of nerve centers throughout the system. They are termed multipolar
because of the greater or lesser number of dendrites given off in
addition to the single axone.
The body of the nerve cell consists of a mass of protoplasm surrounding
a nucleus. The cytoplasm of the nerve cell consists of two distinct
substances: first, neurofibrils; second, perifibrillar substance. In
most nerve cells there is a third substance called chromophilic bodies.
The _neurofibrils_, extremely delicate, are continuous throughout the
cell body and all of its processes. Within the body of the cell they
cross and interlace and probably anastomose.
The _perifibrillar substance_ is a fluid or a semifluid substance which
both in the cell body and in the processes surrounds and separates the
neurofibrils.
The _chromophilic bodies_ are granules or groups of granules which occur
in the cytoplasm of all the larger and of many of the smaller nerve
cells.
THE SPINAL NERVES
The first pair of spinal nerves come out between the atlas and the
occipital bone. Each nerve divides into three branches. The first is the
_anterior branch_ which innervates the dorsalis, the biventer cervicis,
the cervicales, and the caput posticus muscles. The second is the
_recurrent ventralis_ which passes mesially and downward from the
recurrent communicans. It innervates the rectus capitis anticus muscle
and joins a branch of the hypoglossal nerve. There is given off a white
ramus communicans to the sympathetic nerve.
The second spinal nerve emerges from the opening between the first and
the second cervical vertebræ and is similarly disposed as the first
cervical. It gives off a branch called the recurrent ventralis which
fuses with branches of the first cervical and the hypoglossal nerve.
This nerve innervates the complexus muscle.
The succeeding cervical nerves emerge in a similar manner down the neck
and are distributed to the muscles and other structures of the cervical
region. The second, the third, and the fourth cervical nerve gives off
branches which form anastomosing loops with the facial nerve.
According to Gadow the last cervical nerve passes between the last
cervical and the first dorsal segment. There are thus fifteen pair of
cervical nerves.
The first pair of _dorsal nerves_ pass out between the first two dorsal
vertebræ. The first few dorsal nerve branches innervate only the trunk
muscles. Other branches supply the skin and the other adjacent
integument. The ventral branches of the cervical nerves often
communicate with the ventral branches of the first dorsal nerves. These
branches are larger than the superior branches and aid in the formation
of the brachial plexus. Smaller inferior branches are distributed to the
scalenus and other muscles, and extend as far posterior as the
intestines.
The dorsal branches of the spinal nerves in the lumbo-sacral region are
very small, on account of the lack of extensive development of the
muscles of this region. The elements entering into these nerve trunks
are largely vasomotor nerves. In addition to the upper twigs supplying
the skin and the other integument of the region, other branches descend
into the abdominal cavity.
The caudal spinal nerves are not well developed and disappear in the
region of the caudal vertebræ. The dorsal branches innervate the levator
muscles, and the ventral branches the depressor muscles.
There is thus given off, throughout the vertebral column, a pair of
nerves between each two vertebral segments, making as many pairs of
nerves for the region as there are vertebral segments of that region.
=The Brachial Plexus.=—The brachial plexus (Fig. 73, No. 19) arises
principally from the roots of the last three cervical and the first
dorsal spinal nerves. These branches anastomose beneath the deep face of
the scapulo-humeral articulation.
The brachial plexus divides into two distinct parts. The first (Fig. 73,
No. 20) the dorsal called the superior thoracic is distributed to the
serratus and the rhomboideus muscle. The second is the inferior thoracic
(Fig. 73, No. 21), which consists of the main plexus, and which gives
off muscular branches especially to the sterno-coracoideus and to the
main portion passing on as the nerve trunk to the structures of the
wing.
From the superior thoracic plexus there are several secondary plexuses
formed.
The _dorsalis_ or _serratus plexus_. This plexus is located adjacent to
the anterior part of the main plexus and is formed of from two to four
spinal nerve branches. The rhomboideus is supplied by branches which may
be traced to the first root of this plexus. This nerve is called the
rhomboideus superficialis. Another nerve is given off from the middle
part of the dorsal side of the plexus and is called the rhomboideus
profundus. A third, the superficialis serratus, is the largest of the
branches. It breaks up into terminal branches, one going to each
serration, or digitation, of the serratus muscle. Branches from the
anterior portion of this plexus are distributed to the patagii muscles.
The superior brachial plexus gives off the following nerves:
The _subcoraco-scapularis_, which is purely a motor nerve, springs from
the anterior roots of the plexus. It gives off a branch to the following
three muscles: subcoracoideus, subcoracoido-scapularis, and
scapulo-humeralis.
The _scapulo-humeralis_ is distributed to the scapular humeral region.
The _latissimus dorsi_ is located on the peripheral border of the
scapulo-humeralis or supraspinatus. It originates from the second, and
the fourth nerve roots of the plexus. It is located on the dorsal side
of the plexus. It divides into two main branches, one going to the
latissimus dorsi and the other to the scapulo-humeralis muscle. From the
side are given off branches which supply the patagii muscle and enter
into the formation of the dorso-cutaneous plexus.
The _axillaris_ springs from the second and the third root of the
brachial plexus, extends in a lateral direction, passing the ventral and
distal rim of the insertional part of the posterior scapulo-humeralis or
teres et infraspinatus and enters near the capsular ligament of the
shoulder-joint. It gives off to this joint a small branch, called the
recurrent articularis, which passes outward between the triceps
brachialis and the humerus. It lies on the inner side of the major
deltoid, and patagii muscles, and also the skin of the lateral shoulder
and the upper arm region. The recurrent axillaris communicates with the
main branch of the radial nerve.
The _cutaneus brachii superior_ is a small nerve which springs from the
last brachial plexus root and passes between the skin and the triceps
brachii muscle on the dorsal surface of the upper arm, or brachial,
region. It extends down over the elbow region, where it gives off
numerous branches to the skin of these regions, to the meta-patagium and
to the extensor muscles of the upper arm.
The _brachialis longus superior_ is a large nerve trunk which springs
from most of the other plexus roots. It extends around to the dorsal
side of the upper arm and supplies the skin, the feathers, and the
muscles of the forearm and the hand. Branches from this nerve trunk
supply the triceps brachii and other muscles of the region. The main
branch passes between the radius and the ulna, and, passing the
elbow-joint, gives off branches to that joint. Passing on, it divides
into two branches. One of these extends superficially over the upper
part of the condylus ulnaris and supplies the extensor digitorum
communis and the extensor metacarpi ulnaris, or flexor metacarpi
radialis muscles, and, continuing superficially to the ulnar side, is
distributed to the skin of the region. The second, a deep branch,
extends on the ulnar side of the radius over the extensor indicis
longus, and innervates the latter muscle, the extensor pollicis longus,
the extensor pollicis brevis, the adductor pollicis, the interosseous
palmaris, and the flexor digitorum.
The inferior brachial plexus gives off the following branches:
The _supra-coracoideus_, a large nerve which springs from the first main
root of the plexus, and passes outward through the foramen coracoideum
of the sternal ligament.
The _sterno-coracoideus_, a small nerve which extends downward from the
plexus.
The somewhat large _posterior coraco-brachialis_, nerve which springs
from one or two middle roots of the plexus and accompanies the pectoral
nerve.
The _anterior thoracic_, a large nerve which springs from two or three
of the posterior roots of the plexus and extends to the shoulder cavity
where it branches. The anterior branch is distributed to the patagium
and the front part of the pectoral muscles. The posterior branch
supplies muscles along the side of the thorax and extends into the
abdominal muscles.
The _anterior coraco-brachialis_, a small nerve which springs mostly
from the inferior longus brachialis, passes to the distal end of the
tuberculum humeralis radii, then passes backward between the front part
of the humerus and the biceps, where it supplies the anterior
coraco-brachialis.
The _cutaneous brachialis_ et _inferior brachialis_, a small nerve which
springs from the posterior roots of the plexus. It is distributed to the
skin of the region and branches are given off to the patagii and the
ventral wing surface. A few branches extend as far as the upper arm.
The _brachialis longus inferior_ (Fig. 68, No. 2), a continuation of the
main trunk of the inferior brachial nerve, which comes out of all the
plexus roots except the first. It gives off some branches to the
pectoral region and then enters the shoulder cavity. It gives off the
anterior coraco-brachialis, passes down the humerus in an S-shape, and
divides into two branches, the ulnar and the median.
The _ulnar nerve_ (Fig. 67, No. 3) divides into numerous branches which
are distributed to the ulnar side of the forearm and the hand. The ulnar
nerve passes below the skin of the ulnar outside rim of the forearm and
gives branches to the carpi ulnaris, finally supplying the flexor
digitorum; it then passes with the tendon of this muscle downward to the
interosseous dorsalis, the abductor indicis, or flexor minimi brevis,
the flexor pollicis, and the abductor pollicis, or extensor proprius
pollicis.
The _median nerve_ (Fig. 67, No. 2) extends down the arm and gives
branches to the biceps muscle, and to the patagii region. Continuing,
the median nerve supplies the pronator muscle and the brachialis
inferior, or brachialis anticus, and then divides into two branches. The
first and largest branch passes between the two pronator muscles giving
branches to the pronator profundus, or pronator longus, and the flexor
digitorum profundus muscle. This nerve then passes anteriorly along the
tendon to the base of the digit, and, on the dorsal side, joins the
other branch of the median nerve. At this point there is often a plexus,
and at the point of fusion of the ulnar nerve on the ulnar side there
may also be a small plexus. These plexuses mainly supply the skin of
these regions. The second branch, passes forward, crosses the extensor
carpi radialis and lies just below the pronator profundus, or pronator
longus, giving branches to the skin. It passes downward on the radial
side of the ulnar and supplies the muscles of that part; it then
continues down the hand. Branches are given off to the extensor proprius
pollicis, the interosseous dorsalis, the flexor digitorum, and to the
skin between the thumb and the forefinger.
The _intercostal nerves_ (Fig. 70, No. 18) are given off from the spinal
cord of the dorsal region. The superior branches are small and supply
the superior dorsal region. The inferior branches lie one behind each
rib, innervate the intercostal muscles, and give a few twigs to the
superficial thoracic muscles.
=The Lumbo-sacral, or Crural, Nerve Plexus.=—The crural plexus (Fig. 70,
No. 35 and 36) is made up of trunks from the last two lumbar and first
four sacral spinal nerves. There are two portions of this plexus
separated by a considerable distance.
The anterior portion (Fig. 70, No. 35) consists of the lumbar nerves and
a portion of the first sacral nerves. The fusion takes place on the bony
ridge that separates the lumbar from the sacral region.
The posterior portion (Fig. 70, No. 36) consists of a part of the first
sacral and all of the three succeeding nerves.
The anterior nerve of this plexus is distributed to the abdominal
muscles. An anterior branch is given off to the sartorius. A large
cutaneous branch enters between the sartorius and the ilio-trochanteric
eminence, and supplies the outer and the upper surface of the upper
thigh region. Several short branches are given off from the middle, or
main, mass of the crural plexus to the ilio-trochanteric, or gluteal,
muscle. Another nerve passes over the side of the sartorius adjacent to
the femoro-tibialis, or extensor femoris, and supplies the
ilio-tibialis, or gluteus primus. The rest of this section of the crural
nerves pass in a distal direction over the inner and front side of the
cutaneous nerve and enter the ilio-femoralis, or gluteus medius, the
ambiens and the femoro-tibialis muscle.
The _furcalis nerve_ comes out between the last two lumbo-sacral
vertebræ.
The _obturator nerve_ springs from several roots. The anterior root of
this nerve comes from the main trunk of the crural plexus, and its last
root from the furcalis nerve. The obturator nerve extends in a ventral
direction from the plexus, then horizontally on the inner surface of the
abdominal cavity. It gives off twigs to the obturator muscle and then
passes through the obturator foramen. After leaving the abdominal cavity
it gives branches to the accessorius, the obturator, and, the
pubio-femoral, or adductor longus muscle.
The crural plexus gives off another large nerve trunk which extends
downward. Shortly after emerging from the plexus it gives off a muscular
branch to the ilio-femoralis, or gluteus medius muscle. It then passes
between this muscle and the shaft of the femur, to the inner posterior
surface of the thigh, and to the inner surface of the knee. It gives off
a branch to the structures of the knee. At the knee this nerve
terminates in several branches, some of which pass to the inner surface
of the tibial head, the internal lateral ligament, the periosteum, the
internal condyle, and finally to the upper part of the head of the
gastrocnemius muscle.
The main part of this nerve passes downward as the cutaneous nerve,
along the inner surface of the lower thigh.
The _ischiadic plexus_ (Fig. 64, No. 41) has five to six roots, which
fuse into the ischiadic trunk. This main trunk extends out of the
abdominal cavity, through the ischiadic foramen, close behind the
anterior trochanter of the ilium (Fig. 65, No. 1). It gives a branch to
the external ilio-femoral or gluteus medius, and a branch to the
post-acetabular portion of the ilio-tibialis, or gluteus primus muscle.
The main trunk (Fig. 69, No. 13) passes downward to the lower portion of
the thigh. In this course there are two branches which run parallel to
the femoral artery and the femoral vein. It gives off a small muscular
branch to the accessorius, and further down gives off a long slender
branch to the outside of the knee-joint. It gives off a lateral
cutaneous branch (Fig. 66, No. 7) to the posterior outer portion of the
lower thigh. It supplies motor fibers to the external head of the
gastrocnemius muscle.
At the knee region the ischiadica divides into three parts. The largest
branch (Fig. 69, No. 14) passes with the tendon of the ilio-fibularis,
or biceps femoris muscle, through a loop, the biceps band, and lies on
the upper lateral surface of the fibula. It is covered by the external
head of the gastrocnemius muscle. It innervates the three posterior
muscles of the lower thigh, and then divides into two branches. One, the
superficialis peroneus (Fig. 69, No. 16) passes with the profundus
nerve, forming a double trunk and occupying the tibio-fibular groove on
the antero-lateral side. It passes over the transverse ligament and the
tibio-metatarsal joint, and after sending small branches to the
structures of the tibial side of the metatarsus, it ends as a cutaneous
nerve on the upper side of the third and the fourth toe.
The other branch is the _peroneus profundus_, which separates from the
peroneus superficialis and passes downward in company with the tendons,
under the transverse ligament, and then along the anterior upper surface
of the metatarsus, where it innervates the muscles of that region. It
gives branches to the malleolus, to the tendons of the third toe, and to
the median part of the second toe, and supplies the cutaneous structures
of the third and the fourth toe.
The third branch of the _ischiadica_, a long nerve, is given off just
after the ischiadica passes through the loop. It passes downward between
the two peroneal nerves. It is covered by a sheath. It passes over the
posterior outside rim of the intertarsal joint and innervates the tendon
sheath. The main portion of this nerve is located on the anterior
surface of the tendon Achillis, and passes down on the plantar side, and
innervates the periosteum and all plantar foot muscles. It finally
radiates to the plantar surface of the three anterior toes.
The integument of the toes is sparingly supplied with nerves.
The nerve trunks that do not pass through the biceps band, or loop can
be divided into a medial (Fig. 69, No. 15) and a lateral (Fig. 69, No.
17) portion. The medial portion (Fig. 69, No. 15) soon divides into
numerous branches which supply the muscles of the posterior and the
inner portion of the thigh. A rather large, long branch passes downward
along the tendon of the plantar muscle, which lies on the posterior
median edge of the tibia, and gives off twigs to the median and
posterior part of the intertarsal joint, supplying the periosteum and
other structures and the adjacent skin. It passes downward along the
outer side of the medial metatarsal insertion of the tendon Achillis.
The _fifth branch_ is given off from the second trunk and lies
laterally. It is covered by the external head of the gastrocnemius,
passes along the vena saphena, and gives off a short main branch to the
inner side of the intertarsal joint. The main part passes the tibial
side of the joint, becomes subcutaneous, and finally innervates the two
plantar muscles. A lateral ascending inner branch innervates the flexor
perforatus digitorum, and, in company with an outer branch, the external
head of the gastrocnemius, and also the flexor pedis perforatus.
The _plexus pedundus_ is formed from the spinal nerves coming out of the
plexus ischiadicus. These fibers emerge caudalward and are directed
horizontally. They frequently anastomose with each other, especially on
the pubic rim and on the outside of the plexus ischiadicus. These
branches are deeply imbedded in the kidney substance and innervate the
pubio-coccygeus, or depressor coccygis lateralis, the ilio-coccygeus,
the transversalis, the sphincter, and other muscles of this region, and
the skin of the anal region.
THE BRAIN
=The Brain Coverings.=—The cerebro-spinal axis of birds is similar to
that of mammals. The _meninges of the brain_ are three in number, dura
mater, arachnoid, and pia mater. The _dura mater_ is the thickest. It is
constructed of white fibrous connective tissue, and lines the cranial
cavity. Thus it serves as an internal periosteum. It is continuous with
the spinal dura mater at the foramen magnum, and is also prolonged as a
sheath of the nerves. In birds of flight where the air-sacs and
reservoirs are developed to the highest state, Sappy finds that, “just
as the medullary tissue is replaced by air in the bones of birds, so
might it be imagined that the subarachnoid fluid is also replaced by air
around the spinal cord,” and observations justify the correctness of
this statement. The dura mater measures exactly the volume of the marrow
in birds; so that there does not exist between the fibers and the
nervous surface any space for an accumulation of liquid. This anatomical
fact is sufficient to demonstrate the absence of subarachnoid fluid in
the bird. In denying the existence of this fluid, it ought to be added
that in this class of vertebrates, the spinal prolongation is covered by
a triple envelope; and that between the pia mater and the dura mater is
a thin transparent membrane, which is lubricated by a serous fluid. This
fluid however does not collect; it only moistens the arachnoid membrane.
The _arachnoid_ is located between the dura mater above and the pia
mater below. The pia mater adheres closely to the nerve tissue.
The _falx cerebri_ exists in fowls and in turkeys. It has the form of a
segment of a circle, and extends from the middle of the interval of the
openings for the olfactory nerves to the tentorium cerebelli. The falx
cerebelli is absent. The tentorium is small and is sustained by a bony
plate, and there are in addition two folds, one on each side, that
separate the hemispheres from the tubercula quadrigemina. Owing to the
absence of the falx cerebelli, the meninges lie close together. The falx
cerebri is ossified in birds.
=The Brain Structure.=—The brain (Fig. 62, _C_ and Fig. 75, _A_ and _C_)
is made up of three principal parts: the cerebrum, the cerebellum and
the medulla oblongata. In a fowl of medium size the brain weighs about
150 grains.
The pons varolii is absent in birds. The crura cerebelli (Fig. 75, No.
9) are immediately connected with the corpora restiformia. The lower
face of the isthmus is convex posteriorly; in front, the tubercula
bigemina (Fig. 75, No. 4) are united to each other by a comparatively
large transverse cord, formed by the optic nerves intercrossing in the
median line. The superior face of the medulla oblongata is depressed
above to constitute a fourth ventricle; in front of this ventricle are
the tubercula bigemina, or _optic lobes_. These two voluminous tubercles
are separated from each other above, where they embrace the cerebellum,
and are salient on the sides of the lower face. They are hollow
internally, and communicate with the aqueduct of Sylvius. The thalami
optici are not well developed.
A large _transverse fissure_ divides the cerebrum from the cerebellum
(Fig. 75, No. 5). The optic chiasm (Fig. 75, No. 15) behind which lies
the hypophysis (Fig. 62, No. _C_, 20) covers the region of the middle
brain. The large transverse fissure is the dividing line between the
hemispheres and the optic lobes. If the hypophysis be removed there will
be observed a slit surrounded by gray matter which is called the tuber
cinereum et infundibulum (Fig. 75, No. 18). On the pars commissuralis
and the after brain there are visible the roots of the fifth, the ninth,
the tenth, and the twelfth pairs of cranial nerves. Close beside the
median furrow which extends to the front part of the long furrow of the
spinal cord there is observed the third pair of cranial nerves, which
have their origin in the middle brain. The fourth pair of cranial nerves
extend from the roof of the middle brain on both sides between the
middle brain and the optic lobes. These nerves finally emerge and become
visible on the ventral surface.
The sixth nerve is visible near the middle furrow and almost in the
middle of the pyramids, and near the roots of the fifth, the seventh,
the ninth, the tenth, and the twelfth pair of cranial nerves.
THE DIVISIONS OF THE BRAIN
(Thalamus
(Pineal body
(Infundibulum
(Hypophysis
Forebrain (Optic tract and chiasm
(Cerebral hemispheres
(Olfactory lobes
(Third ventricle
(Lateral ventricles
(Peduncles of the cerebrum
Midbrain (Optic lobes
(Aqueduct
(Medulla oblongata
Hindbrain (Cerebellum
(Fourth ventricle