NEPOMUC. See JOHN OF NEPOMUK.

NE'POS, CORNELIUS, a Roman historian, born in the 1st c. B. C., but the place and precise time of his birth are unknown. He was the friend of Cicero and Catullus. The only work of N.'s which has survived (if indeed it be his), is a series of twenty; five generally brief biographies of warriors and statesmen, mostly Greeks. These biographies are distinguished by the purity of their Latinity, the conciseness of their style, and their admirable exhibition of character, but sufficient care has not been exercised in the examination of authorities, nor is the relative importance of things duly regarded. Until the middle of the 16th c., these biographies, on the strength of the titles given in the various MSS., were generally ascribed to Æmilius Probus, a writer who lived in the latter part of the 4th c.; but in 1569, an edition was put out by the famous Dionysius Lambinus, who pronounced the so-called Lives of Æmilius Probus to be in reality the lost work of Cornelius Nepos, De Viris Illustribus. His weightiest argument is drawn from the excellence of the Latin, and the chastity of the style, so unlike the corrupt and florid language of the Decline. Many critics hold that these Lives ought to be regarded as an abbreviation of the work of N. by Probus. This hypothesis is not without its difficulties, but it is perhaps the least objectionable of any. There are many editions, among which may be mentioned those of Van Staveren (Leyd. 1773), of Tzschucke (Gott. 1804), and of Bremi (Zur. 1820); and the book is in general use as a school-book. It has been very frequently translated into English and other languages.

NEPTUNE, an ancient Italian god. It is doubtful whether he was originally a marine deity at all,

Neptune.

approach, while the monsters of the deep, recognising their lord, made sportive homage round his watery path. But he sometimes presented himself at the assembly of the gods on Olympus, and in conjunction with Apollo, built the walls of Troy. In the Trojan war he sided with the Greeks; nevertheless he subsequently shewed himself inimical to the great sea-wanderer Ulysses, who had blinded his son Polyphemus. He was also believed to have created the horse, and taught men its use. The symbol of his power was a trident, with which he raised and stilled storms, broke rocks, &c. According to Herodotus, the name and worship of Poseidon came to the Greeks from Libya. He was worshipped in all parts of Greece and Southern Italy, especially in the seaport towns. Isthmian games were held in his honour. Black and white bulls, boars, and rams were offered in sacrifice to him. with a trident, and with horses or dolphins, often N. was commonly represented along with Amphitrite, in a chariot drawn by dolphins, and surrounded by tritons and other

sea-monsters.

The

over which he ruled, he is sometimes figured asleep As befitted the fluctuating element or reposing, and sometimes in a state of violent agitation.

NERBU'DDAH, a river of Hindustan, rises in the Vindhya Mountains, at a height of from 3000 to 4000 feet above sea-level, in lat. 22° 40′ N., long. 81° 52′ E. It flows west, past Jabalpur (190 miles from its source), where the great depression between the Vindhya Mountains on the north and the Satpura Mountains on the south, known as the Valley of the N., begins. The other principal towns on its banks are Hoshangabad, Burwani, and Barneh. At Hoshangabad it is 900 yards wide, and from five to six feet in depth. At Barneh it begins to expand into a wide estuary, and after flowing 30 miles further, it falls into the Gulf of Cambay. Entire length about 800 miles, of which 90 miles are navigable for river-boats.

NERCHI'NSK, an important mining town of Russia, Eastern Siberia, in the Trans-Baikal Territory, on the Nercha, a tributary of the Shilka, in lat. 51° 58' N., long. 116° 35' E., 4707 miles from St Petersburg. It was founded in 1658, and has from 4000 to 5000 inhabitants. The district, of which N. is the centre, yields 2166 pounds of gold yearly, together with large quantities of silver, lead, and iron, and precious stones. The only tinmines in the empire are worked here. The soil in the vicinity is fertile, and the climate mild and agreeable.

NEREIS, a genus, and NEREIDE, a family of Annelida, having a long slender body, a distinct with tentacles and eyes; the whole body

5

Nereis.

covered with tubercles, and the gills lobed and tufted. They are all marine, and generally hide under rocks or in the sand. They swim actively,

by rapid and undulating inflections of the body, and by the aid of numerous oars arranged along the sides; each formed of a stout footstalk, numerous bristles, and a flap. The proboscis is thick, strong, and armed with two jaws.

NE'REITES, the name given to animals which have left their impress on the Silurian Rocks, and which exhibit a form similar to the modern Nereis. They occur on the surface of the laminae of fine shales, over which, when it was soft, the creature moved, leaving a long and tortuous trail, which is generally found to terminate in a more defined representation produced apparently by the body itself, although every trace of it has disappeared. See ICHNOLOGY, fig. 2.

NERI, PHILIP DE, a saint of the Roman Catholic Church, and founder of the Congregation of the Oratory (q. v.), was born of a distinguished family in Florence, July 21, 1515. His character, even in boyhood, foreshadowed the career of piety and benevolence to which he was destined, and he was commonly known among his youthful companions by the name of 'good Philip.' On the death of his parents, he was adopted by a very wealthy uncle, with whom he lived for some time at San Germano, near Monte Cassino, and by whom he was recog. nised as his destined heir. But he relinquished all these prospects, for a life of piety and charity, and having come to Rome in 1534, he there completed his philosophical and theological studies, and won the esteem and reverence of all by his extraordinary piety, and his benevolence and activity in every good work whether of charity or of religion. Although he did not receive priest's orders till 1551, he had already been for years one of the most earnest and devoted in all the pious works of Rome for the instruction of the poor, the care of the sick, and the reclamation of the vicious; and in 1550, in unison with several of his friends, he established a confraternity for the care of poor pilgrims visiting Rome, and other houseless persons, as well as of the sick generally, which still subsists, and which has numbered among its associates many of the most distinguished members of the Roman Catholic Church. This confraternity, however, is chiefly noteworthy as having been the germ of the far more celebrated CONGREGATION OF THE ORATORY (q. v.), which was founded by St Philip in concert with his friends Baronius and Tarugio, both afterwards cardinals, Sabriati, and some others. Besides the general objects above indicated, and the spiritual duties designed for the personal sanctification of the members, the main object of this association was the moral instruction and religious training of the young and uneducated, who were assembled in chapels or oratorios, for prayer and for religious and moral instruction. As a further means of withdrawing youth from dangerous amusements, sacred musical entertainments (thence called by the name of oratorio) were held in the oratory, at first consisting solely of hymns, but afterwards partaking of the nature of sacred operas or dramas, except that they did not admit the scenic or dramatic accompaniments of these more secular compositions. Religious and literary lectures also formed part of his plan, and it was in the lectures originally prepared for the Oratory that, at the instance of N., the gigantic Church History of Baronius had its origin. The personal character of N., the unselfish devotedness of his life, his unaffected piety, his genuine love of the poor, his kindly and cheerful disposition, and, perhaps, as much as any of the rest, a certain quaint humour, and a tinge of what may almost be called drollery which pervaded many of his sayings and doings, contributed to popularise his institute,

and to engage the public favour for himself and his fellow-labourers. He himself enjoyed the reputation of sanctity and of miracles among his fellowreligionists almost beyond any of the modern saints; and he may still be described as emphatically the popular saint of the Roman people. He lived to an and in the active discharge to the last of all the extreme age in the full enjoyment of all his faculties, charitable duties to which his life had been devoted. He died at the age of 80, May 26, 1595. He was canonised by Gregory XV. in 1622. His only literary remains are his Letters (8vo, Padua, 1751); the Constitutions of his congregation, printed in 1612; which are printed in the collection of Rime Oneste. some short spiritual treatises, and a few sonnets

NE'RIUM. See OLEANDER.

His

NE'RO, Roman emperor from 54 A.D. to 68 A.D., was born at Antium, on the coast of Latium, 15th December 37 A.D., and was the son of Cn. Domitius Ahenobarbus and of Agrippina, the daughter of Germanicus Cæsar, and sister of Caligula. mother becoming the wife of the Emperor Claudius, Claudius adopted him (50 A.D.), and his name, originally L. Domitius Ahenobarbus, was changed to Nero Claudius Cæsar Drusus Germanicus. After the death of Claudius (54 A.D.), the Prætorian Guards, at the instigation of Afranius Burrhus, their prefect, declared him emperor, instead of Claudius's son Britannicus, and their choice was acknowledged both by the senate and the provinces. His reign began with the semblance of moderation and good promise, under the guidance of Burrhus and his tutor Seneca the philosopher; but the baleful influence of his mother, together with his own moral weakness and sensuality, frustrated their efforts, and he soon plunged headlong into debauchery, extravagance, and tyranny. He caused Britannicus, the son of Claudius, to be treacherously poisoned at the age of 14, because he dreaded him as a rival, and afterwards (59 A.D.) caused his own mother Agrippina (with whom he was latterly on bad terms) to be assassinated, to please his mistress Poppea Sabina (the wife of his principal boon-companion Otho, afterwards emperor), in order to marry whom he also divorced and afterwards put to death his wife Octavia (aged 20), the sister of Britannicus. The low servility into which the Roman senate had sunk at this time, may be estimated from the fact that it actually issued an address congratulating the hateful matricide on the death of Agrippina. N. himself, on the other hand, confessed that he was ever haunted by the ghost of his murdered mother. The affairs of the empire were at this time far from tranquil. In 61 A.D., an insurrection broke out in Britain under Queen Boadicea, which was, however, suppressed by Suetonius Paulinus. The following year saw an unsuccessful war against the Parthians in Armenia. At home, matters were not much better. The emperor was lampooned in verse; the senate and priesthood, alike venal, were also satirised by audacious malcontents; Burrhus, a valuable friend, died; and even Seneca, though not a great moralist, out of his books, thought it only decent to remove from court. In July 64, occurred a great conflagration in Rome, by which two-thirds of the city were reduced to ashes. N. himself is usually believed to have been the incendiary. It is said that he admired the spectacle from a distance, reciting verses about the burning of Troy, but many scholars are doubtful whether he really had any hand in it. At all events he laid the blame on the Christians-that mysterious sect, who, like the Jews in the middle ages, were the cause of all otherwise inexplicable calamities, and persecuted them with great fury. Moreover, he rebuilt the

city with great magnificence, and reared for himself on the Palatine Hill a splendid palace, called, from the immense profusion of its golden ornaments, the Aurea Domus, or Golden House; and in order to provide for this expenditure, and for the gratification of the Roman populace by spectacles and distributions of corn, Italy and the provinces were unsparingly plundered. A conspiracy against him failed in the year 65, and Seneca and the poet Lucan fell victims to his vengeance. In a fit of passion he murdered his wife Poppaa, by kicking her when she was pregnant. He then proposed to Antonia, the daughter of Claudius, but was refused, whereupon he caused the too fastidious lady to be put to death, and married Statilia Messallina, after killing her husband. He also executed or banished many persons highly distinguished for integrity and virtue. His vanity led him to seek distinction as a poet, a philosopher, an actor, a musician, and a charioteer, and he received sycophantic applauses, not only in Italy, but in Greece, to which, upon invitation of the Greek cities, he made a visit in 67. But in 68, the Gallic and Spanish legions, and after them the Prætorian Guards, rose against him to make Galba emperor, and N. fled from Rome to the house of a freedman, Phaon, about four miles distant. The senate, which had hitherto been most subservient, declared him an enemy of his country, and the tyrant ended his life by suicide, 11th June 68. One is sorry to learn that such a wretch had a taste for poetry, and was skilled in painting and modelling.

NERVA, M. COCCEIUS, a Roman emperor, elected by the senate after the murder of Domitian, 18th September 96. He was born 32 A.D., of a family belonging to Narnia, in Umbria, and twice held the honour of consulship before his election to the dignity of emperor. He displayed great wisdom and moderation, rectified the administration of justice, and diminished the taxes; but finding himself, upon account of his advanced age, not vigorous enough to repress the insolence of the Prætorian Guards, he adopted M. Ulpius Trajanus, then at the head of the army of Germany, who succeeded him on his death, 27th January 98. After his decease, he obtained an apotheosis.

NERVOUS SYSTEM, THE, is composed in all vertebrated animals of two distinct portions or systems-viz., the cerebro-spinal and sympathetic or ganglionic.

The cerebro-spinal system includes the brain and spinal cord (which form the cerebro-spinal axis), and the cranial and spinal nerves. It was termed by Bichat the nervous system of animal life, and comprises all the nervous organs concerned in sensation, volition, and mental action.

The sympathetic system consists essentially of a chain of ganglia connected by nervous cords, extending from the cranium to the pelvis, along each side of the vertebral column, and from which nerves with large ganglionic masses proceed to the viscera and blood-vessels in the cavities of the chest, abdomen, and pelvis. It was termed by Bichat the nervous system of organic life, since it seems to regulate-almost or quite independently of the will the due performance of the functions of the organs of respiration, circulation, and digestion. The essential parts of the cerebro-spinal axis are described in the articles BRAIN, CEREBRUM and CEREBELLUM, and SPINAL CORD. The brain and spinal cord are covered and protected by three membranes or meninges, as they are frequently termed-viz., the dura mater, the arachnoid, and the pia mater. The dura mater is a strong fibrous membrane, which supplies the cranial bones with

710

blood in early life, and adheres firmly to their inner surface. It is less closely attached to the bony walls of the spinal canal. Inside the cranium it gives off processes (such as the falx cerebri, tentorium cerebelli, and falx cerebelli) which divide and support different parts of the brain; it gives a strong fibrous sheath to every nerve; and by splitting into two layers at certain points, it forms receptacles for venous blood, which are termed SINUSES (q. v.). The arachnoid (so called from its being supposed to be as thin as a spider's web) is a serous membrane, and, like all serous membranes, is a closed sac, consisting of a parietal and a visceral layer. The parietal layer adheres to the inner surface of the dura mater, to which it gives a smooth, polished appearance; while the visceral layer somewhat loosely invests the brain and spinal cord, from direct contact with which, however, it is separated by the intervention of the pia mater and some loose areolar tissue. In most regions there is an interval between the visceral layer of the arachnoid and the pia mater, which is called the sub-arachnoid cavity, and is filled during life by the cerebro-spinal fluid. This fluid, which varies in quantity from two to ten ounces, keeps the opposed surfaces of the arachnoid in close contact, and affords mechanical protection to the nervous centres which it surrounds, and guards them against external shocks. It is accumulated in considerable quantity at the base of the brain, where it serves for the protection of the large vessels and nerves situated there. In fracture of the base of the skull, the draining away of this fluid, often in very large quantity, through the external auditory It is doubtless secreted by the pia mater, which is meatus, is often one of the most significant symptoms. the immediate investing membrane of the brain and blood-vessels, held together by an extremely fine spinal cord. areolar tissue. It dips down between the convolutions and fissures of the brain, and is prolonged into the interior, forming the velum interpositum and the choroid plexuses of the fourth ventricle. It is by means of this membrane that the blood-vessels are conveyed into the nervous substance. We now proceed to notice the nerves connected with the cerebro-spinal centre or axis. These are usually described in two classes-the spinal and the cranial or encephalic. The former class consists of all those which arise from the spinal cord, and emerge from the spinal canal through the intervertebral foramina; while the latter includes those which arise from some part of the cerebro-spinal centre, and emerge through foramina in the cranium or skull.

This membrane consists of minute

The Spinal Nerves (exclusive of the spinal accessory nerve, which, from the fact that it emerges from the skull, is usually ranked among the cranial nerves) are thirty-one on either side, there being a pair for each pair of intervertebral foramina (whose formation is described in the article VERTEBRA AND VERTEBRAL COLUMN), and for the foramina between the atlas (the first or highest vertebra) and the occipital bone at the base of the skull. Every spinal nerve arises from the cord by two roots, an anterior and a posterior, of which the latter is distinctly the larger. Each root passes out of the spinal canal by a distinct opening in the dura mater. Immediately after its emergence, a ganglion is seen on the posterior root, and in the anterior surface of this ganglion the anterior root lies imbedded. Just beyond the ganglion, but not at all previously, the nervous fibres of both roots intermingle, and a compound nerve results. The trunk thus formed separates immediately after it has passed through the intervertebral canal into two divisions-the anterior and posterior-each of which contains

Fig. 1.-Roots of a Dorsal Spinal Nerve, and its union with the Sympathetic:

e, c, anterior fissure of the spinal cord; a, anterior root; p, posterior root with its ganglion; a', anterior division or branch; p', posterior branch; s, sympathetic; e, its double junction with the anterior branch of the spinal nerve by a white and a gray filament, the respective natures of which are subsequently described.-From Todd and Bowman. distributed to the skin and the muscles. The anterior branch communicates with the sympathetic nerve, as is shewn in the figure. The mode of connection of the roots of the nerves with the cord is noticed in the article SPINAL CORD. These nerves are arranged in classes, according to the regions of the spine in which they originate, and we thus speak of eight cervical, twelve dorsal, five lumbar, and six sacral nerves on either side.

The discovery of the separate functions of the anterior and posterior roots of the spinal nerves, which has been characterised as the first important step towards a right understanding of the physiology of the nervous system, was made by our distinguished countryman Sir Charles Bell, although there is reason to believe that Magendie, without any knowledge of Bell's experiments, arrived at similar conclusions at nearly the same time. The original experiments consisted in laying open the spinal canal in rabbits, and irritating or dividing the roots of the spinal nerves. It was observed that irritation of the anterior roots caused muscular movement, and that the posterior roots might be irritated without giving rise to any muscular action; while division of the posterior roots did not impair the voluntary power over the muscles. Hence it was inferred that the anterior roots were motor (or conveyed motive power to muscles), and the posterior roots not motor; but it was not fully determined what degree of sensibility remained in parts supplied from the divided roots. Numerous physiologists arrived at similar results to those of Bell; but the most conclusive experiments are those of Müller, who operated on frogs, in which, from the great width of the lower part of the spinal canal, the roots of the nerves can be exposed with great facility. In these experiments, it was found that irritation of the anterior root always excited muscular contraction, while no such effect followed irritation of the posterior root; that section of the anterior root caused paralysis (or loss of power) of

motion, while section of the posterior root caused paralysis of sensation; and that when the anterior roots of the nerves going to the lower extremity were cut on one side, and the posterior roots on the other, voluntary power without sensation remained in the latter, and sensation without voluntary motion in the former. The obvious conclusion to be derived from these experiments is, that the anterior root of each spinal nerve is motor, and the posterior sensitive. (In place of the terms sensitive and motor, the terms afferent and efferent are now frequently used. The functions of the nerves being to establish a communication between the nervous centres and the various parts of the body, and vice versâ; an afferent nerve communicates the impressions made upon the peripheral nervous ramifications to the centres, while an efferent nerve conducts the impulses of the nervous centres to the periphery.)

The Cranial Nerves, although twelve in number on either side, were arranged by Willis (Cerebri Anatome; cui accessit Nervorum Descriptio et Usus, 1664), whose system is still generally adopted, in nine pairs, which, taken from before backwards in the order in which they are transmitted through the foramina at the base of the skull, stand as follows: 1st, Olfactory; 2d, Optic; 3d, Motores Oculorum; 4th, Pathetic; 5th, Trifacial; 6th, Abducentes; 7th, Portio Dura or Facial, Portio Mollis or Auditory; 8th, Glossopharyngeal, Par Vagum or Pneumogastric, Spinal Accessory; 9th, Hypoglossal.

They may be subdivided into three groups, according to their functions-viz. Nerves of Special Sense-the Olfactory (see NOSE), Optic (see EYE), and Auditory (q. v.); Nerves of Motion, or Afferent Nerves-the Motores Oculorum, Pathetic, Abducentes, Facial, and Hypoglossal; and Compound Nerves-the Trifacial, Glossopharyngeal, Pneumogastric, and Spinal Accessory.

The reason why no nerve of Taste is included in the above arrangement amongst the nerves of special sense will be subsequently seen; and we proceed briefly to notice the functions of the motor cranial nerves.

The 3d, 4th, and 6th pairs-the Motores Oculorum, Pathetic, and Abducentes-together make up the apparatus by which the muscles of the orbit (the four Recti, the superior and inferior Oblique, and the Levator Palpebra) are called into motion, and are sufficiently noticed in the article EYE.

The Facial Nerve, or the Portio Dura of the 7th pair, is divisible into three stages. The first stage is the intercranial, from its origin to its exit from the cranial cavity, in association with the Portio Mollis, or Auditory Nerve (q. v.), at the internal auditory meatus. The second stage is contained in the Aqueduct of Fallopius, a bony canal lying in the petrous portion of the temporal bone. In this stage it anastomises with other nerves, and thus sensory fibres are introduced into it from the 5th pair and other sources, which make irritation of some of its branches to cause pain. The third stage commences with the emergence of the nerve through the stylo-mastoid foramen. The nerve now lies in the parotid gland (which is not shewn in the figure), and after giving off the posterior auricular, and a few smaller branches, finally divides into the temporal, facial, and cervical branches (see 3, 5, and 9 in fig. 2). This diverging distribution of the nervous branches over the face forms the pes anserinus of the older anatomists, from the supposed resemblance to the expanded foot of a goose. Careful dissection of this nerve shews that the great majority of its fibres are distributed to muscles; and indeed, if we except the muscles of mastication, which receive their motor power from the 3d division of the 5th

Fig. 2.-Distribution of the Facial Nerve and of the Branches of the Cervical Plexus:

1, the facial nerve at its emergence from the stylo-mastoid foramen; 3, temporal branches communicating with (4) the frontal branches of the fifth or trifacial nerve; 5, infraorbital branches, communicating with (6) the infra-orbital branches of the fifth nerve; 7, maxillary branches communicating with (8) the mental branch of the fifth nerve; 9, cervico-facial branches; 15, the spinal accessory nerve giving off a branch to the trapezius muscle.

the aspect of the countenance and the balance of the features depend. The power of closing the eyelids depends upon this nerve, as it alone supplies the orbicularis palpebrarum; and likewise that of frowning, from its influence upon the corrugator supercilii. Anatomy indicates that this nerve is the motor nerve of the superficial muscles of the face and ear, and of the deep-seated muscles within the ear. This conclusion is abundantly confirmed by comparative anatomy. For wherever the superficial muscles of the face are well developed, and the play of the features is active, this nerve is large. In monkeys it is especially so. That extremely mobile instrument, the elephant's trunk, is provided with a large branch of the facial as its motor nerve. In birds, on the other hand, it is very small.'-Todd and Bowman, Physiological Anatomy and Physiology of Man, vol. ii. p. 107.

Before Sir Charles Bell commenced his experiments on the functions of the nerves, it was believed that the facial was the nerve of sensibility of the face, and it was on several occasions divided with the view of relieving tic douloureux, of which it was supposed to be the seat. But the operation, of course, yielded no relief, and always inflicted a permanent injury, since it was succeeded by paralysis of the facial muscles, with total loss of control over the features and over the closing of the eye, on the side on which the operation was performed.

The treatment of facial palsy which is often, especially if it arises from cold, a very temporary affection, although usually a very alarming one to the patient and his friends, is described in the article PARALYSIS.

The Hypoglossal Nerve (derived from the Greek words hypo, under, and glotta, the tongue) escapes from the cavity of the skull by the anterior condyloid foramen, and passes outwards and forwards around

Fig. 3.-This figure illustrates the Anatomy of the Side of the Neck, and shews, inter alia, the Nerves going to the Tongue:

1, portion of temporal bone, shewing the external auditory meatus and mastoid and styloid processes; 5, the tongue; 13, the common carotid artery; 14, the internal jugular vein; 15 and 16, the external and internal carotids; 17, the gustatory branch of the fifth nerve; 20, the glossopharyngeal nerve; 21, the hypoglossal nerve; 22, the descendens noni; 24, the pneumogastric nerve, lying between the carotid artery and the jugular vein; 25, the facial nerve.

gives off the long anastomosing branch known as the Descendens noni.

Experiments on living animals, comparative anatomy, and pathological investigations, alike indicate that this is the motor nerve of the tongue. In cases of paralysis of this nerve, the power of articulation is much injured or totally destroyed; and this is often one of the first symptoms which lead the physician to apprehend serious cerebral lesion.

We now proceed to the consideration of the Compound Nerves, beginning with the Trifacial or Fifth Nerve, This nerve, as was first pointed out by Sir Charles Bell, presents a remarkable resemblance to the spinal nerves in its mode of origin; for it arises by two roots, one large and the other small, and on its larger root, as on the posterior and larger root of the spinal nerves, is a distinct ganglion; the two roots being quite distinct until after the formation of the ganglion, when the lesser one coalesces with the lowest branch, which emerges from the_ganglion to form the inferior maxillary nerve. This ganglion, which is known as the Gasserian Ganglion, and which is formed upon the larger root of the nerve, lies upon the upper surface of the petrous portion of the temporal bone, and is of a somewhat triangular form, with its base directed forwards and outwards. From this base there proceed three nerves-viz. the ophthalmic, on the inside; the superior maxillary, in the middle; and the inferior maxillary, externally. The first two of these nerves consist exclusively of fibres