terminates anteriorly in the pars commissuralis and the pars
peduncularis. As the spinal cord approaches the head there is a gradual
swelling, or lateral thickening, which merges into the medulla
oblongata. The superior and the inferior surface of the medulla
oblongata (Fig. 75, No. _A_, 1 and _C_, 17) are flattened. There is a
shallow furrow and a slight swelling at the point where the hypoglossal
nerve emerges. The central canal of the spinal cord gradually comes
closer to the upper surface and communicates with the fourth ventricle,
at which point the posterior raphe is shortened and the sulcus
longitudinalis posterior becomes shallow. At the point where the central
canal terminates in the fourth ventricle there is a V-shaped point
called the _calamus scriptorius_ (Fig. 75, No. 2). The _fourth
ventricle_ is located on the upper wall of the medulla oblongata below
the cerebellum, and is bounded laterally by the peduncles of the
cerebellum. It is marked posteriorly by the calamus scriptorius and
anteriorly by the valve of _Vieussens_.
The _valve_ of _Vieussens_ is located at the posterior end of the
aqueduct of Sylvius. The posterior part of the fourth ventricle is
marked by grooves or furrows. Extending along the central part of the
floor of the fourth ventricle there is a sulcus, or groove, called the
sulcus centralis, which divides the superior pyramids. On the median
lateral sides there are two points of gray substance which form the alæ
cinereæ. The roots of the pneumogastric, the glosso-pharyngeal, and the
spinal accessory may be traced to the alæ cineræ and the gray matter of
the ridges of the medulla oblongata. The sixth pair of the cranial
nerves emerge from the medulla near the sulcus centralis and to the side
of the auditory nerve. Part of the trigeminus emerges from the rim of
the furrow next to the ridge. At the outer border of the medulla are
observed the thick ends of the roots of the pneumogastric, the spinal
accessory, the glosso-pharyngeal, and the auditory facialis.
On both sides of the sulcus longitudinalis inferior are found the
_inferior pyramids_, or _pyramidal columns_. These pyramids become
expanded near the origin of the third pair of nerves and merge into the
cerebral peduncles, or crura cerebri.
The pyramidal fibers may also be traced to the optic lobes. The roots of
the abducens are found at a point between the crura cerebri and the
optic lobes. The third pair lies to the side of this, and the roots of
the trigeminus are adjacent to those of the third. A bundle of fibers
from this region pass into the cerebellum and form the crura cerebelli;
others pass into the cerebellum from the side, spreading out in
fan-shaped radiation, and forming the white central substances
peculiarly arranged, called the arbor vitæ, or _tree of life_. A third
bundle fuse with the crura cerebelli anteriorly, and pass into the
peduncles of the cerebrum, or crura cerebri.
The gray ganglionic substance forms columns which extend the whole
length of the medulla oblongata and into the cerebri. The medulla
oblongata contains numerous centers which preside over various visceral
functions as deglutition, respiration, thermotactic, secretory, cardiac,
and digestion.
In the medulla as in the spinal cord there are five main groups of
cells. First, the posterior or upper horns, from which come the somatic
sensory nerves. Second, Clark’s cells, located centrally, which are the
origin of the ganglionated splanchnic nerves. Third, cell groups of the
lateral horns which are the center for the non-ganglionated splanchnic
nerves, and certain other nerves for the viscera, including the enteric
muscles. Fourth, cell groups of the anterior horns, which are the center
of all somatic voluntary muscles. Fifth, groups of single cells probably
belonging to the posterior horns which are probably centers for other
splanchnic nerve fibers.
[Illustration:
FIG. 77, _A_.
_G_, A section through the tuberculum bigeminum. 1, Pia mater. 2,
Nerve fibers. 3, Fine granular ground substance. 4, Thin layer of
small cells. 5, Fine granular ground substance. 6, A second thin
cell layer. 7, Fine granular ground substance. 8, Third thin cell
layer. 9, Fine granular ground substance. 10, Fourth widely extended
cell layer. 11, A fine granular layer. 12, A thick layer of spindle
cells. 13, A layer of medullated nerve fibers. 14, Ependemal cells.
15, The sinus.
_H_, A transverse section through the medulla oblongata, 1, 3, 4 and 6
are ganglionic centers. 2, The dorsal groove. 8, The ventral
fissure. 5, The central canal. 7, Fiber tracts.
_I_, A transverse section through the cervical portion of the spinal
cord. 1, Ventral septum. 2, Dorsal septum. 3, The central canal. 4,
The dorsal horn. 5, The ventral horn.
_J_, A section through the wall of the oil gland, or rump gland. 1,
The stroma. 2, The tubular glands some of which are branched.
_K_, A section through a spike of a comb of a cock. 1, The epithelial
layer. 2, A dense fibrous subepithelial layer. 3, A second fibrous
vascular layer. 6, A denser central core supporting the large
arteries and veins. 4, A vein. 5, An artery.
_L_, A section through a lobe of the wattle of a hen. 1, The
epithelial layer. 2, A dense vascular layer. 3, A vein. 4, An
artery. 5, A loose fibrous vascular structure.
]
The _gray matter in the cord_ is in the form of two commas placed with
their backs together with the central canal passing between them. The
central canal is located in the center of the cord. As the cord nears
the medulla oblongata the form of the gray matter changes. The second,
the third, and the fifth ganglion centers are arranged laterally, and
show a distinct side-horn group. The central canal separates the two
Clark’s columns; later the side-horn group enlarges and extends
ventrally around the anterior horn group and joins on the other side in
half-moon shaped formation.
The _central ganglionic mass of the spinal cord_, the center for the
enteric visceral system, is divided into three complex parts. First,
there are cell groups on both sides of the posterior raphe. Second, the
half-moon shaped ventral groups extending parallel peripherally
described above. Third, lying between groups one and two a complex
group, which possesses a large collection of cells. These furnish
splanchnic nerve centers, the anterior supplying motor nerves and
carrying motor impulses outward, and the superior sensory which carry
impulses to the centers. The cell group of the superior horns takes a
sidewise peripheral position (Gadow).
The _cerebellum_ (Fig. 62, _C_, 2) is located above the medulla
oblongata and posterior to the cerebrum. The cerebellum consists
principally of a median lobe called the worm. The lateral lobes are
conical and rudimentary. The under part of the worm forms the roof of
the fourth ventricle.
On the upper surface of the cerebellum are numerous transverse markings
(in the hen 13 or 14) which divide the lobe into leaves. When the
cerebellum is cut lengthwise there is observed on the sectioned surface
the peculiar arrangement of the white and the gray matter, the arbor
vitæ, mentioned above. There is a small cavity in the cerebellum which
communicates with the fourth ventricle.
The _cerebellar cortex_ shows four parts, namely: First, the central
part composed of white medullated fibers, between which are arranged
neuroglear cells. Second, a rather reddish layer of cells of different
sizes. These cells are embedded in a fine ground substance and are about
0.003 mm. in diameter. Third, a plain layer of large pear-shaped
ganglion cells, the cells of Purkinje. Fine branches extend outward from
the apex of these cells. Fourth, an outer gray layer which consists of
small multipolar nerve cells with some neuroglear cells. The color of
these three layers, according to Gadow, depends upon the color of the
plumage; that is, if the plumage is dark, this layer is dark; if light,
the layers are light. The marked coloring is said to be most distinct in
the second layer.
The anterior portion of the cerebellum consists of medullated nerves
whose fibers run crosswise. These are the extensions of the true
cerebellar fibers, which become fewer and fewer as they proceed
anteriorly. Finally, a few remaining fibers running crosswise fuse as
the commissura Sylvii, and extend into the lobi optici. A layer of nerve
cells extends under these medullated nerve fibers.
A bundle of fibers extending from the medulla oblongata. These fibers
extend into the optic lobes, form the crura cerebri, or peduncles of the
cerebri, and also form the inferior wall, or floor, of the aqueduct of
Sylvius. The crura are sometimes spoken of as the partes pedunculares.
These contain gray matter. The following division is made of the
ganglion of this region. First, a group of cells near the base of the
peduncles, which are divided from the ventral rim by the arciform
fibers. This ganglionic formation may be considered an extension from
the medulla oblongata. Second, a group of the ganglion cells of the
lobus optici. Third, a group of cells lying near the lobus opticus,
which give rise to the ascending roots of the trigeminus. Fourth, a
group of ganglionic cells from which spring the roots of the motores
occuli, and which lies near the middle line and under the sulcus
centralis. Many ganglion cells are found to the right and to the left of
the sulcus centralis, which show different arrangement of the cells at
different levels. Fifth, a group of irregular cells which extends
centrally on the superior walls of the third ventricle and to the
superior part of the lobus opticus.
The microscopic structure of the roof of the lobus opticus, or corpus
bigeminum, shows that the layers are arranged parallel to the pia mater
and are divided into the following parts:
Externally, the pia mater.
An outer layer of very fine nerve fibers, which lies just below the pia
mater.
A layer of fine granular ground substance.
A thin layer of small cells the diameter of each of which is 0.038 mm.
A layer of fine granular ground substance.
A second thin cell layer.
A zone of fine granular ground substance.
A third thin cell layer.
A fine granular ground substance.
A fourth widely extended cell layer.
A fine granular layer.
A somewhat thick layer of spindle-shaped cells.
A layer of medullated nerve fibers.
This last is the inner layer of nerve fiber. On the inner surface of
this layer we find the ependymal cells, the cells that line the cavity
of the optic lobes. The commissura Sylvii form the covering of the lobi
optici and join these two lobes. This commissure is formed from the
upper and the lower layer of nerve fibers. In the commissure these
fibers cross each other.
The _hypophysis_ (Fig. 62, No. _C_, 20) lies back of the chiasm, or
optic commissure, and below the middle of the third ventricle. The
infundibulum, a pedicle-like structure, connects the hypophysis with the
third ventricle of the cerebri. It contains a cavity and forms an
extension of the third ventricle. The hypophysis is divided into two
lobes, an anterior and a posterior.
The _third ventricle_ communicates anteriorly with the lateral
ventricles of the cerebri through the foramen of Monro, and posteriorly
with the fourth ventricle through the aqueduct of Sylvius. The posterior
walls of the third ventricle are relatively thick and form the posterior
brain commissure. The wall is thin from the commissure to the chiasm.
Within it lie two ridges, which connect the two hemispheres with the
corpus callosum, and, farther anteriorly, with the anterior cerebral
commissure. The space from the third ventricle and to the anterior
commissure is called the laminæ terminales.
The _pineal gland_, located between the cerebri and the cerebellum, lies
against the choroid plexus, which covers the highest point of the third
ventricle.
The _cerebrum_ is divided by a deep longitudinal fissure into two
hemispheres. The cerebrum is shaped somewhat like the heart on a playing
card. The lower face is somewhat flattened. The upper and lateral sides
are not provided with convolutions or sulci, but are smooth. The fissure
of Sylvius is faintly marked on the inferior face. The olfactory lobes
appear well developed and are relatively large for the size of the
brain. They lie close together on the extreme anterior portion in the
median line.
The _olfactory cerebral crura_ emerge from the cranium at the upper
angle between the posterior wall, the roof, and the septum of the orbit,
and pass through the olfactory foramina and in grooves on the upper part
of the septum; then passing forward they penetrate the frontal structure
and are finally distributed over the turbinated mucous membrane.
The _corpus striatum_ is large; it occupies nearly the entire floor of
the ventricles.
The _anterior commissure_ is found between the hemispheres. Its middle
part lies in the lamina terminalis on the division between the anterior
and the middle brain, and close to the thalami optici. Its side
extensions are rounded masses called the nervi amygdales.
The _corpus callosum_, quite rudimentary, lies on the posterior dorsal
rim of the anterior commissure.
The _corpus striatum_ forms a thin broad ridge, which passes ventrally.
This structure consists of twenty-five large pyramidal cells which lie
in the posterior lateral dorsal section, and of from ten to fifteen
pyramidal cells which are located in the rest of the section. In
addition to these two groups there are many cells about 5 millimeters in
diameter. These lie, six or more, in a nest imbedded in neuroglia. There
is a thin layer of spindle cells near the ventral line. The outer nerve
nest, or nerves amygdalis, lies in the posterior lateral ventral part.
It is covered by a thin membranous layer. Its cells are pyramidal in
shape and are from 10 to 15 millimeters in diameter. The cells terminate
in spindle form toward the periphery. Nerve fibers extend from the
anterior commissure.
The outer wall of the corpus striatum consists of the following layers:
An outer white layer consisting of fine medullated nerve fibers imbedded
in a ground substance containing numerous nerve cells.
A layer of ganglionic cells, consisting of pyramidal cells from 10 to 15
millimeters in diameter, of other round cells 20 millimeters in
diameter, and finally of cells only 5 millimeters in diameter. This
layer forms a reddish line in the dorsal portion of the corpus striatum.
These layers form part of the median and the posterior cerebral wall of
the lateral ventricle. The lateral ventricle is closed except for a
slit-like opening behind the posterior commissure. This is the foramen
of Monro, through which the lateral ventricle communicates with the
middle, or third, ventricle.
The _choroid plexus_ is found at the base of the lateral ventricle.
The wall of the lateral ventricle becomes thinner near the region of the
transverse commissure on the surface toward the middle brain. At this
point the pia mater and the ependyma, or the lining cells of the
ventricular membrane, form the major part of the wall.
The domestic fowl does not have the hippocampus.
The wall of the middle ventricle consists of an outer white layer, which
is arranged similarly to that of the corpus striatum. The ventral rim of
the wall is formed by a spiral band which consists of fine medullated
fibers, in which are imbedded a few cells.
The _processus cerebri mammillares_ are also called the tubercula
olfactoria. From these structures extend forward the olfactory nerves,
the posterior roots of which may be traced to the walls of the lateral
ventricles.
The structure of the processus cerebri mammillaris is made up of five
layers as follows:
An outer layer, which consists of colorless olfactory fibers extending
in all directions.
A granular ground substance layer, in which are imbedded a few cells.
A thicker granular layer, on the inner edge of which are twenty large
pyramidal cells with processes pointing outward.
A layer of closely packed cells, in which are fine medullated nerve
fibers. These cells are about 5 millimeters in diameter.
An innermost layer of ependymal cells.
Near the hemisphere the peripheral nerve fibers and nerve cells
disappear, leaving only the ground substance of the processus to aid in
the formation of the hemisphere.
There is a long bundle of fibers on the lower surface of the hemisphere,
which blends with the substance of the olfactory fibers.
The peduncles of the cerebrum (Fig. 62, No. 21) are the slightly
diverging columns of nerve tissue, which form the anterior continuation
of the medulla oblongata and disappear under the optic tracts and the
chiasm.
THE SYMPATHETIC NERVOUS SYSTEM
The sympathetic nervous system commences anteriorly at the large
superior cervical nerve ganglion. This ganglion, anteriorly, brings into
communication the glosso-pharyngeal, the pneumogastric, or vagus, and
the sympathetic system.
The following nerve trunks emerge from the superior cervical nerve
ganglion: a large trunk fusing with the vagus and directed downward,
accompanying and surrounding the carotid artery; a second large trunk
fusing with the recurrent pharyngeus and the glosso-pharyngeus; a third
trunk which merges with the hypoglossal; finally several trunks which
are distributed to the head.
The _temporo-lacrimalis_, one of the large sympathetic nerves of the
head, as it extends from the cervical nerve ganglion, receives branches
from the ganglion radicis vagi. This nerve, passing between these two
ganglions, extends horizontally forward and outward through a foramen,
crossing the glosso-pharyngeus. Near the Fallopian canals it crosses the
facial nerves, lies supero-laterally, and receives a short branch from
the facial nerve whose fibers are traceable to the geniculate ganglion.
It also receives a branch from the recurrent maxillaris.
This nerve accompanies the external ophthalmic artery forming around it
a network of fibers, called the external ophthalmic plexus. It lies
outside and downward from the optic nerve, sends fine branches to the
external ophthalmic artery and to the masseter artery, and extends along
with a small branch of the superior maxillary nerve to the skin of the
outside rim of the eye cavity.
The _ophthalmic plexus_, a second trunk extending to the head, enters in
its course, anteriorly, the lacrimal plexus. Its fibers also supply the
lacrimal gland, and finally anastomose with the second recurrent branch
of the trigeminus.
The sympathetic _caroticus cephalicus_ nerve, a third trunk extending to
the head, after emerging from the large superior cervical nerve
ganglion, receives some small branches from the glosso-pharyngeal
ganglion, and then enters, in a horizontal manner, a foramen, the
canalis caroticus externus, located in the lower part of the
basi-occipital and the sphenoid bone. The anterior opening of this
foramen, or canal, is close to the posterior part of the pterygoid bone.
Inside this canal the sympathetic caroticus cephalicus receives a small
branch which extends in a straight line from the basal part of the
facial nerve. After receiving this branch, the nerve trunk, passing to
the ear drum, lies close to the petrosum and the sphenoid. It is covered
by the masseter muscle. The carotic ganglion is located at the point of
the fusion of the caroticus cephalicus with the main trunk. At this
point the caroticus cephalicus divides into two branches. The first is
the superior recurrent nerve, which lies close to the upper surface of
the alæ of the sphenoid, passes between the obliquus externus and the
orbital wall, around the eyeball, and finally into the orbital septum
and the internal muscles of the eye. It communicates with the ophthalmic
nerve and sends twigs to the lacrimal gland, the gland of Harder, the
upper eyelid, and the nose. The orbito-nasale ganglion is located where
these nerves communicate near the nasal region. The second branch of the
sympathetic caroticus cephalicus is the inferior recurrent nerve. This
branch passes forward and dorsally from the pterygoid bone to a point
where the upper rim of the rising wings of the jaw bone meets the
sphenoid rostrum. In this course, some branches are given off to the
pharynx, one branch near the lacrimal gland, and a branch which
communicates with the superior maxillary nerve just before the nerve
enters the jaw bone. The spheno-palatine ganglion is at the point of
fusion.
The terminal branches of the spheno-palatine are distributed to the hard
palate, the nose, and the lacrimal gland.
From the large cervical nerve ganglion and near the roots of the
caroticus cephalicus are given off a few small nerve fibers which pass
alone to the pharynx or accompany the jugular nerve branches, and fuse
with the main trunk of the caroticus cephalicus nerve.
From the large superior cervical nerve ganglion the sympathetic trunk
extends downward toward the thorax. It is covered deeply with muscles.
This portion is known as the cervical sympathetic nerve trunk. A trunk
lies on each side of the cervical vertebræ. The large thoracic nerve
ganglion, the inferior cervical, is located along this trunk at the
entrance of the thorax. From this ganglion there is given off the
recurrent cardiacus which supplies the heart. The end branches of the
sympathetic nerve trunk blend with the pneumogastric nerve. The inferior
nerve ganglion is the ganglion cardiacum. Near this ganglion is found a
nerve plexus in which there are imbedded peripherlistic ganglia. This
thoracic plexus, accompanying the collica artery, passes to the
abdominal region, supplying the intestines and taking part in the
formation of the abdominal plexus. The abdominal plexus is located near
the anterior portion of the kidneys. Its fibers are directed mainly
downward to the visceral organs. The large intestine, the rectum and the
copulatory organs receive branches. These latter branches take part in
the formation of the pedunda nerve plexus. Some branches of this plexus,
follow the branches of the posterior mesenteric artery.
The _thoracic trunk_ (Fig. 64, No. 42) of the sympathetic is double. The
anterior portion gives off an anterior splanchnic nerve, or plexus (Fig.
64, No. 43) which accompanies the celiac axial artery to the gizzard and
liver, communicating with the pneumogastric. The posterior splanchnic
nerve is intimately combined with the adrenal body, and the testes, or
the ovary (Fig. 64, No. 45). Intestinal branches accompany those of the
mesenteric arteries (Fig. 64, No. 46). Other branches supply the
kidneys, and communicate with long branches of the spinal nerves
destined for the cloaca and adjacent parts, and thus form a plexus
similar to that found in mammals.
FUNCTIONS OF THE NERVOUS SYSTEM
According to function, the nerves composing the trunks are divided into
afferent and efferent.
The _afferent nerves_ are those that convey the impulses from the
periphery of the body to the nerve center, which are located in the
brain or in the spinal cord. The impulses conveyed are those of special
senses, as sight, hearing, taste, touch, and smell. Impulses producing
sensation pleasurable or painful come from the skin, the muscle, or the
viscera.
The _efferent nerves_ are those which convey impulses from the nerve
centers to the periphery. These impulses may be motor as those going to
the muscle cells of the skeletal muscles, the viscera, or the
blood-vessels. These motor impulses make movements in these organs
possible. In the blood-vessel they result in the control of the caliber
of the vessel. These impulses may be of an inhibitory character, as in
slowing the heart. They may be secretory impulses stimulating the gland
to activity or regulating metabolism.
The _ganglia_ are nerve centers which receive and generate impulses; the
nerve trunks are filaments which convey impulses. The gray matter of the
cord described above is the ganglionic portion, and the outer white
matter is made up of nerve fibers which convey impulses from one part of
the cord to another, or to and from the brain.