Chapter XI: Part II: 1, The os occipitale (1, dorsal; 2, two lateral; 3, ventral (10)

_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.

The nerves that have their roots in the spinal cord superiorly, are 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.

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_.

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.

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.

In the newly formed chick the first indication of feathers is the formation of papillæ, which is constituted by the upward growth of the dermis, or the sensitive and vascular parts of the skin. Then the skin immediately around the papilla sinks downward, so that later the papilla is inclosed in a follicle of the skin. The epidermis over the papilla is the same as over the rest of the surface. The horny outer layer of the epidermis forms for the growing feather a protective sheath which is cast off as the feather is formed. The feather proper develops from the underlying germinative layer, which as the feather develops, forms a cylinder of cells. The lower part of the cylinder is in touch with the papilla; this later becomes the quill (Fig. 80, No. 2). The upper part of the cylinder develops the web portion of the feather. As soon as the feather is fully developed the papilla, which has projected into the quill and nourished it, is withdrawn, and the quill becomes filled with a pithlike material forming septa, which extend in different directions.

Once a year, usually in the late summer or in the fall, the entire feather coat is changed. This process is called _molting_. During this time the bird appears in a somewhat depressed condition, the hen almost always ceases laying. Birds also molt in the spring, to a limited extent. The male at this time takes on the so-called breeding plumage, which is the most beautiful of the year. It has recently been established by Rice, that the young fowl, in reaching a stage of egg production, molts five times before the laying period begins.

The =structure of the skin= of fowls is similar to that of the skin of mammals. The skin consists of two layers the outer portion, or _epidermis_, and the inner true skin, the _cutis_, _corium_, or _dermis_. If we study a section of skin from the shank region of a fowl we find the outer portion is differentiated into two distinct regions, the rete Malpighii and the stratum corneum. The _stratum corneum_ is the outer horny layer. The cells making up this portion are fusiform, flattened, and in regular rows. The nuclei in these layers of cells are not pronouncedly visible, and the outline of the cells not clear in a section such as used in our ordinary methods of study. The corneum is a compact mass of remnant cells which have lost the appearance and their texture of living cells. It thus becomes modified into scales upon the skin surface.

Between the stratum corneum and the stratum Malpighii there is another zone which consists of the cells undergoing a transitional stage from a cubical shape to a more flattened appearance, and gradually becoming more granular and hyaline.

The various parts of the epidermis are in close genetic relationship to one another. The upper layer of epithelium is constantly being desquamated. This casting off is compensated for by a continuous upward pushing of its lower elements. Cell proliferation occurs in the basal cells and in adjacent cellular strata of the stratum germinativum, or stratum Malpighii, where the elements are often seen in process of mitosis, or cell division. The young cells are gradually pushed upward. During their course they assume the general characteristics of the elements composing the layers through which they pass. This process is as follows: each cell changes first into a cell of the stratum Malpighii; then, when it commences the formation of keratohyalin, it changes into a cell of the stratum granulosum; later still, into a cell of the stratum lucidum; and finally into an element of the stratum corneum, where it gradually loses its nucleus, cornifies, and at last drops off.

The mesodermic portion of the skin consists of loose, subcutaneous connective tissue containing some fat. The amount of adipose tissue in the subcutaneous layer is subject to great variation; there are a few places in which there is little or no fat. The upper portion of this layer contains a few elastic fibers, which interlace and run in all directions. Numerous round or oval cells are found in the upper region. The lower and middle portions of the corium are richly supplied with blood-vessels terminating into capillaries, which penetrate the portions bordering the epidermis. Nerves giving off terminal branches also occur.

The various _colors_ of the skins of fowls are due to the distribution of various quantities of two colors, orange-yellow and brownish-black.

The yellow pigment is probably carotin and xanthophyll, two pigments contained in association with the chlorophyll of plants, which the bird obtains in its feed. These coloring matters were formerly called lipochrome; but as lipochrome may be any coloring matter of fat, it is not sufficiently definite. This yellow pigment, when present, is diffused through all parts of the cell. When dilute, it gives a yellow hue; when concentrated, orange. It is found in the epidermis and in the fatty masses of and beneath the corium, and is probably identical with the yellow color of fat in other portions of the body and in the yolk of the egg. A yellow shank, in a heavily laying hen, soon loses a part of its pigment; this is also noted of the coloring matter in other parts of the body. This fact indicates that the coloring matter of the fatty part of the egg yolk is from the same source as that of the fat. The draught of this substance is more than normally and the reserve is being drawn upon. Feed rich in zanthophyll and carotin cause intense yellow colored yolks, and feeds poor in these substances cause the yolks to be a pale yellow. Cotton seed meal contains two pigments, one a yellow crystalline substance and the other a brownish resinous substance. Both of these pigments are probably deposited in the egg yolk. The eggs of some hens contain a large quantity giving a light brownish-yellow color to the yolk.

The brown or black-brown pigment is carried in microscopic pigment granules, which may be scattered through the ordinary cells or may be confined to special pigment cells. The former are confined to the epidermis; the latter may be found in both layers, but infrequently in the epidermis. When granules are present in the flattened cells of the corium, they occupy the nuclear region. They lie in short thin lines while those of the under portion of the Malpighian layer occur in oval groups. Where these granules occur in the rete layer, they cluster around the nuclei. In the colored skin there are dark pigment granules found in the corium and to a less extent in the rete layer. Hanau has described a definite cellular body which he found densely packed among the black-brown granules. There is a central body which sends out branches in all directions. In very dark skinned shanks these ramifying strands interlace, and form a compact network, which in many cases is so thick as to give the impression of a homogeneous mass. Here and there occur round or oval pigmented bodies, which Hanau concluded were the star-shaped cells with their pseudopod-like appendages contracted. Pigment cells commonly lie around blood-vessels, clearly indicating their course. They frequently form a compact tube, but more often are limited to fragments which only partly enclose the vessels. Pigment cells occur in several well-defined localities: in the upper portion of the cutis among the closely interwoven strands of connective tissue; in the region bordering the blood-vessels; in proximity to nerves, in nerve endings and in surrounding fat masses. Isolated granules are frequently scattered throughout the lower section of the corium. Barrows concludes that the lower bodies of pigment play little or no part in the color of the external shank, as they lie far below the opaque connective tissue. Melanin pigment granules are always contained in the pigment cells. When found in the Malpighian layer, the pigment cells are of an oval form.

Immediately below the epidermis in the shank skin, extends a space less in width than the row of columnar cells, which is devoid of pigment. The brown pigment is melanin, which in large quantities takes on a black hue.

White skin does not contain superficial pigment. Melanin has been observed in the study of white shank skin, but it lay at considerable depth or in quantities insufficient to be noticeable. In some breeds of fowls, as the Mottled Houdan, there are areas in which much melanin is irregularly deposited, which circumstance gives the leg its mottled appearance.

The yellow color is the result of a deposit of the yellow pigment in the fat of the shank. This may be deposited in both layers of the skin, or in the corium alone. When present it is diffused throughout the entire cell as well as throughout the intercellular substance. In young birds the Malpighian layer contains much yellow pigment. Old hens have only small quantities in the corneum. In breeds with normal yellow shanks old hens that have never laid eggs show a deep orange color in both the dermis and epidermis.

In blue shanked birds melanin is present only in the corium. The black pigment is seen through the semi-transparent Malpighian stratum, making it appear bluish instead of brown or black.

The black shank color results when melanin appears in the epidermis. Two forms of black pigment occur in the epidermis: granules in both layers and pigment cells in the rete Malpighii.

The green shank is produced where there is pigment in the epidermis and numerous melanin pigment cells in the upper corium. It is an optical effect due to melanin lying under the semi-transparent yellow epidermis. There is no melanin in the epidermis.

In the beak the corium of the skin is represented by a thin layer located between the =periosteum= and the stratum Malpighii. Numerous blood-vessels pass into it, and in the soft horn-like skin, occur sensory nerve fibers.

The nails originate from the epidermis, of which they are a modification. The nails of the toes are bent downward. They present a convex dorsal surface and are concave ventrally. The dorsal surface consists of a horny plate, which is set in a nail matrix. The ventral portion merges with the sides of the upper half, and, as the lower portion is the softer and wears faster, the nail has a sharp point and edge. The matrix is formed by a growth of the Malpighian layer of the cutis. A fold of skin lies over its posterior part. The epidermic cells of the dorsal, or nail, part, and the base of the ventral part grow fast. The outer cell layer gradually becomes horn-like.