Chapter X: Part II: 1, The os occipitale (1, dorsal; 2, two lateral; 3, ventral (9)

The _ophthalmic division_ of the fifth nerve is the smallest of the three branches. It emerges directly from the Gasserian ganglion, and passes through a narrow bony canal in the base of the brain, below the pathetic and the abducens, and through the foramen ophthalmicum. It then enters the eye cavity above the optic nerve, and on the wall of the cavity extends downward, and with regard to the eyeball, dorsally lying close to the rectus internus muscle and close to the surface of the eyeball. It passes along the olfactory nerve, extends under the superior oblique muscle, and finally reaches the inner angle, or canthus, of the eye. It here divides into the recurrent externa and the ethmoidalis. The ethmoidalis branch is a straight continuation of the ophthalmic. It extends along close to its fellow of the opposite side, and, over the vomer, splits into two branches. The smaller of these branches breaks through the bone cells of the jaw, continues upon its ventral surface in a furrow extending forward, and terminates in the gum region. It supplies the gum and point of the beak. The larger branch enters into the cell substance extending to the tip of the beak, in its course sending out a number of fine filaments, which spread out along the outer surface. At this point these two branches may fuse. The larger branch is endowed with the sense of touch.

The recurrent externa divides shortly after leaving the main trunk of the eye cavity passes over the lacrimal gland. It gives two branches to this gland and one to the membrana nictitans, and innervates the upper eyelid. It then emerges from the eye cavity, passing over the os lacrimale, and gives one or more branches to the integument of this region, including the comb. This branch is large in birds with a large comb. Branches pass in front of the lacrimal bone through the outer nasal cavity and into its deeper structure.

Shortly after the ophthalmic branch has entered into the eye cavity and before it crosses the optic nerve, it gives a fine branch to the motor occuli.

The second and the third branches of the fifth cranial nerve are mixed. They contain elements of both the portio major and the portio minor. These two branches come from the lower part of the outer region of the ganglion, and pass together through a cavity which is located between the os petrosum and alæ and basis sphenoid, and then branch.

The ramus secundus, or _superior maxillary division_, of the trifacial, is the second branch and passes into the orbit below the optic nerve and the eyeball. It is called the recurrent infra-orbitale. It gives an ascending branch to the gland of Harder, one to the conjunctiva, one to the membrana nictitans, and one to the eyelids. It also gives a branch to the skin below the eye and to the angle of the mouth. This latter branch is called the _recurrent subcutaneous_. These two branches communicate with the recurrent nasal ciliaris of the first trigeminus branch. The second branch passes below the nasal opening, then passes on and forms the alveolar nerve on the side between the gums. It sends several recurrent posterior branches to the elevations on the back part of the mouth. It extends forward to the point of the beak.

The _inferior maxillary division_ of the fifth is larger than the other two. It is directed downward and outward then upward to the temporal region. At the temporal region it divides into five parts. It gives a branch to the temporalis muscle, one to the pterygoid muscle, and one to the mylo-hyoideus. The main branch gives twigs to the parotid gland region and enters into the dental canal of the lower jaw. Numerous filaments break through the lower jaw bone, and then spread out on the skin and rim of that bone. The largest branch, called the recurrent maxillaris externa, comes out near the coronoid process. The rest of the trunk extends to, and comes out of, several foramina at the anterior point of the jaw bone.

These filaments are contained in grooves or cavities of the lower jaw and terminate in touch buds.

The trifacialis fuses with the other cranial nerves and with the sympathetic nervous system. Some of these fusions are as follows: The recurrent ophthalmica fuses with the orbito-nasalis ganglion. Indirect fusion of the recurrent maxillary division of this nerve takes place near the Gasserian ganglion through the sympathetic nerves, the temporo-lacrimalis, the facial nerves, the large cervical ganglion, and indirectly with the glosso-pharyngeal and the vagus. This fusion has been called the superior recurrent branch of the trigeminus, or trifacialis.

There is a direct fusion of the superior recurrent maxillary division of the fifth just before entering the upper jaw. This fusion is with the spheno-palatine ganglion and the sympathetic carotidis cephalica, and also with the large cervical ganglion.

=The Abducens= (Fig. 62, No. _C_, 13).—This nerve originates in the somatic and motoric column along with the other nerves of the eye muscles. Its nucleus lies in the circle of the pars commissuralis of the cerebellum. The nerve then passes ventrally, as does the motoris oculi and leaves the brain along the median line. This is a comparatively large nerve and passes somewhat laterally and ventrally from the foramen opticum through a canal in the sphenoid. It then enters the orbital cavity. Some muscular twigs are given off to the quadratus and to the pyramidalis muscle. This nerve also innervates the external rectus muscle of the eye. The abducens anastomoses with the ramus ciliaris and the ophthalmic nerve and gives fine nerve twigs to the ramus ciliaris externus of the ciliary ganglion.

=The Facialis and the Acusticus.=—The facial nerve originates with the auditory (Fig. 62, No. _C_, 14 and 15) in a very vaguely known manner, from the cerebellum. It divides into three parts.

The first of these probably comes from the complex ganglion with the posterior roots of the auditory. This part belongs to the somatic sensory group of nerves. From this same group originates the auditory, which spreads out into the cochlea and takes the impressions of sound. This nerve is short and thick, and, at the point where it loses its medullary covering on entering the cochlea, there is developed a ganglion. This ganglion is similar to the spinal ganglia.

The second part is provided with one root which originates mesially and ventrally, from the deeper ganglion cells. Some of the fibers from this root constitute the vestibular branches. They accompany the auditory and supply the anterior part of the ear labyrinth and the semicircular canals. The larger part of the fibers of this trunk make up the intermediate part of the facial. The geniculate ganglion is formed at their fusion. The sympathetic spheno-palatine nerve emerges from this ganglion, coming out of the aqueduct of Fallopius.

The third part, called the portio dura, is the main facialis. It is located opposite the auditoria intermedia. Its roots may be traced to the complex ganglion, from which they take a direction ventrally from the median portion.

The facialis, after emerging from the aqueduct of Fallopius, takes a curved course and partly fuses with the sympathetic temporo-lacrimalis on the upper posterior wall of the ear drum. It is here accompanied by the carotid and the cephalic arteries. It leaves the ear drum through an opening in the quadrate bone, giving off a large branch to the digastricus muscle and a small one to the stapedius of the columella auris. The facialis trunk is quite large. It then passes downward along the quadrate bone, where it receives a large branch from the glosso-pharyngeus. This nerve also gives branches to the mylo-hyoideus and the stylo-hyoideus muscle, crosses laterally the glosso-pharyngeus, and finally fuses with branches of the subcutaneous and with the first, the second, the third, and the fourth cervical nerves. After this fusion the nerves innervate the skin of the anterior of the neck and the constrictor colli muscle.

The facialis anastomoses as follows: with a fine branch of the sympathetic temporo-lacrimalis, indirectly with the ramus trigeminus, with the large cervical nerve ganglion and with the spheno-palatine ganglion and nerve.

=The Vagus Group.=—The ninth, the tenth, and the eleventh cranial nerves are by some anatomists called the vagus group, first so called by Willis. They are made up of both sensory and motor nerves.

=The Glosso-pharyngeus.=—The roots of the glosso-pharyngeus (Fig. 62, No. _C_, 16; Fig. 76, No. 13) emerge, along with those of the vagus, from the medulla oblongata, and enter, as a short trunk, the foramen jugulare et caroticum. Between these two nerves there is usually found a thin portion of bone. The trunk of the glosso-pharyngeus forms a ganglion in the foramen jugulare et caroticum where it receives connecting branches from the near-by ganglion radicis and vagus nerve. The glosso-pharyngeus passes out of the foramen jugulare et caroticum above the large superior sympathetic nerve ganglion, with which it communicates. The glosso-pharyngeus passes diagonally over the ramus temporo-lacrimalis of the sympathetic system, and then receives a very strong branch from the vagus (Fig. 76, No. 12). It sends a short branch to the recurrent lacrimalis of the sympathetic system. At this point there is formed a reddish-yellow ganglion, the petrosal ganglion, which is similar to the petrosal ganglion of mammals. Frequently it is found close below the large superior cervical nerve ganglion. The petrosal ganglion is frequently connected by special fibers with the cervical nerve ganglion, the large superior cervical nerve ganglion and the ganglion radicis vagi. The glosso-pharyngeus is divided into the following branches.

First, the recurrent pharyngeus which gives branches to the upper part of the throat and which is tortuous in its course. It receives a branch from the superior cervical nerve ganglion and gives off branches to the salivary glands and papillæ of the posterior tongue region.

Second, the recurrent lingualis, which passes with the lingual artery over the hyoid bone to the base of the tongue and to the papillæ. Another branch passes below the tongue bone and supplies the tongue and the pharynx. The glosso-pharyngeal is a mixed nerve.

FIG. 76.—Photograph of spinal cord of a hen. 1, Cerebrum. 2, The
olfactory nerve terminating at 3, the turbinated bones. 4, The
superior maxillary division of the fifth nerve dividing. 5, The eye
with the upper part laid open. 6, The cervical segment. 7, The
lumbo-sacral segment. 8, The lungs. 9, The kidneys lying above the
peritoneum. 10, The terminal filament of the spinal cord. 11, The
hypoglossal nerve. 12, The vagus nerve. 13, The glosso-pharyngeal
nerve. 14, The branch from the vagus.
]

=The Vagus.=—The vagus, or pneumogastricus (Fig. 18, No. 5; Fig. 62, No. _C_, 17; Fig. 76, No. 12), a mixed nerve, forms a ganglion in the foramen through which it passes. The foramen lies between the os petrosum and os occipitale, close to the foramen jugulare, above and to the inside of it.

The ganglionic root of the vagus communicates with the superior cervical nerve ganglion. The vagus, after coming out of the foramen, takes up branches of the spinal accessory, passes to the superior nerve ganglion, crosses the carotid artery, and then, accompanied by the internal jugular, it connects by a branch with the hypoglossal. It then communicates with the petrosal ganglion, and receives a long branch from the superior cervical ganglion. It then extends along the neck with the jugular vein and often fuses with the sympathetic ganglionic plexuses. As it passes, it is interwoven with the glosso-pharyngeus, but each nerve element retains its own individuality. Extending down the neck, on entering the thoracic cavity, it lies between the plexus brachialis and the carotid artery. Then it passes below the subclavian artery, and between the bronchial tubes, the aorta pulmonalis, and the subclavian vein. Then ventrally it rests upon the glands, and the right and the left vagi fuse or unite. From here they radiate down to the stomach in fan-shape, and, continuing, they fuse with the sympathetic system.

The other branches of the vagus are:

The first branch is the recurrent laryngeus, which supplies the lower end of the bronchial tubes, and the esophagus, then enters above the bronchus near the origin of the subclavian artery as the recurrent cardiacus.

The second are the recurrent pulmonale which pass into the lungs, each fusing with its fellow from the opposite side, and giving off branches inferior to the vena cava including a branch to the heart.

The third are the recurrent hepatici which pass through the diaphragm and are distributed to the liver.

=The Accessorius Spinalis.=—This is a very small motor nerve (Fig. 62, No. _C_, 18). It comes out between the dorsal and the ventral root of the third cervical nerve. It lies close to the neck and extends anteriorly, receiving roots from the first and the second cervical nerve. It passes through the occipital foramen into the brain cavity and then enters the ganglion radicis vagi. It passes out of the cranial cavity through the foramen jugulare. It then partly fuses with the vagus and partly, as a fine branch, with the subcutaneous colli.

=The Hypoglossus.=—The hypoglossus is a motor nerve (Fig. 62, No. _C_, 19). It originates anterior to the eleventh pair of cranial nerves, and from the same ganglion as the abducens and the motor oculi. It leaves the medulla oblongata from its ventral surface, and passes with a posterior and an anterior branch out of the cranial cavity through two separate foramina in the os occipitale laterale. The posterior branch, much smaller than the anterior, passes between the cranial bones and the cervical sympathetic nerve; it then passes at right angles through the rectus capitis anticus, and, while closely following the carotid artery, it fuses with the anterior branch. The larger, anterior branch, much stronger than the posterior, sends a short strong branch to the complexus muscle. The rest of this branch, which probably has sympathetic elements, is disposed as is a spinal nerve. It crosses the sympathetic nerve, and at this point forms a typical cervical sympathetic ganglion. It also forms a short loop with the recurrent ventralis of the first cervical plexus. From this loop, after it has given off several branches to the muscles of the neck, it gives off one or two strong branches, which fuse with each other and with the posterior thin branch, thus forming the trunk of the hypoglossus. It receives many elements from the first cervical nerves.

The hypoglossus sometimes communicates with the vagus, crosses over the latter, and divides itself into two main branches, as follows:

First, the _recurrent laryngo-lingualis_, which passes between the cornua of the os hyoideum and the larynx to the anterior part, and furnishes the principal tongue muscles. It extends along the inferior surface of the tongue and fuses with the one of the other side and extends to the free tip of the tongue. This nerve probably receives sensory elements from the second root and from the confluent of the first cervical nerve. This form of anastomosis is well marked in birds with thick tongues, as ducks.

Second, the _recurrent laryngeus_ furnishes the muscles of the superior larynx and of the tongue skeleton. It extends downward and also furnishes the muscles of the trachea. It follows the course of the jugular vein and at the entrance of the chest supplies the furcula. The nerve passes into the thorax, downward along the side of the bronchial tubes, and supplies innervation to all the muscles of the inferior larynx.

THE SPINAL CORD

=Structure of the Cord.=—The spinal cord is called the myelon or medulla spinalis. It is a comparatively large, white, irregularly cylindrical cord, flattened from above downward. It extends from the foramen magnum at the base of the medulla to the caudal portion of the spinal canal where it terminates in a fine filament. In order to allow considerable motion of the spinal column without danger of injury to the spinal cord, the cord is loosely suspended in the canal. The meninges, or coverings, are continued from the brain. In addition to these there are to be found arteries, veins, and nerves entering and others making their exit from the canal and from the cord. At the cervico-dorsal juncture, where the brachial plexus is given off, the cord is enlarged. There is another enlargement at the point where the lumbo-sacral plexus is given off. At this point, superiorly, there is a longitudinal cavity called the _sinus rhomboidalis_. This sinus contains a gelatinous substance. There is a pair of nerves given off at each intervertebral foramen. Thus there are as many pairs of spinal nerves as there are vertebral segments. The inferior root is the motor root and carries the impulses from the cord to the periphery. The superior root is the sensory branch and carries the impulses from the periphery to the cord, and thence the impulse is carried to the centers in the brain. The sensory and the motor roots are of about equal size. The inferior have more numerous filaments. The ganglion on the superior or sensory root is relatively large. In the sacral region the sensory and the motor branches pass through their own bony canal.

White _rami communicantes_ are given off at each intervertebral foramen to the sympathetic system and gray rami are received from the sympathetic ganglia. Thus there is established a direct communication between the sympathetic and the spinal system. There is a ganglion on the superior root just outside the spinal cord.

The ganglionic portion, or _gray matter_, is arranged in the center of the cord in two comma-shaped parts forming an X. The white matter, or the material forming the fibers, is arranged around the central ganglionic portion.

The spinal cord may be divided into two lateral symmetrical halves. There are two longitudinal fissures, one on the upper and the other on the lower half. The upper called the _superior median fissure_ is narrow but deep. The one on the inferior side, called the _inferior median_, is usually more pronounced. The superior parts of the gray matter are called the superior cornua, or horns; the inferior parts the inferior cornua, or horns. The center of the cord is pierced by a _central canal_ which communicates with the fourth ventricle at the calamus scriptorius.

STRUCTURE OF THE NERVE TRUNKS AND GANGLIA

The protoplasmic processes called _dendrites_ have a similar structure to the cell body. The dendrites branch dichotomously, become rapidly smaller, and usually end at no great distance from the cell body.

The _axone_ or _axis cylinder process_, differs from the cell body and from the dendrites. It does not contain chromophilic granules. It consists entirely of neurofibrils and perifibrillar substance. It emerges from the cell at an enlargement known as the axone hill. This hill is free from chromophilic bodies.

Nerves are divided into two kinds, medullated and non-medullated.

_Medullated nerves_ are divided into two kinds: medullated nerves with a neurolemma, and medullated nerves without a neurolemma.

Medullated nerves with a neurolemma consists of an axone, a medullary sheath, and a neurolemma. A delicate membrane called an axolemma, or periaxial sheath envelops the axone. The medullary sheath called myelin, is semifluid, somewhat resembling fat. The outer covering is the neurolemma, or sheath of Schwann. It is a delicate, structureless membrane which incloses the myelin. There is under this sheath an occasional oval nucleus. At intervals there are constrictions, or nodes, called the constrictions of Ranvier. The part between the nodes is called the internode.

The medullated axones without a neurolemma are the medullated nerve fibers of the central nervous system.

The non-medullated axones, or _non-medullated nerve fibers_, are divided into non-medullated axones with and those without, a neurolemma.

The non-medullated axone without a neurolemma is merely a naked axone. They are confined to the gray matter and to the beginnings and endings of sheath axones, all the latter being uncovered for a short distance after leaving the nerve cell body and also just before reaching their terminations.

The long axones serve to make connections with the peripheral, or distant, nerve cell, muscle cell, or gland cell; while the shorter axones of certain neurones divide into terminal branches in the immediate vicinity of its cell body, presumably to come into relation with other nerve cells in the same or adjacent groups.

FIG. 77.—Histological structure of tissues.

1. Non-striated or involuntary muscle cell. _a._ 1, Nucleus; 2,
Protoplasm. _b._ Transverse section showing nuclei in the center of
the cells that are cut through the nuclear zone. _c._ A
cross-section of the connective tissue which binds together the
muscle cells. _d._ A section showing the so-called intercellular
bridges.

2. A cross-section of the rectus abdominalis. 1, The endomysium which
binds the cells into primary fasciculi or bundles. 2, The perimysium
which surround the bundles of fasciculi. 3, The epimysium which
surrounds the muscle. These binding structures are white fibrous
connective tissue.

3. A longitudinal section at the juncture of a muscle and tendon. _a_,
The juncture between the muscle and tendon. Note the many nuclei in
both muscle and tendon. This is voluntary or striated muscle which
make up the dermal, dermo-osseous and skeletal muscles.

4. A longitudinal section of heart muscle. _a_, Connective-tissue
cell. _b_, Nucleus of a muscle cell. _c_, Cement line between the
muscle discs. _d_, The cell or segment.

5. A bipolar ganglionic nerve cell. _a_, The nucleus. _b_, The
nucleolus. _c_, The fibrillar structure. _d_, The medullary sheath.

6. A diagram showing the scheme of the peripheral nerve trunk. _a_,
The neuraxis of the peripheral sensory neurone. _b_, The spinal
ganglion of the superior or sensory root. _c_, The dendrite or
peripheral nerve fiber of the sensory nerve. _d_, The nerve trunk.
_f_, The sympathetic nerve ganglion connected with the spinal cord
through the white and the gray ramus communicans. _e_, The neuraxis
of the sympathetic neurone. _g_, Neuraxis or trunk of the motor
neurone or nerve cell. _h_, The anterior horn of the gray matter of
the spinal cord.

7. A diagram of a peripheral sensory neurone. _a_, The neuraxis which
ends in the spinal cord or brain. _b_, The T-shaped division of
Ranvier. _c_, The dendrite or sensory nerve fiber in the nerve
trunk. _d_, The nucleus and nucleolus of the cell _e._, _f._ the
axis cylinder process of the cell. _g_, The telodendrions or
terminal branches of the dendrite or axis cylinder.

8. A schematic diagram of the sensory motor reflex. _a_, The
telodendria. _b_, The dendrite. _c_, Nerve cell of the motor
neurone. _d_, The motor neurone. _f_, The muscle fiber. _g_, The
neuraxis of both sensory and motor neurones, the upper being the
sensory. _h_, The nerve cell in the sensory ganglion. _i_, The
sensory neurone or axis cylinder (nerve fiber). _j_, The skin with
peripheral telodendrion of sensory neurone.
]

_Neurones_ are devoted to the maintenance of functions. Reproductive neurones are so arranged as to receive afferent nerve impulses from other tissues; emissive neurones give off efferent nerve impulses. The former are sensory neurones; the latter are motor neurones if connected with muscles, and excito-glandular if connected with gland cells. The basis, then, of the nerve system is a series of neurones, with projecting and association processes, coördinated for the purpose of performing specific actions manifested either by motion, by trophic changes, or by the apperception of stimuli of a chemic, mechanic (tactile and auditory), thermal, or photic nature.

The whole of the nerve structure is composed of the nerve tissue and supporting connective tissue. The neurones constitute the nerve tissue, while the supportive tissue is composed of neuroglia and of white fibrous tissue derived either from the investing membrane or from the sheaths of its numerous vascular channels.

The neurones, or nerve cells, exhibit marked variations as to external characters, dimensions, and form. The neurone presents a swollen cell mass and a nucleus; it is known as the ganglion cell. From this cell body are given off a number of processes of two distinct kinds: first, protoplasmic processes, which are commonly branched, called dendrites; second, a single, thinner, and paler process; the axis cylinder process.

The bodies of cells vary in size from 4 to 100 microns or more in diameter. The largest cells occur in the inferior horn of the spinal cord, in the spinal ganglia, in the large pyramidal cell layer of the cerebral cortex, in the Purkinjean cell layer of the cerebellum, and in Clark’s column of the spinal cord. Very small cells occur in the olfactory bulbs, in the granular layer of the cerebral and cerebellar cortex, and in the gliosum cornuale of the cord.

Nerve cells are classified according to the number of processes arising from the cell body, and neurones are referred to as unipolar, bipolar, and multipolar.

_Unipolar cells_ are met with frequently in early stages of embryonic development, but are rare in adults, occurring only in the retina, in the olfactory bulb, and within the baskets of the Purkinjean cells of the cerebellum. The cells of the cerebro-spinal ganglia, except the cochlear and vestibular, are apparently unipolar, but they are developmentally and functionally of bipolar nature.

_Bipolar cells_ are found almost exclusively in the peripheral sensory system, as in the olfactory membrane, in the retina, in the cochlear and vestibular ganglia, and in the cerebro-spinal ganglia of the embryo.

_Multipolar cells_ are the most numerous and form the principal elements 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

The _medulla oblongata_ (Fig. 62, No. _C_, 1; Fig. 75, _A_ and _C_) terminates anteriorly in the pars commissuralis and the pars peduncularis. As the spinal cord approaches the head there is a gradual swelling, or lateral thickening, which merges into the medulla oblongata. The superior and the inferior surface of the medulla oblongata (Fig. 75, No. _A_, 1 and _C_, 17) are flattened. There is a shallow furrow and a slight swelling at the point where the hypoglossal nerve emerges. The central canal of the spinal cord gradually comes closer to the upper surface and communicates with the fourth ventricle, at which point the posterior raphe is shortened and the sulcus longitudinalis posterior becomes shallow. At the point where the central canal terminates in the fourth ventricle there is a V-shaped point called the _calamus scriptorius_ (Fig. 75, No. 2). The _fourth ventricle_ is located on the upper wall of the medulla oblongata below the cerebellum, and is bounded laterally by the peduncles of the cerebellum. It is marked posteriorly by the calamus scriptorius and anteriorly by the valve of _Vieussens_.

The _valve_ of _Vieussens_ is located at the posterior end of the aqueduct of Sylvius. The posterior part of the fourth ventricle is marked by grooves or furrows. Extending along the central part of the floor of the fourth ventricle there is a sulcus, or groove, called the sulcus centralis, which divides the superior pyramids. On the median lateral sides there are two points of gray substance which form the alæ cinereæ. The roots of the pneumogastric, the glosso-pharyngeal, and the spinal accessory may be traced to the alæ cineræ and the gray matter of the ridges of the medulla oblongata. The sixth pair of the cranial nerves emerge from the medulla near the sulcus centralis and to the side of the auditory nerve. Part of the trigeminus emerges from the rim of the furrow next to the ridge. At the outer border of the medulla are observed the thick ends of the roots of the pneumogastric, the spinal accessory, the glosso-pharyngeal, and the auditory facialis.

On both sides of the sulcus longitudinalis inferior are found the _inferior pyramids_, or _pyramidal columns_. These pyramids become expanded near the origin of the third pair of nerves and merge into the cerebral peduncles, or crura cerebri.

The pyramidal fibers may also be traced to the optic lobes. The roots of the abducens are found at a point between the crura cerebri and the optic lobes. The third pair lies to the side of this, and the roots of the trigeminus are adjacent to those of the third. A bundle of fibers from this region pass into the cerebellum and form the crura cerebelli; others pass into the cerebellum from the side, spreading out in fan-shaped radiation, and forming the white central substances peculiarly arranged, called the arbor vitæ, or _tree of life_. A third bundle fuse with the crura cerebelli anteriorly, and pass into the peduncles of the cerebrum, or crura cerebri.

The gray ganglionic substance forms columns which extend the whole length of the medulla oblongata and into the cerebri. The medulla oblongata contains numerous centers which preside over various visceral functions as deglutition, respiration, thermotactic, secretory, cardiac, and digestion.

In the medulla as in the spinal cord there are five main groups of cells. First, the posterior or upper horns, from which come the somatic sensory nerves. Second, Clark’s cells, located centrally, which are the origin of the ganglionated splanchnic nerves. Third, cell groups of the lateral horns which are the center for the non-ganglionated splanchnic nerves, and certain other nerves for the viscera, including the enteric muscles. Fourth, cell groups of the anterior horns, which are the center of all somatic voluntary muscles. Fifth, groups of single cells probably belonging to the posterior horns which are probably centers for other splanchnic nerve fibers.

FIG. 77, _A_.

_G_, A section through the tuberculum bigeminum. 1, Pia mater. 2,
Nerve fibers. 3, Fine granular ground substance. 4, Thin layer of
small cells. 5, Fine granular ground substance. 6, A second thin
cell layer. 7, Fine granular ground substance. 8, Third thin cell
layer. 9, Fine granular ground substance. 10, Fourth widely extended
cell layer. 11, A fine granular layer. 12, A thick layer of spindle
cells. 13, A layer of medullated nerve fibers. 14, Ependemal cells.
15, The sinus.

_H_, A transverse section through the medulla oblongata, 1, 3, 4 and 6
are ganglionic centers. 2, The dorsal groove. 8, The ventral
fissure. 5, The central canal. 7, Fiber tracts.

_I_, A transverse section through the cervical portion of the spinal
cord. 1, Ventral septum. 2, Dorsal septum. 3, The central canal. 4,
The dorsal horn. 5, The ventral horn.

_J_, A section through the wall of the oil gland, or rump gland. 1,
The stroma. 2, The tubular glands some of which are branched.