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NERVE (Lat. nervus, Gr. vevpov, a bow...

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Originally appearing in Volume V19, Page 397 of the 1911 Encyclopedia Britannica.
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NERVE (Lat. nervus, Gr. vevpov, a bowstring), originally a sinew or tendon (and still so used in the phrase " to strain every nerve "), but now a term practically confined to the fibres of the nervous system in anatomy, though consequentially employed as a general psychical term in the sense of courage or firmness, and sometimes (but more usually " nervousness ") in the opposite sense. In the present article the anatomy of the nerves is dealt with; see also NERVOUS SYSTEM, MUSCLE AND NERVE, NEUROPATHOLOGY, &c. I. CRANIAL The cranial nerves are those which rise directly from the brain, and for the most part are concerned with the supply of the head. With one exception they all contain medullated fibres (see NERVOUS SYSTEM). Twelve pairs of these nerves are recognized, and they are spoken of as often by their numbers as by their names. The following is a list: (I) Olfactory; (2) Optic; (3) Oculo-motor or Motor oculi; (4) Trochlearis or Patheticus; (5) Trigeminal or Trifacial; (6) Abducens; (7) Facial; (8) Auditory; (9) Glosso-pharyngeal; (to) Vagus or Pneumogastric; (1r) Spinal accessory; (12) Hypoglossal. The first, or olfactory nerve, consists of the olfactory bulb and tract, which are a modified lobe of the brain and lie beneath the sulcus rectus on the frontal lobe of the brain (see fig. I). At its posterior end the tract divides to become continuous with the two extremities of the limbic lobe (see BRAIN), while at its anterior end is the bulb from which some twenty small non-medullated nerves pass through the cribriform plate of the ethmoid to supply the sensory organs in the olfactory mucous membrane (see OLFACTORY ORGAN). The second or optic nerve consists of the optic tract, the optic commissure or chiasma, and the optic nerve proper. The optic tract begins at the lower visual centres or internal and external geniculate bodies, the superior quadrigeminal body and the pulvinar (see fig. I), but these again are connected with the higher visual centre in the occipital lobe by the optic radiations (see fig. 2). In the chiasma some of the fibres cross and some do not, so that the right optic tract forms the right half of both the right and left optic nerves. In addition to this the fibres coming from the internal geniculate body of one side cross in the chiasma to the same body of the opposite side, forming Gudden's,commissure. The optic nerve passes through the optic foramen in the skull into the orbit, where it is penetrated by the central artery of the retina, and eventually pierces the scelerotic just internal to the posterior pole of the eyeball. Its final distribution is treated in the article EYE. The third or oculomotor nerve rises from a nucleus in the floor of the aqueduct of Sylvius (see BRAIN, fig. 8), and comes to the surface in a groove on the inner side of the crus cerebri (fig. I); it soon pierces the dura mater, and lies in the outer wall of the cavernous sinus, where it divides into an upper and lower branch. Both these enter the orbit through the sphenoidal fissure, the upper branch supplying the superior rectus and levator palpebrae superioris muscles, the lower the inferior and internal rectus and the inferior oblique, so that it supplies five of the seven orbital muscles. The fourth or trochlear nerve is very small, and comes from a nucleus a little lower than that of the third nerve. It is specially remarkable in that it crosses to the opposite side in the substance of the valve of Vieussens of the fourth ventricle, after which it winds round the outer side of the crus cerebri (fig. i) and enters the outer wall of the cavernous sinus to reach the orbit through the sphenoidal fissure. Here it enters the superior oblique muscle on its orbital surface. The fifth or trigeminal nerve consists of motor and sensory roots. The motor root rises from a nucleus in the upper lateral part of the floor of the fourth ventricle, as well as by a descending (mesencephalic) tract from the neighbourhood of the Sylvian aqueduct (see fig. 3). The large sensory root goes to a sensory nucleus a little external to the motor one, and also, by a spinal or descending root, to the substantia gelatinosa Rolandi as low as the second spinal nerve (see fig. 3). The su rficial origin of the fifth nerve is from the side of the pons (see f g. I), and the two roots at oncepass into a small compartment of the dura mater, in front of the apex of the petrous bone, known as Meckel's cave; here the large crescentic Gasserian ganglion is formed upon the sensory root, and from this the three branches come off, earning the nerve its name of trigeminal. The first of these divisions is the ophthalmic, the second the maxillary, and the third the mandibular, while the motor root only joins the last of these. The first or ophthalmic division of the fifth runs in the outer wall of the cavernous sinus, where it divides into frontal, lachrymal and nasal branches. They all enter the orbit through the sphenoidal fissure. The frontal nerve divides into supraorbital and supratrochlear, which pass out of the upper part of the anterior opening of the orbit and supply the skin of the forehead and upper part of the scalp as well as the inner part of the eyelids. The lachrymal nerve supplies that gland and the outer part of the upper eyelid. The nasal nerve gives off a branch to the ciliary or lenticular ganglion, which lies in the outer part of the orbit, and through which, as well as through its own long ciliary branches, it supplies the eyeball with sensation. It leaves the orbit through the anterior eth- moidal canal, and lies for a short distance on the cribriform plate of the ethmoid ; it then enters the nasal cavity through the nasal slit and supplies this cavity, as well as the surface of the nose as far as the tip, with ordinary sensation. The second or maxil- lary division of the fifth nerve leaves the skull through the foramen rctundum, and then runs across the roof of the spheno-maxillary fossa; here the spheno-maxillary or Meckel's ganglion hangs from it by two roots. The nerve then runs in the floor of the orbit, giving off superior dental branches, until it emerges on to the face at the infraorbital foramen, where it divides into palpebral, nasal and labial branches, the names of which indicate their distribution. The third or mandibular division of the fifth leaves the skull through the foramen ovale, and .at once gives off a set of motor branches for the muscles cf mastication; these are derived from the motor root of the fifth, except that for the buccinator, which really supplies only the skin and mucous membrane in contact with the muscle. After the motor branch is given off, the third division of the fifth divides into, lingual, inferior dental and auriculo-temporal. The lingual is joined by the chorda tympani branch of the facial nerve, and then passes to the anterior two-thirds of the tongue. In its course it passes deep to the submaxillary gland, and here the small sub- maxillary ganglion is connected with it by two roots. The inferior dental nerve gives off a small motor branch to the mylohyoid and posterior belly of the digastric muscles, and then enters a canal in the lower jaw, where it gives off twigs to all the lower teeth. A mental branch comes out through the mental foramen to supply the skin of the chin. The auriculo temporal nerve rises by two roots, which embrace the middle meningeal artery, and runs backward and then upward close to the lower jaw joint to supply the parotid gland, the skin on the outer side of the ear, and the side of the scalp. At its beginning it communicates with the otic ganglion, which lies just internal to it below the foramen ovale, and also receives a communication from the nerve to the internal pterygoid muscle. The sixth or abducent nerve rises from a nucleus in the floor of the fourth ventricle deep to the eminentia teres (see fig. 3). It appears on the surface of the brain just below the pons and close to the middle line (see fig. I), soon after which it pierces the dura mater and runs in the floor of the cavernous sinus to the sphenoidal fissure. Entering the orbit through this, it quickly supplies the external rectus muscle. The seventh or facial nerve begins in a nucleus which is about the same level as that for the sixth, but much deeper from the floor of the fourth ventricle as well as farther from the middle line (see fig. 3). The fibres of. the facial loop round the nucleus of the sixth, and then emerge in the triangular interval between the medulla, pans and cerebellum, close to the eighth nerve. and having the pars Oculo-motor nerve Trochlear nerve Trigeminal nerve Abducent new Facial nerve Pars intermedia Auditory new Glosso-pharyngeal nerve Vagus nerve Spinal accessory nerve (accessory) Spinal accessory nerve (spinal) Hypoglossal nerve Hypoglossal nerve Spinal cord Vermis of Cerebellum (cut) Olfactory tract Broca's area Olfactory tubercle Mesial root of olfactory erve Lateral root Optic chiasma Ant. perforated spot Temporal lobe (cut) Optic tract Oculo-motor nerve Trochlear nerve Taenia semicircularis Trigeminal nerve Ext. geniculate body Abducent nerve Int. geniculate body Pulvinar Facial nerve Pars intermedia Auditory nerve Lateral ventricle Mid. cerebellar peduncle Glosso-pharyngeal nerve Vagus nerve Spinal accessory nerve (accessory) Spinal accessory nerve (spinal) Occipital lobe (cut) From D. J. Cunningham, in Cunningham's Text-book of Anatomy. lobes, and the cerebellum on the left side removed, to show the origins of the cranial nerves. intermedia between (see fig. O. Entering the internal auditory meatus with these structures the facial nerve soon passes into a canal in the petrous bone known as the aqueductus Fallopii, and in this it makes a sudden bend and forms the geniculate ganglion, from which the great superficial petrosal branch to Meckel's ganglion is given off. The canal ends at the stylo-mastoid fora- men on the base of the skull, and here the nerve enters the parotid gland, in which it forms a plexus called the pes anserinus. From this, branches pass to all the muscles of the face except those of mastication. In the aqueduct the pars intermedia joins the seventh, and, beyond the geniculate ganglion, leaves it as the chorda tympani, o which runs through the tympanum (see EAR) to join the lingual branch of the fifth. It is probable that the pars intermedia, geniculate ganglion and chorda tym pani, represent the sensory root of the facial nerve. Just out-1O96 side the stylo- mastoid foramen From D. J. Cunningham, in Cunningham's Text-Book of the facial gives Anatomy. off the poslei for FIG. 2.-Diagram of the Central Connexions auricular branch of the Optic Nerve and Optic Tract. to the occipitalis and posterior auricular muscles, as well as a branch of supply to the etylohyoid and posterior belly of the digastric muscles. The eighth or auditory nerve is in two bundles, cochlear and vestibular. The former comes from the cochlear nuclei which lie deep to the acoustic tubercle in the floor of the fourth ventricle (see fig. 3), while the latter rises from the dorsal nucleus, nucleus of Deiters and the nucleus of the descending root, which are more deeply placed. The nucleus of Deiters is connected with the cerebellum, and is concerned in maintaining the equilibrium (q.v.) of the body, while, as is pointed out in the article BRAIN, the cochlear nuclei are connected with the inferior quadrisphenogeminal body o lodybe by by the the aulateralditory fillet as radiations. asons. with The the vestibular internal ts~~r,, ,E 7 niculate body, while this body again is connected with the her auditory centre in the grey cortex of the temporo- cocHLcon N TUB, wcusr,cuM ago so \ root passes in front of the restiform body (see fig. 3), and the P''An' cochlear behind that body. Together they enter the internal ruci.cus AMe auditory meatus, and, at the end of it, pierce the lamina cribrosa. the vestibular nerve supplying the utricle and superior and external semicircular canals, the cochlear nerve the posterior canal, the saccule and the cochlea (see EAR). The ninth or glossopharyngeal nerve is chiefly, if not entirely, sensory, and its deep termination in the brain is the solitary bundle (see fig. 3; and BRAIN, fig. 4). It appears on the surface between the olive and restiform body (see fig. I), and leaves the skull through the posterior lacerated foramen ; as it does so two ganglia, the jugular and petrous, are formed on it, after which It runs downward and forward, between the internal and external carotid arteries, and eventually reaches the back of the tongue (see TONGUE). On its way it supplies the tympanum, the stylopharyngeus muscle, though there is grave doubt as to whether these fibres are not really derived from the facial nerve, contributions to the pharyngeal plexus, the tonsil and part of the epiglottis. From D. J. Cunningham, in Cunningham's Text-Book of Anatomy. . The tenth nerve or vagus has sensory and motor fibres; the FIG. 3.-Deep Origins of Cranial Nerves from the Fourth Ventricle. former go to the solitary bundle mentioned in the description of the last nerve (see fig. 3), while the latter come from the dorsal nucleus and nucleus ambiguus, both of which are found deep to the lower half of the fourth ventricle. The nerve appears on the surface between the olive and restiform body and just below the ninth (see fig. I). It leaves the skull through the posterior lacerated foramen, and, like the glossopharyngeal, has two ganglia developed on it; the upper of these is the ganglion of the root, and the lower the ganglion of the trunk (see fig. 4). From the former the auricular branch or Arnold's nerve (see EAR) comes off, while from the latter are given off the pharyngeal branches to the pharyngeal plexus (fig. 4, Ph.) and the superior laryngeal branch which is the sensory nerve of the larynx (fig. 4, S.L.). Between the two ganglia the accessory part of the eleventh nerve joins the tenth, and it is from this communication that the motor twigs to the pharynx, larynx, alimentary and respiratory tracts are derived, as well as the inhibitory fibres of the heart. In the neck the vagus accompanies the carotid artery and internal jugular vein, and here it gives off superior and inferior cardiac branches. The left inferior cardiac branch passes to the superficial, while the three others go to the deep cardiac plexus. The nerve now enters the thorax, passing between the subclavian artery and vein. On the right side its recurrent laryngeal branch loops under the subclavian artery (fig. q., R.), and runs up to supply all the muscles of the larynx except one (see RESPIRATORY SYSTEM). In the thorax the left vagus passes in front of the arch of the aorta, under which the left recurrent laryngeal loops, and on both sides a thoracic cardiac branch is given to the deep cardiac plexus. Both vagi pass behind the root of their own lung, and break up to form the posterior pulmonary plexus after giving off some branches for the much smaller anterior pulmonary plexus; they then reach the oesophagus, where they again break up into an oesophageal plexus or plexus gulae. As the diaphragm is approached the two nerves become distinct again, but the left one now lies in front and the right behind the food tube, so that, when the stomach is reached, the left vagus supplies the front of the organ and communicates with the hepatic plexus, while the right goes to the back and communicates with the coeliac, splenic and renal plexuses. The eleventh or spinal accessory nerve is entirely motor, and consists of a spinal and an accessory part. The former rises from the anterior horn of the grey matter of the spinal cord as low as the fifth cervical nerve. Its fibres come to the surface mid-way between the anterior and posterior nerve-roots, and run up through the foramen magnum to join the accessory part, the deep origin of which is the lower part of the nucleus ambiguus. The accessory part, as has been noticed, joins the vagus, while the spinal part pierces the sterno-mastoid muscle and runs obliquely downward and backward across the posterior triangle of the neck to enter the trapezius; both these muscles are in part supplied by the nerve. The twelfth or hypoglossal nerve is motor, and rises from a nucleus in the floor of the fourth ventricle deep to the trigonum hypoglossi (see BRAIN, fig. 3). It emerges from the brain between the anterior pyramid and the olive (see fig. I), and leaves the skull in two bundles through the anterior condylar foramen. Soon after this it is closely bound to the vagus, and, in front of the atlas, receives an important contribution from the loop between the first and second cervical nerves. The nerve then passes downward until it reaches the origin of the occipital artery, round which it loops, and then runs forward on the surface of the hyo-glossus to the muscles of the tongue. As it bends round the occipital artery it gives off its descendens hypoglossi branch, which derives its fibres from the communication with the first cervical already mentioned. This branch runs down and forms a loop with the communicans cervicis branch from the second and third cervical nerves, and from this loop (ansa hypoglossi) many of the depressor muscles of the hyoid bone and larynx are supplied. Farther forward special branches are given off to the thyro-hyoid and genio-hyoid muscles, and these, like the descendens hypoglossi, are derived from the first and second cervical loop, thus leaving all the true muscles of the tongue to be supplied by the medullary part of the nerve. For the embryology and comparative anatomy of the cranial nerves, see NERVOUS SYSTEM. Il.SPINAL The spinal nerves are those which arise from each side of the spinal cord and are distributed to the trunk and limbs, though some of the upper ones supply the lower parts of the head and face. As is shown in the article NERVOUS SYSTEM, the division between cranial and spinal nerves is rather one of convenience than of any real scientific difference. There are generally thirty-one pairs of these nerves, which are subdivided according to the part of the vertebral column through which they pass out; thus there are eight cervical (abbreviated C.), twelve thoracic (Th.)—formerly called dorsal, five lumbar (L.), five sacral (S.) and one coccygeal (Coc.). As the thoracic nerves are the simplest and most generalized in their arrangement, a typical one of these, say the fourth or fifth, will be first described. The nerve is attached to the spinal cord by two roots, of which the ventral is purely efferent or motor and the dorsal purely afferent or sensory. On the dorsal root is a fusiform ganglion which lies in the foramen be- tween the vertebrae through which the nerve passes. The two roots then join together to form a mixed nerve (see fig. 5), but very soon divide once more into anterior (ventral) and posterior (dorsal) primary divisions. These, however, each contain sensory and motor fibres. Just before it divides in this way the mixed nerve gives and receives its rami communicantes with the sympa- thetic (see NER-
End of Article: NERVE (Lat. nervus, Gr. vevpov, a bowstring)
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