-sacs._—The two abdominal air-sacs located on each
side of the abdominal cavity, when inflated with air, form enormous
bladder-like structures. They are related laterally with the abdominal
wall and internally with the abdominal viscera. The anterior extremities
are in communication with the mesobronchi and are somewhat inflected to
pass under the fibrous arches extending from the spine to the pelvis.
Anteriorly these sacs adjoin the diaphragm, the testes in the male, and
ovary in the female, and to the parietes of the abdomen and those of the
pelvis. Below and in front, they rest on a fibrous septum, which in all
birds divides the abdominal cavity into two smaller cavities: one
anterior, representing the abdomen and containing the liver; the other
posterior, representing the pelvis and containing the gizzard and the
intestines. The anterior portion overlies the posterior part of the
lobes of the liver, the proventriculus, the spleen, and the gizzard. The
kidneys are located above these air-sacs. Dorsal to the sacs is also a
part of the intestines and in the female the oviduct. The abdominal
air-sacs are attached by a ligament-like structure in their medial,
their anterior, and their lateral margin. The posterior, the dorsal, and
the ventral margin are free. Mesially this attachment is to the
mesentery, connecting the left cæcum to the dorsal margin of the
gizzard, and also to the mesentery of the proventriculus. The anterior
attachment is to the body wall and extends in front of the end of the
ovary and the adrenal glands. At the antero-lateral part of the body
cavity the attachment extends in a widening band along the lateral side
of the ovary and of the oviduct, as far back as the caudal margin of the
sac. The lateral attachment is related to the kidney, the dorsal
ligament of the oviduct, and the abdominal wall.
Each of these abdominal sacs has three extensions: one suprarenal and
two femoral.
The _suprarenal extension_ leaves the principal sac at the
postero-external part of the kidney, extends upward, and forward, and
expands over the surface of the kidney. At the internal border of the
kidney, this prolongation extends between the transverse processes of
the sacral vertebræ, reaches a height of the first dorsal vertebra,
forms a triangular canal located above the sacrum in the sacral channel,
and is separated from its fellow by a series of corresponding spinous
processes.
The two _femoral extensions_, an anterior, small, and a posterior,
large, extend from the abdominal air-sac at the cotyloid cavity, leave
the pelvis through the bony passage occupied by the crural vessels,
extend around the coxo-femoral articulation, and terminate in a blind
extremity. In some birds, particularly in birds of prey and ostriches,
there are prolongations extending into the femur, entering through a
foramen at the anterior part of the great trochanter.
_Summary of Bones Supplied by Each Air-sac._—The thoracic air-sac
communicates on each side of the thorax with twelve bones, including the
four sternal ribs. It supplies air to the clavicles, which are
perforated at both their extremities, and to the coracoids, which are
perforated just below their scapular extremity. The sternum is supplied
through two series of openings, the middle ones that conduct air into
the sternal ridge and the lateral ones, eight in number and very small,
correspond to the intercostal spaces. The sternal ribs are penetrated by
small foramina at their inferior extremities. From the subscapular
extension the scapulæ receive air through one or two foramina at their
anterior extremity. The humeral prolongation supplies the humerus
through a foramen located at the upper edge of the humeral fossa, at the
infero-internal part of the articular head.
The cervical air-sac furnishes air to all the cervical vertebræ, to all
the dorsal vertebræ, and to all the vertebral ribs. The anterior parts
of the vertebræ of the neck are supplied with air through the passage
accommodating the vertebral artery. The posterior parts of the vertebræ
are supplied by extensions from the interspinal canal. The first
extensions obtain entrance to the anterior segments by one or more
openings of the inner wall of the intertransverse canals; the median
extensions penetrate the posterior segments by two openings, a right and
a left, situated on the inner wall of those segments. The first dorsal
vertebra is supplied with air in the same manner, by the middle and the
lateral canals of the neck. This air, after passing through the first
vertebra, leaves by a lateral exit to enter a small air-sac. From this
it passes into the superior part of the second vertebra, escapes from
this through its lower portion, to be received into a lateral sac, and
so on to the last dorsal vertebra. These sacs also supply the vertebral
ribs with air, which enters them by very small openings located on their
spinal extremities.
The diaphragmatic air-sacs do not have communications with the bones.
The abdominal air-sacs communicate with the sacrum, the coccygeal
vertebræ, the iliac bones, and the femurs. The air passing through the
sacrum, the coccyx, and the ilium comes directly from the suprarenal
extensions; the air which fills the femoral cavity comes from the
femoral extensions.
In some birds these air spaces are more greatly developed than in
others. The bones that are always aerated in all birds are the cervical
and the dorsal vertebræ, the sternum, and the humeri. Those aerated in
some kinds only are the furculum, the scapulæ, the vertebral and the
sternal ribs, the sacrum, the coccyx, and the femurs. Those that are
never aerated are the bones of the forearm, the hand, the leg, and the
foot.
The service of air to the bones in most parts of the body by the
air-sacs, as just shown, is in special cases otherwise rendered. The
Eustachian tubes furnish air to the bones of the cranium and to the
upper jaw; while the lower jaw receives air from the pneumatic foramen
situated upon each ramus behind the tympanic articulation, and from an
air cell which surrounds the joint.
The cavities of the embryonic bones, which afterward become pneumatic,
are filled with marrow. Selenka states that the invasion of the bones by
the air is a late development, and that in the humerus this invasion
occurs after the twenty-second day in the life of the chick.
Hunter and Compar, who have made extensive researches, consider the
function of the air-sacs as threefold.
First, the air-sacs are subsidiary respiratory organs, which aid in
ridding the blood of waste products and in taking in oxygen.
Second, they aid mechanically the actions of respiration in birds.
During the act of inspiration the sternum is depressed, the angle
between the vertebral and the sternal ribs is made less acute, and the
thoracic cavity proportionately enlarged; the air then rushes into the
lungs and into the thoracic receptacles, while those of the abdomen
become flaccid. When the sternum is raised, or approximated toward the
spine, part of the air is expelled from the lungs and the thoracic
air-sac through the trachea, and part is driven into the abdominal
receptacles, which are thus alternately enlarged and diminished with the
expansion and the contraction of the thorax. Hence the lungs,
notwithstanding their fixed condition, are subject to due compression
through the medium of the contiguous air receptacles, and are affected
equally and regularly by every motion of the sternum and of the ribs.
Third, they reduce decidedly the specific gravity of the whole body.
This must necessarily follow from the large spaces filled with air as
well as from the absence in the bones of marrow and other fluids. The
air-sacs by their position also render equilibrium more stable.
ANGIOLOGY
=The Circulatory Apparatus.=—The circulatory apparatus consists of two
tubular systems: the blood vascular system and the lymphatic system.
The blood vascular system consists of the heart, the arteries, the
veins, and the capillaries.
The heart is the central, propelling organ. The arteries form a series
of efferent tubules, which, by branching, constantly increase in number
and decrease in caliber, and which serve to carry the blood from the
heart to the tissues. The capillaries are extensions from these latter
tubules into which the arteries empty, and through the walls of which
the interchange of elements between the blood and the other tissues
takes place. The veins form a system of converging tubules which receive
the blood from the capillaries, decrease in number and increase in size
as they approach the heart, and return the blood to that organ.
The lymphatic system consists of capillaries and veins alone. As in the
blood system, the lymph capillaries collect the effete material and pour
it into the lymph veins, and these in turn, carry it to the large blood
veins adjacent to the heart.
Both these systems have one and the same continuous lining, which
consists of a single layer of endothelial cells. In the heart this
lining is called the endocardium, and in the vessels, the endothelium.
It forms a perfectly smooth surface.
THE HEART (Fig. 21, No. 7)
The heart of the domestic fowl is located in the median line of the
thoracic cavity. It is more anterior and mesial than in mammals. Its
axis is parallel with the axis of the trunk. The lungs being confined to
the dorsal part of the trunk, the lower part of the heart is not
surrounded by them, but extends backward, the apex resting in the
anterior part of the anterior median fissure of the liver.
The heart has the form of an acute cone (Fig. 50, No. 1), the apex of
which is bluntly rounded.
The heart is surrounded by a sero-fibrous sac, the _pericardium_. This
sac adheres to the cervical air reservoirs anteriorly and to the
diaphragmatic septum posteriorly. It is composed of two membranous
layers: the _parietal_, external, dense, and fibrous; and the
_visceral_, internal, and serous. The pericardial sac has no direct
attachment to the heart, except at the upper extremity where it
surrounds the large vessels emerging from it. The serous layer is
reflected over the outer portion of the heart, where it is called the
epicardium. The function of the pericardium is to prevent friction
during the beating of the heart. It contains a small amount of serous
fluid for perfect lubrication. This fluid is called the liquor
pericardii.
Internally the heart has four cavities: two auricles and two ventricles.
The _right ventricle_ is more crescent-shaped than in solipedes, and in
a manner envelops the left ventricle in front and to the right, though
it does not reach the point of the heart. The _right auricle_ is larger
than the left. The _auriculo-ventricular_ valve is not tricuspid as in
mammals. This valve instead of being formed as usual by a membranous
curtain, with margins retained by cords fixed to the walls of the
ventricles, is composed of a wide muscular leaf, which appears to be a
portion of the inner wall of the ventricle detached from the
_interventricular septum_. This septum is convex; and the
auriculo-ventricular orifice is an oblique slit situated between it and
the muscular valve in question; so that, when the heart wall contracts
at the systole, the valve is applied against this septum and closes the
passage. The bicuspid, or auriculo-ventricular valve of the left side
usually has two segments, though occasionally there may be three. The
_fossa ovalis_ is a depression behind the posterior semi-lunar valve in
the septum of the heart. The membranous septum closing the foramen ovale
is complete and strong but thin and transparent. The right auricle
receives the blood from the two venæ cavæ coming from the anterior
extremity, and from the posterior vena cava. These empty into a sinus.
The left auricle has two vessels, the pulmonary veins which bring blood
to it from the lungs.
=Structure of the Heart.=—The heart is lined by a serous membrane, the
_endocardium_, which is a continuation of the endothelium of the
blood-vessels. There are a few muscular pillars in the inner wall,
called the _columnæ carnæ_. To give the heart its pumping power, it is
made up of contractile tissue, a specialized kind of muscle called
_heart muscle_. It is involuntary-striated and occupies an intermediate
position, both morphologically and embryologically, between smooth
involuntary muscle and striated voluntary muscle (Fig. 74, No. 4). It,
like striated voluntary muscle, is both transversely and longitudinally
striated. Heart muscle cells are short, thick cylinders, which are
joined end to end to form long fibers. By means of lateral branches the
cells of one fiber anastomoses with cells of adjacent fibers. Each cell
of heart muscle contains one centrally located nucleus. There is no
distinct sarcolemma, but the sarcoplasm is more dense near the surface
of the cell, which gives it the appearance of an enveloping cell wall.
There is a zone free from fibrillæ around the nucleus. The longitudinal
fibrillæ, which make up the cell, are held together by a cement-like
substance.
The main mass of the heart wall, called _myocardium_, consists of the
specialized muscular tissue just described. The myocardium differs in
thickness in different parts of the heart wall. It is thickest in the
left ventricle and thinnest in the auricles. The left ventricle forces
the blood through the systemic circulation and hence must be thicker to
give it more power than is needed for the right ventricle, which forces
the blood only through the lungs. The auricles are thinnest of all; for
they receive the blood and pass it only to the chambers below. The
_auricular appendages_ at the base of the heart in fowls are not so well
marked as in mammals. The auricular muscles consist of an outer coat
common to both auricles, the fibers of which are transverse and of an
inner coat, independent for each auricle, the fibers of which are
longitudinal. Between the two coats, occur bundles of muscle the fibers
of which run in various directions. The disposition of the muscle tissue
of the ventricles is much more complicated. It is composed of several
layers of fibers intricately interwoven.
The _endocardium_, covering the inner surface of the myocardium, forms a
serous lining of all the chambers of the heart. At the arterial and
venous openings it is continuous with and similar in structure to the
intima of the vessels. The endocardium consists of two layers, an
external layer closely attached to the myocardium and consisting of
mixed fibers, including those of elastic tissue and smooth muscle cells;
and an inner, single layer of endothelial cells, spoken of above.
The heart is supplied with nutrient blood by the two _coronary
arteries_, which are given off from the common aorta just above the
semi-lunar valves.
A right, or anterior, and a left, or posterior, coronary from their
point of origin, turn in a ventral direction between the root of the
aorta and the pulmonary artery, the right going to the right coronary
groove and the left to the left coronary groove in the crown furrow.
From here they send branches into the heart. The anterior, right,
coronary, or coronaria dextra, the larger, is given off from the
inferior wall of the aorta. It divides into a ramus superficialis and a
ramus profundus. The ramus superficialis enters the crown furrow and
divides into two or three branches on the right heart wall. These
branches extend to the apex of the heart. Twigs from this artery along
its course extend into the muscular wall reaching the posterior of the
coronary groove where they anastomose with those of the left coronary,
the ramus profundus, and with other branches from the same artery. The
ramus profundus, larger than the preceding, gives off fine branches into
the walls of the aorta and of the pulmonary artery, then enters from
behind into the right wall of the septum ventriculorum, extends into the
apex of the heart, and supplies the septum, or right inner chamber wall
with the last branch, this breaking through the posterior wall of the
auricular appendage.
The posterior, _left coronary_, or _coronaria sinistra_, originates from
the dorsal wall of the aorta, proceeds as one branch on the upper
surface of the left auricular appendix, and then extends between the
left appendix and the pulmonary artery to the ventral surface of the
heart. On the left side it supplies the wall of the pulmonary artery and
gives off a ramus profundus. It sometimes divides into two parts and
supplies the ventral wall of the right chamber and then extends to the
left wall of the septum medium. The rest of the coronaria sinistra
enters into the left crown furrow as the ramus superficialis, which
provides the left and dorsal upper surface of the left chamber to the
apex. From this furrow it extends into the left chamber and the left
appendage, and finally fuses with the ramus superficialis of the
coronaria dextra.
THE BLOOD-VESSELS
The blood-vessels consist of arteries, veins and capillaries.
=The Structure of the Capillaries and Arteries.=—The capillaries are
minute vessels which connect the _arterioles_, or terminal arteries,
with the _venules_, or terminal veins. They are only from 6 to 14
microns in diameter. Their walls consist of a single layer of
endothelial cells, which are somewhat elongated in the long axis of the
vessels. Their edges are serrated, and are united by a small amount of
intercellular cement-like substance. Capillaries branch without
diminution in caliber, and these branches anastomose to form capillary
networks, the meshes of which differ in size and shape in different
tissues and organs. The largest meshed networks occur in the serous
membranes and in the muscles; and the smallest occur in the glands, such
as the liver.
The _walls of the arteries_ are thick and stand open when empty, owing
to the elastic tissue contained in their walls, while the walls of the
veins collapse when empty, owing to their containing a smaller amount of
elastic tissue. The arterial wall is provided with three coats: _tunica
intima_, or inner coat; _tunica media_, or median coat; and _tunica
adventitia_, or outer coat.
The _tunica intima_ consists of a single layer of endothelial cells,
continuous with and similar to that forming the walls of the
capillaries. In passing from the capillaries to the arterioles, there is
first a thin coat, or sheath-like layer, of connective tissue around the
outside of the endothelial tubes. Further along, isolated smooth muscle
cells arranged in a circular manner occur between the endothelial layer
and the layer of connective tissue, this structure forming vessels
called precapillary arteries. Further along still, the muscle cells form
a complete layer; in this section the vessels are called arterioles and
are made up of three coats: the inner endothelial, the middle muscular,
and the outer fibrous.
In arteries of medium size the intima consists of the endothelial layer,
a layer of delicate white and elastic fibers, connective-tissue cells,
and the membrana elastica interna, or an outer layer, the elastic layer,
of the intima.
The _media_ consists of a thick coat of circularly arranged smooth
muscle cells, its thickness depending largely upon the size of the
vessels. There is also a small amount of fibrillary connective tissue,
which supports the muscle cells. Elastic tissue is present in the media,
the amount depending on the size of the vessel, the larger the vessel
the more elastic tissue there is present. In the large arteries coarse
elastic fibers intermingle with the finer ones. When much elastic tissue
is present the muscle cells are separated into more or less well-defined
groups.
The _adventitia_ is composed of loose connective tissue with some
elastic fibers. A few smooth muscle cells are present, which, as are
also the elastic fibers, are arranged longitudinally. The adventitia,
blending with the connective tissue surrounding the arteries, serves to
anchor the vessels to the surrounding structure.