filaments from both roots, and possessing, as will be immediately shewn perfectly different functions. These divisions, of which the anterior is considerably the larger, proceed to the anterior and posterior parts of the body respectively, and are

P

Fig. L-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 Pneu mogastric, 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

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. 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). 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

This

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 communi

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

of the Neck, and shews, inter alia, the Nerves going to the Tongue:

1, portion of temporal bone, shewing the external auditory meatus and inastoid 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

from the ganglionic root, while the third-the inferior maxillary-is composed of fibres from both roots, and is therefore a compound nerve. From the mode of distribution, as well as from that of origin, it is inferred that the ophthalmic and superior maxillary are purely sensory, while the inferior maxillary is a motor and sensory nerve. (We have not inserted a special figure of this complicated nerve; the frontal branch of the ophthalmic division is, however, shewn in No. 4, fig. 2, while the infraorbital branches of the superior maxillary division, and the mental branches of the inferior maxillary division, are shewn in Nos. 6 and 8 of the same figure; while the gustatory or lingual branch of the last-named division is shewn in No. 17, fig. 3. The nasal branches also shewn in one of the diagrams! illustrating the article NoSE.) Experiments on living animals confirm the inference that have been drawn on anatomical grounds. Division of the ophthalmic or of the superior maxillary nerve, induces loss of sensibility without any serious impairment of muscular power; but when the inferior maxillary nerve, on either side, is divided, the power of mastication is destroyed on that side, and the sensibility of the tongue and of the lower part of the face on that side is lost.

The lingual or gustatory branch of the inferior maxillary is distributed to the mucous membrane and papillæ at the fore part and sides of the tongue, where it acts both as a nerve of common sensibility and of taste. (The consideration of the respective parts which this nerve and the glossopharyngeal play in the sense of taste, is considered in the articles TONGUE and SENSE OF TASTE.)

The trifacial nerve is the seat of the affection known as tic-douloureux, and described in the article NEURALGIA. It is in the dental branches of this nerve that toothache is situated; and in the process of teething in young children, the irritation of these branches, consequent upon the pressure of the teeth, often gives rise to convulsions, by being conveyed to the medulla oblongata, and exciting motor nerves by reflex action.

The Glossopharyngeal Nerve is principally an afferent or sensory nerve, but has a small motor root. It escapes from the cranium in association with the pneumogastric and spinal accessory nerves, through the same foramen as that through which the jugular vein emerges. It then descends by the side of the pharynx, and after anastomosing with the facial and pneumogastric nerves, and giving off a branch to the tympanum of the ear, terminates in branches to the mucous membrane of the base of the tongue, of the palate, tonsils, and pharynx, and in twigs to the digastric and stylopharyngeal muscles; so that its distribution is almost entirely to sentient surfaces (see fig. 3, No. 20). From a careful examination of the investigations of Dr John Reid and others regarding the functions of this nerve, Todd and Bowman arrive at the following conclusions: 1. It is the sensitive nerve of the mucous membrane of the fauces and of the root of the tongue, and in the latter situation it ministers to taste and touch, as well to common sensibility; and being the sensitive nerve of the fauces, it is probably concerned in the feeling of nausea, which may be so readily excited by stimulating the mucous membrane of this region.' 2. Such are its peripheral organisation and central connections, that stimulation of any part of the mucous membrane in which it ramifies, excites instantly to contraction all the facial muscles supplied by the pneumogastric and the facial nerves; and the permanent irritation of its peripheral ramifications, as in the case of sore throat, will affect other muscles supplied by the facial nerve likewise. It is therefore an excitor of

the movements necessary to pharyngeal deglutition.' -Op. cit. vol. ii. p. 119.

The Pneumogastric Nerve, or Par Vagum, is dis. tributed to so many important organs (the larynx, heart, lungs, stomach, &c.), and is of such great physiological importance, that a special article is devoted to its consideration.

The Spinal Accessory Nerve is more remarkable for its peculiar course than in any other respect. It rises from the spinal cord at the level of the fifth or sixth cervical nerve, passes upwards between the anterior and posterior roots of the cervical nerves into the skull, and emerges from the cranial cavity with the two preceding nerves. It is chiefly distributed to the trapezius muscle. See Fig. 2, No. 15.

In the above remarks on the cranial nerves, we have omitted all notice of their points of origin, as that subject is sufficiently noticed in the article BRAIN. We shall now briefly notice the mode in which the extremities receive their nerves. These nerves are derived from the spinal nerves, through the intervention of what is termed in anatomy a plexus. Four or five nerves proceed from the spinal cord for a certain distance, without any communication with each other. They then divide, and from the conjunction of the adjacent branches new nerves result, which again subdivide and interchange fibres. From the net-work or plexus thus formed nerves emerge, each of which is composed of fibres derived from several of the original branches. The most important of these plexuses are found in the regions of the neck, the axilla, the loins, and the sacrum, and are known as the cervical, brachial, lumbar, and sacral plexuses. The Brachial Plexus is formed by communication

1,

Fig. 4. A diagram shewing the Brachial Plexus of Nerves of the left side, with its branches. Front view. the brachial plexus; 2 and 3. the anterior and posterior thoracic nerves; 4, the phrenic nerves going to the diaphragm; 7 and 9, the external and internal cutaneous nerves; 10, the origin of the median nerve (which receives its name from taking a course along the middle of the forearm to the palm of the hand); 12 and 13, branches of this nerve; 14, the point at which it passes under the annular ligament, and divides into its terminal branches, which are distributed to the thumb and to all the fingers except the little finger and the outside of the ring-finger, which are supplied by 15) the ulnar nerve, whose terminal branches are shewn at 18; 19, the musculospiral nerve (the largest of the plexus); 23, 24, the radial nerve, one of the branches of the musculo-spiral.

17

181

between the anterior roots of the last four cervical nerves and the first dorsal nerve. These nerves are

nearly equal in size, and their mode of distribution is sufficiently explained by the diagram. The branches emerging from this plexus supply the shoulder and the arm; and the names of the most important of these branches are given in the description attached to the figure.

The Lumbar and Sacral Plexuses, with the nerves of the lower extremity, are shewn in fig. 5. The

20

Fig. 5.-A diagram shewing the Lumbar and Sacral Plexuses, with the Nerves

of the lower extremity. 1, the first four lumbar nerves which, with the branch from lumbar plexus; 2, the four upper sacral nerves, which, with the last lumbar, form the sacral plexus: 6, the ante rior crural or temoral nerve; 7, 8, 9, 10, its branches; 11, Its terminal branch, the long or internal saphenous; 13, the gluteal nerve; 15, the lesser

the last dorsal, form the

ischiatic nerve; 16, the greater ischiatic or sciatic nerve (the largest nerve in the body), dividing at about the lower third of the thigh, into 17, the popliteal nerve, and 18, the peroneal nerve; 19, muscular branches of the popliteal, given off in the posterior region of the knee; 20, the posterior tibial nerve, dividing, at 21, into the internal and external plantar nerves, which are distributed to the sides of the toes, in precisely the same manner as the median and ulnar nerves are distributed

to the fingers; 22, the external saphenous nerve; 23 and 24, the two terminal branches of the peroneal nerve -viz. the anterior tibial and the musculo-cutaneous nerves.

description attached to the diagram sufficiently explains the mode of formation and the distribution of the branches of these plexuges.

The general arrangement of the sympathetic system, or, as it is sometimes termed, the sympathetic nerve, has been already noticed at the beginning of this article. Its cephalic portion consists of four ganglia on either side-viz., (1) the Ophthalmic, or Lenticular Ganglion; (2) the Spheno-palatine, or Meckel's Ganglion; (3) the Otic, or Arnold's Ganglion; and (4) the Submaxillary Ganglion. They are all closely connected with the branches of the trifacial nerve. The cervical portion contains three ganglia, the dorsal twelve, the lumbar four, the sacral five, and the coccygeal one, which, instead of lying on the side of the vertebral column, is placed in front of the coccyx, and forms a point of convergence for the two ganglionated cords which run from the cervical to the sacral region parallel to one another. Each ganglion may be regarded as a distinct nervous centre, from which branches pass off in various

directions. In addition to the cords of communica tion between the ganglia, certain sets of nerves may be usually traced-viz. (1) visceral nerves, which generally accompany branches of arteries to the viscera (the lungs, heart, kidneys, liver, spleen, and intestine, &c.); (2) arterial branches, distributed to arteries in the vicinity of the ganglia; and (3) branches of communication with the cerebral and spinal nerves, an example of which is shewn in fig. 1.

The distribution of the sympathetic nerve on the right side is shewn in fig. 6. The only nerve that our limited space will permit us to notice is the great splanchnic. This nerve arises by separate roots from the 5th, 6th, 7th, 8th, and 9th thoracic ganglia. These roots (see the figure) unite to form a large round cord, which passes obliquely downwards and forwards, and after entering the abdomen by piercing the diaphragm, ends in a large and complex ganglion, the semilunar ganglion, which lies upon the side and front of the aorta, at the origin of the coeliac axis. The semilunar ganglia, with the nerves entering and emerging from them, combine to form the solar plexus, which, from the mass of nervous matter which it contains, has been termed the abdominal brain. It is in consequence of the existence of this great nervous centre, that a blow in the region in which it lies always inflicts a severe nervous shock, and not unfrequently causes death.

Experiments and clinical observations lead to the conclusion, that the sympathetic system supplies motor power to many of the internal viscera, especially the heart and the intestinal canal; that it also contains sensitive fibres, as is shewn by the sufferings of patients during the passage of a gall-stone or a renal calculus through a duct, whose sole nervous energy is derived from this system; that it presides over the process of secretion in the most important glands; and that it operates on the blood-vessels in causing them to contract, while the cerebro-spinal nerves produce the opposite effect.

On examining different parts of the nervous system under the microscope, we find that the nervous matter is distributed in two forms, the vesicular and the fibrous. The vesicular matter is gray in colour, and granular in texture, contains nucleated nerve cells, and is largely supplied with blood; it is immediately associated with mental actions, and is the seat in which the force manifested in nervous action originates. The fibrous matter is, in most parts, white and composed of tubular fibres, though in some parts it is gray and consists of solid fibres; it is less vascular than the former, and is simply the conductor of impressions made upon it. When these two kinds of matter are united together into a mass they form a nervous centre, such as the brain or spinal cord, while the nerves passing to and from them are composed of threads of fibrous matter. The nervous matter of both kinds is a soft, unctuous substance, with very slight tenacity; the softness being in a great measure due to the large quantity of water which it contains.

The fibrous form is the most extensively diffused throughout the body. It forms a large portion of the nervous centres, and is the main constituent of all the nerves. It occurs in two varieties -viz. as the tubular fibre, or the nerve tube, and the gelatinous fibre, the latter being of comparatively rare occurrence, and being found chiefly in the sympa thetic system.

When a tubular fibre is viewed by reflected light, it presents a beautiful pearly lustre, and appears to be homogeneous. But if viewed by transmitted light, with a sufficient magnifying power, indications of structure become visible. Externally, there is the