sensory; that is, they convey the sensory impulses from the periphery to
the cord ganglion and to the brain. They have a ganglion just outside
the cord.
The nerves that have their roots in the spinal cord inferiorly; that is,
they convey motor impulses from the nerve centers to the periphery. The
function of the inferior roots is to supply all the voluntary muscles as
well as the oviduct, the intestines, and other hollow viscera, including
the blood-vessels with the power of movement. Many of these fibers pass
to the sympathetic ganglion and are distributed as sympathetic nerve
fibers.
The cord is divided into different tracts, that is, certain groups of
fibers convey certain kinds of impulses.
The superior column of the cord conveys to the cerebrum such impressions
as temperature, pressure, and muscular tension.
The fibers of the lateral columns carry sensations of pain.
The fibers of the direct cerebellar tract carry impulses which result in
the maintenance of the equilibrium of the body.
All the voluntary impulses originate in the cerebrum, pass through the
cerebellum and travel direct to the bulb; they then pass over to the
opposite side, and travel by the crossed pyramidal tract to the
multipolar cells of the inferior horn of the spinal cord, and transmit
the impulse through motor fibers that originate at that point in the
cord.
All sensory impulses enter the brain on the side opposite their origin,
and all motor impulses leave the brain on the side opposite that to
which they are distributed. Injury of the motor area of the right side
of the brain leads to paralysis of the left side of the body.
An impulse of the vasomotor nerve travels in the lateral column of the
cord.
A nerve impulse may originate in the brain and be modified in passing
through a ganglion in the spinal cord or in the sympathetic system.
The system of _reflex action_ is as follows: first, an efferent nerve
which conveys the impulse from the periphery to a nerve center; second,
a ganglion, or nerve center, to receive the impulse and generate other
impulses; third, an efferent nerve to convey the impulse from the nerve
center to the periphery.
The following is an example of reflex action. The foot of a fowl is
pierced with a pin. The sensory impulse is conveyed by sensory nerve
fibers to the nerve centers. In this center the ganglionic nerve cells
generate a motor impulse which is sent back through the motor nerve
fiber to the muscles controlling the part. The result is that the muscle
contracts and jerks the foot.
There are many _functional centers_ located in the medulla oblongata.
Destruction of this part of the system results in instant death of the
bird. In addition to furnishing a path for fibers carrying impulses from
the body to the cerebrum, it furnishes a large number of centers for
such functions as respiration, swallowing, secretion, temperature, and
vasomotor and cardiac activity.
The _function of the cerebellum_ is principally that of coördination. It
brings about harmony and rhythm in muscular movements. If the cerebellum
be removed, the bird can no longer walk. It has lost, with this removal,
all power to coördinate.
The _cerebrum_ of the bird has no convolutions, and the gray, or
ganglionic, portion is thin, indicating low power of intelligence. There
are certain areas presiding over other functions, as motor areas,
sensory areas, and so on.
A careful study of the brain of the fowl shows us that the centers
presiding over sight and smell are well developed.
The olfactory bulbs are the centers of the sense of smell.
The optic thalamus is the center of the sense of sight.
The sympathetic nerve system transmits impulses to the involuntary
muscular structure of all organs, including those of the intestinal
tract, the blood-vessels, and perhaps also the glands.
ESTHESIOLOGY
THE SENSE ORGANS
The five special senses are seeing, smelling, tasting, hearing, and
feeling.
THE ORGAN OF SIGHT
The sense of sight in the bird is well developed. The eye (Fig. 26, _D_,
_E_ and _F_), the organ of sight, is relatively large, round laterally,
and rather flattened antero-posteriorly. It is located at the side of
the head. The eyeball is only slightly movable. The septum interorbital
separates the two eyeballs laterally. As in mammals, the eyeball has
three coats, which are from inside to outside, the retina, the choroid,
and the sclera. The =sclerotic coat= is completed anteriorly by the
cornea with which it forms a union called the _corneo-scleral juncture_.
Around the cornea the sclerotic coat contains a ring of osseous scales
varying in number from twelve to twenty. The sclera may become ossified
posteriorly, forming an osseous sheath around the optic nerve. The
_pecten_ is a vascular comb-like membrane stretching from the nervous
opticus to the crystalline lens. The choroid coat is always black. The
pupil in the hen is also black and round. The iris contains striated
muscular fibers. The _membrana nictitans_, located at the inner angle is
well developed. It is moved by two muscles (Fig. 26, No. _B_, 7, 8, 10).
The lacrimal gland and the gland of Harder are present. There is no
meibomian gland.
The _tears_ are secreted by the lacrimal gland and are drained away from
the fore part of the eyeball by two small canals which extend into a
lacrimal sac. From this sac there extends a tube into the nasal cavity
called the lacrimal duct.
The lower lid is the larger and often incloses a small cartilaginous
plate. The _conjunctiva_ is a true mucous membrane which covers the
anterior portion of the eye cavity attaching to the cornea-scleral
juncture.
The =choroidea= is rich in pigment. On its inner surface lies the dark
pigment layer of the retina. The corpus ciliare, that part of the
choroid coat bearing the ciliary processes, consist of numerous folds.
The ciliary muscles are arranged obliquely. Each consists of three
digitations.
The _iris_ is covered on the posterior side with pigment, the color of
which determines the color of the eye. The yellow pigment of the iris
has been said to be due to carotin and xanthophyll and the black pigment
to melanin. The enlarging and the contracting of the pupil is brought
about by muscles. The reduction of the pupillary caliber is due to the
sphincter pupillary muscle. It is said by some anatomists that the
muscles controlling the caliber of the pupil in the bird furnish
voluntary motion and that the capability of accommodation of the eyes is
greater in birds than in mammals.
The =retina= contains no blood-vessels; otherwise the structure is
similar to that of mammals. The crystalline lens is flattened on the
corneal side and is convex posteriorly. The lens epithelium develops
into fibers in the parts close to the equator, and are almost
perpendicular to the eye axis. The corneal portion is relatively small.
The =sclerotic coat= is dense and white. It is divided into three
layers. It is thin and flexible, and somewhat elastic posteriorly. It
has an internal layer of hyaline cartilage. Anteriorly its form is
maintained by the circle of osseous plates mentioned above. These
plates, interposed between the exterior and the middle layer, are
located immediately behind the cornea. The scales are thin and of oblong
quadrate shape, being elongated from before backward.
The choroid coat is a membrane loosely cellular and highly vascular. It
is impregnated by a black pigment. Opposite the bony circle the choroid
separates into two layers. The external layer is the thinner and adheres
at first firmly to the sclerotic; it passes forward to become continuous
with the iris. The inner layer is thicker than the external. The two
layers are made up of radiating fibers which terminate anteriorly in the
ciliary processes, the ends of which are adherent to the capsule of the
crystalline lens.
The iris is delicate in structure. It is composed of a fine network of
interlacing fibers.
The ciliary nerves and blood-vessels run in the form of single trunks
between the choroid and the sclerotic, and terminate anteriorly in a
ring-shaped plexus for the supply of the iris and the muscular circle of
the cornea. As stated before, the pupil in the fowl is round, but in the
goose it is elongate transversely; and in the owl, a vertical oval.
The optic nerve approaching the sclerotic coat becomes altered into a
conical extremity, which enters a sheath and is directed downward and
obliquely forward. The extremity of the optic nerve in the interior of
the eye presents a white narrow streak. Branches of the opthalmic artery
enter the eye between the lamina of the retina, along the whole extent
of the oblique slit, and penetrate the fold of the pecten upon which
they form a delicate ramification.
The crystalline lens is of soft texture. It is inclosed in a capsule and
is nearly round. It adheres very firmly in the depression in the
anterior part of the vitreous humor. The capsule is lodged between two
layers of hyaloidea, which as they recede from each other, leave around
its circumference the sacculated canal of Petit.
The cornea is of horny consistency and is transparent. Light thus
rapidly passes through it to the posterior part of the eye.
The vitreous chamber, lying back of the crystalline lens, contains a
clear jelly-like substance.
THE ORGAN OF HEARING
The ear, the organ of hearing, has in the fowl no conchal cartilage. The
_external auditory meatus_, or canal opening, is found on each side of
the head, and is usually guarded by a few stiff, short feathers. In some
kinds of birds these feathers are capable of being erected so as to
direct the waves of sound into the inner ear. The outer entrance of the
ear contains glands. This canal is short. It leads to the drum, which is
somewhat convex from the outside, and which has a membrane forming a
complete curtain stretched over the outer part. The irregularly formed
drum has connection with the air-containing cavity of the skull, and by
a thin cartilaginous canal, the Eustachian tube, with the pharynx. The
_auditory ossicles_ are represented by only a single bone, called the
_columella_, which most closely represents the stapes of mammals. This
is attached by processes of cartilage to the tympanic membrane. Owen
considers these cartilages as representing the malleus of mammals.
Huxley on the other hand considers them to represent the incus.
Originating from the processes is a small muscle which is attached to
the drum. This is by Shufeldt considered the Tensor tympani. The drum
cavity through the fenestra vestibularis and cochlearis is connected
with the labyrinth.
The =inner ear= consists of a membranous labyrinth, surrounded by a
spongy bony structure—the bony labyrinth. In it are recognized the
vestibulum, the three half-circle shaped canals, and the cochlea. The
superior semicircular canal is the largest. The acoustic nerve enters at
the end of the canals near the ampullæ. The nerves are supported in
delicate vascular membranes lining the canals and slightly projecting
into the ampullæ.
[Illustration:
FIG. 78.—Diagram of the inner ear. 1, The integument. 2, The superior
semicircular canal. 3, The external canal. 4, The horizontal canal.
5, The ampulla. 6, The obtuse osseous conical cochlear cavity. 7,
The Eustachian tube. 8, The tympanum. 9, Filaments of the auditory
nerve.
]
The =vestibulum= is a small irregular cavity which communicates with the
arcades and the cochlea, and through the fenestra vestibularis with the
drum cavity. The endolymph of the vestibulum contains microscopic
crystals of calcium carbonate. The semicircular canals are relatively
larger and thicker than in mammals. The ampullæ in the upper and back
part are separated by walls. The cochlea is an obtuse conical tube-like
structure slightly curved at the blind extremity with the concavity
directed backward. It contains a membranous lining. At this point the
cochlea is broadened and accommodates a branch of the auditory nerve.
This nerve spreads out in fine filaments upon the surface of the tubes.
The hollow space of the cochlea is divided by a spiral partition, making
two chambers, the scala vestibuli and the scala tympani. These walls
extend from the beginning of the cochlea.
The =tympanic cavity= is formed by the occipital, the basi- and
alisphenoids, the petrosal portion of the temporal bone. It represents
the stapedial canal leading to the foramen ovalis and the pneumatic
apertures by which the air from the Eustachian tube is conducted to the
precranial diploë.
THE ORGAN OF SMELL
The nose is the seat of the peripheral portion of the organ of the sense
of smell. The terminals of the olfactory nerves, which receive the
impressions of odors, are broadened in the mucous membrane of the walls
of the anterior nares. There are no ethmoidal volutes, or sieve-like
structure, in birds. The nerve extends from the anterior portion of the
brain in cone-shape, finally dividing into filaments which are
distributed over the mucous surface of the turbinated bones.
THE ORGAN OF TASTE
The most important part of the organ of taste is the _tongue_. In birds
the dorsum of the tongue is covered by a thick stratum corneum, a heavy
layer of stratified squamous epithelium. The tongue, therefore is not in
birds so well adapted for the perception of taste as it is in mammals.
The lingual branch of the trigemini is lacking in birds. This fact makes
the ninth pair, or the nervi glosso-pharyngei, the exclusive nerves of
taste. There is an opinion ventured by one anatomist that, since the
first and the second branch of the fifth pair, or trigemini, have
terminal filaments in the hard palate, they may furnish fibers for the
sense of taste. There are many taste cells on the tongue and on the
dorsal palate.
THE ORGAN OF TOUCH
The peripheral parts of the organ of touch in the fowl are the skin and
the feathers.
In a few birds special touch and taste perception can be supplied by the
edge and point of the beak.
In the skin of birds are found numerous sensory nerve endings for
tactile sense.
The sensory nerves, which provide the sense of touch, usually terminate
in one of two forms. The first are the Herbst’s bodies, which in many
respects are similar to the Picinian bodies.
Herbst’s _touch corpuscles_ are found on all parts of the skin; they are
especially numerous in the region of the tail and of the wing. In the
wing they are particularly numerous in the region of the flight
feathers. They occur in large numbers in the periosteum of the anterior
tibial region and in the mucous membrane of the cloaca and of the
generative organs. They are numerous in the conjunctiva and on the
surface of the tongue. They are also found in the gums and in the beak.
Herbst’s corpuscle is made up of a central fiber-shaped part with a
smooth extension of the axis cylinder of its nerve (Fig. 29, _G_). This
central fiber is surrounded by a peculiar protoplasmic body. Outside of
this there is a double row of cubical cells which surround the axial
part. These are close together. Outside of these there occurs a
concentric lamellar layer, which contains cells. In the periphery these
lamellæ become more distinct and contain larger but fewer cells. The
capsule is made up of very thin layers, which are continued into the
perineural layer. Each body contains a thin outer zone. The nerve fiber
passes in a regular manner from the axis cylinder, the myelemma, and the
sheath of Schwann, and as a delicate nerve twig is surrounded by a
sheath consisting of several perineural layers. As it enters the center
of the touch corpuscle, it loses its myeline sheath near the base of the
body, and the terminal fiber becomes flattened. Its rim is directed
toward the two rows of cubical cells, and it ends in a rounded knob.
The touch corpuscles are the largest in the mucous membrane of the
cloaca and smallest in the skin. Hess has found these touch corpuscles
in the large filiform papillæ on the side of the tongue and a few on its
lower surface. They are also found in the soft skin of the edges and
inner borders of the beak.
STRUCTURE OF APPENDAGES
The _skin of the fowl_ is very thin and does not contain oil glands or
sweat glands. In the fowl the oil is supplied by a tail or rump gland,
the _glandula uropygii_ (Fig. 35, No. 16). This gland is round or oval
in shape, and about the size of a pea and consists of two lobes. It is
of the tubular variety with a teat, which, in most instances has two
openings. A medium septum divides the gland into two halves. The oil
secreted by the columnar epithelial cells is collected in a body cavity
located in the center of the gland; this has a duct extending to the
surface. The bird by squeezing out a quantity of this oil into its beak
oils the feathers, passing the beak over them, one by one. The oil
renders the feathers practically impervious to water. It is necessary
for the bird to give proper attention to its plumage in order that the
feathers appear in prime condition. Should there be a disease of the oil
gland, or should the bird become ill and neglect its toilet the result
is an unkempt appearance of the plumage, the feathers becoming rather
rough and more or less injured by the weather.
The _subcutis_ is well developed and furnishes to the cutis great
capability for movement, which is necessary for the rising and falling
of the feathers. The corneum is very thin. Papillary bodies are present
only in a very few places, such as the region about the eye and on the
toes. Where the toes touch the ground in walking there are large
wart-like thickenings of the epithelium. In most birds the shanks are
unfeathered. The epidermis on the feathered parts of the skin is thin,
dry on the surface, and abounds in continuous scales. The stratum
corneum is very thick on the horny sheath of the beak, on the top of the
toes, on the spurs of the cock, and on the scale plates that cover the
skin on the shanks. The feathers of birds serve the same protective
purpose as hair on mammals. In cold weather the skin muscles controlling
the feather movements contract; thus the feathers become ruffled much as
we observe the hair standing erect on horses under similar conditions.
By increasing the dead air space around the body, the radiation of heat
is retarded, and the body kept warmer. The corneum of the skin is not
usually rich in blood-vessels; however, in the domestic fowls there is
in the comb, in the wattles, and similar appendages of the head, a thick
vascular network.
The _beak_ and the _claws_ are modified skin; they are true horn
material. At the base of the beak there is often formed scales, which
surround the nostrils, in whole or in part, and have a naked, or waxy,
appearance, which is known as the _cera_.
[Illustration:
FIG. 79.—Photomicrograph of the section of skin from the sole of the
foot of a hen. 1, Horny stratified squamous epithelium. _a_, Stratum
corneum. _b_, Stratum lucidum. _c_, Stratum germinativum. 2,
Connective tissue supporting membrane, pars reticularis. 3,
Blood-vessels.
]
The feathers may be considered of two chief kinds, the quill feathers
and the clothing feathers. The most rudimentary of the latter are known
as down. A _quill feather_ consists of two principal parts: the quill,
or calamus, and the vane, or vexillum. The quill is continuous with the
central shaft, called the rachis, the two forming the stem of the
feather. Projecting outward from the stem on each side are a large
number of pointed and very flexible _barbs_. These barbs are located
nearly at right angles to the quill and have extending from them at
right angles smaller processes or _barbules_. These barbules hook
together the barbs and give the web its form. In some feathers, as the
hackle, and in the wing bar feathers of the male of some breeds, there
is a portion of the upper outer edge of the feather not provided with
barbules, which fact gives the feather its characteristic appearance.
The down feathers are loose and fluffy. In this kind of feather the
shaft is weak, and the barbs are not provided with barbules. The barbs
like the shaft may be considered weak. These feathers give great warmth
to the body. There are fiber feathers appearing as hair-like filaments,
and called _filo-plumæ_. These are found scattered over the body; they
are particularly abundant in the region of the head and the neck.
At each end of the quill is a small opening, or _umbilicus_. Inside the
barrel of the feather there is _pulp_, which in young feathers is very
vascular; the vessels entering by the proximal umbilicus are buried,
along with part of the quill, in a papillated follicle of the skin. From
this follicle the feather is developed. At the base of the shaft a
secondary rudimentary quill is usually formed, which may be represented
by a mere tuft of down. On the same general principle the smaller
feathers are constructed. These cover the body, the upper parts of the
legs, and the head, while the larger feathers and quills are confined to
the wings and the tail. The longest quill feathers are those arising
from the hand, called the _primaries_. Those arising from the forearm
are called the _secondaries_. Those that are developed from the proximal
part of the arm are called the _tertiaries_. The rudimentary pollux
carries some feathers which form the _alula_, or bastard wing. The
scapularies are feathers covering the scapula and the humerus. Covering
the bases of the larger flight feathers are wing coverts consisting of
several rows of small feathers. The quill feathers of the tail are
called the _rectrices_. The rectrices have considerable mobility; their
bases are covered by a row of tail coverts.
The pedal digits of the natatores are joined by a membrane, covered with
scaly skin, which forms the web foot.
The feathers in many parts of the body are developed in rows, there
being intervals, or elongated skin areas, between these groups of
several rows, which are not provided with feather papillæ, but which are
covered over by the feathers developed in front of these spaces. The
definite feather lines, or areas, have been called _pterylæ_. The
intervening tracts devoid of feathers are called the _apteria_.
The first outer covering of the bird, or baby chick, is a temporary one
consisting of fasciculi of long filaments of down. These fasciculi on
their first appearance, are enveloped in a sheath, which soon becomes
ruptured and are entirely cast off by the time the baby chicks are ready
to be taken from the nest or the incubator.
[Illustration:
FIG. 80.—Photomicrograph of the skin of the neck of a S. C. White
Leghorn hen. 1, The skin possessing an outer stratum, the stratum
corneum, next the stratum lucidum, next the stratum granulosum and
underneath the stratum germinativum. Beneath the outer dark band
representing the upper skin strata is the pars reticularis of the
derma. 2, Papilla showing from inside to outside the hyaline feather
wall, the stratum corneum, the Malpighian layer of the follicle, the
corium. Inside the feather is noted the pulp-like material.
]
The down fasciculi, each emerging from its small quill, are succeeded by
the feathers, which they apparently guide through the skin.
Feathers do not spring from all parts of the body alike; especially
devoid of feathers are those parts where chafing and friction is
greatest, as under the wings and in the groin.
At the end of the quill there is a small opening, the _inferior
umbilicus_, into which projects a papilla of the dermis. Where the quill
emerges from the skin there is another small opening, the _superior
umbilicus_, from which springs frequently a small feather called the
_hyporachis_. The shaft, or _rachis_, has a groove extending along that
surface which lies next the body.