NERVE (Lat. nervus, Gr. νεῦρον, 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:—
(1) 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; (10) Vagus or Pneumogastric; (11) 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. 1). 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. 1), 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.
| From D. J. Cunningham, in Cunningham’s Text-Book of Anatomy. |
| Fig. 1.—View of the Under Surface of the Brain, with the lower portion of the temporal and occipital lobes, and the cerebellum on the left side removed, to show the origins of the cranial nerves. |
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. 1); 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. 1) 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 superficial origin of the fifth nerve is from the side of the pons (see fig. 1), and the two roots at once pass 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 ethmoidal 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 maxillary division of the fifth nerve leaves the skull through the foramen rotundum, 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 of 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 submaxillary 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. 1), 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, pons and cerebellum, close to the eighth nerve, and having the pars intermedia between (see fig. 1). 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.
| From D. J. Cunningham, in Cunningham’s Text-Book of Anatomy. |
| Fig. 2.—Diagram of the Central Connexions of the Optic Nerve and Optic Tract. |
The canal ends at the stylo-mastoid foramen 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, 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 tympani, represent the sensory root of the facial nerve. Just outside the stylo-mastoid foramen the facial gives off the posterior auricular branch to the occipitalis and posterior auricular muscles, as well as a branch of supply to the stylohyoid 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 quadrigeminal body by the lateral fillet as well as with the internal geniculate body, while this body again is connected with the higher auditory centre in the grey cortex of the temporo-sphenoidal lobe by the auditory radiations. The vestibular root passes in front of the restiform body (see fig. 3), and the cochlear behind that body. Together they enter the internal 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. 1), 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.
The tenth nerve or vagus has sensory and motor fibres; the 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. 1). 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. 4, 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.
| From D. J. Cunningham, in Cunningham’s Text-Book of Anatomy. |
| Fig. 3.—Deep Origins of Cranial Nerves from the Fourth Ventricle. |
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.
| From A. M. Paterson, in Cunningham’s Text-Book of Anatomy | |||
| Fig. 4.—The Distribution of the Pneumogastric Nerve. | |||
| Va.R, | Right vagi. | I.C, | Internal, and |
| Va.L, | Left vagi. | E.C, | External carotid arteries. |
| r, | Ganglion of the root and | Ca1, | Superior cervical cardiac |
| connexions with | branch. | ||
| Sy, | Sympathetic, superior | Ca2, | Inferior cervical cardiac branch. |
| cervical ganglion. | R.L, | Recurrent laryngeal nerve. | |
| G.Ph, | Glosso-pharyngeal. | Ca3, | Cardiac branches from |
| Acc, | Spinal accessory nerve. | recurrent laryngeal nerves. | |
| m, | Meningeal branch. | Ca4, | Thoracic cardiac branch |
| Aur, | Auricular branch. | (right vagus). | |
| t, | Ganglion of the trunk | A.P.Pl, | Anterior, and |
| and connexions with | P.P.Pl, | Posterior pulmonary | |
| Hy, | Hypoglossal nerve. | plexuses. | |
| C1, C2 | Loop between the first | Oes.Pl, | Oesophageal plexus. |
| two cervical nerves. | Gast.R, | and Gast.L, Gastric | |
| Sy, | Sympathetic. | branches of vagus (right | |
| Acc, | Spinal accessory nerve. | and left). | |
| Ph, | Pharyngeal branch. | Coe.Pl, | Coeliac plexus. |
| Ph.Pl, | Pharyngeal plexus. | Hep.Pl, | Hepatic plexus. |
| S.L, | Superior laryngeal nerve. | Spl.Pl, | Splenic plexus. |
| I.L, | Internal laryngeal branch. | Ren.Pl, | Renal plexus. |
| E.L, | External laryngeal branch. | ||
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. 1), 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.
II. 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.
| From A. M. Paterson, in Cunningham’s Text-Book of Anatomy |
| Fig. 5.—Scheme of the Distribution of a Typical Spinal Nerve. |
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 between 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 sympathetic (see Nervous System).
The anterior primary division, runs round the trunk, between the ribs, forming an intercostal nerve and giving off a lateral cutaneous branch, when the side of the body is reached, which divides into anterior and posterior secondary branches. The rest of the division runs forward, supplying the intercostal muscles, as far as the edge of the sternum, when it ends in an anterior cutaneous branch to the front of the chest. The dorsal primary division divides into an external (lateral) and internal (mesial) branch through which the skin and muscles of the back are supplied.
 | |||
| From A. M. Paterson, in Cunningham’s Text-Book of Anatomy. | |||
| Fig. 6.—The Distribution of Cutaneous Nerves on the front of the Trunk. On one side the distribution of the several nerves is represented, the letters indicating their nomenclature. | |||
| G.A., | Great auricular nerve. | I.H, | Intercostohumeral. |
| S.C, | Superficial cervical nerve. | I.C, | Internal cutaneous. |
| S.Cl, | Supraclavicular nerves. | M.S, | Cutaneous branch of musculo- |
| Acr, | Acromial. | spiral nerve. | |
| Cl, | Clavicular. | E.C, | External cutaneous nerve. |
| St, | Sternal. | G.C, | Genito-crural nerve. |
| T. 2-12, Lateral and anterior | M.C1,2., Middle cutaneous nerve. | ||
| branches of thoracic nerves. | I.C1, | Branch of internal cutaneous | |
| I.H, | Ilio-hypogastric nerve. | nerve. | |
| I.I, | Ilio-inguinal nerve. | P, | Branches of pudic nerve. |
| Circ, | Cutaneous branch of cir- | S.Sc, | Branches of small sciatic |
| cumflex nerve. | nerve. | ||
| L.I.C, | Lesser internal cutaneous nerve. | ||
It will be seen from the foregoing that the thoracic nerves are almost completely segmental in their distribution, in other words, each supplies a slice of the body, but in the other regions this segmental character is masked by the development of the branchial skeleton and the limbs. In the cervical region the first cervical or suboccipital nerve comes out between the occiput and atlas and does not always have a posterior root. When it has not, it obviously can supply no skin. Its anterior primary division joins those of the second, third and fourth cervical nerves to form the cervical plexus, from which the skin of the side of the neck and lower part of the head and face are supplied by means of the small occipital, great auricular, superficial cervical, suprasternal, supraclavicular and supra-acromial nerves (see fig. 7) as well as those muscles of the neck which are not supplied by the cranial nerves. The phrenic nerve, which comes chiefly from the fourth cervical, deserves special notice because it runs down, through the thorax, to supply the greater part of the diaphragm. The explanation of this long course (see Diaphragm) is that the diaphragm is formed in the neck region of the embryo. The posterior primary division of the second cervical nerve is very large, and its inner (mesial) branch is called the great occipital and supplies most of the back of the scalp (fig. 7). The fifth, sixth, seventh and eighth anterior primary divisions of the cervical nerves as well as a large part of that of the first thoracic are prolonged into the arm, and in the lower part of the neck and armpit communicate with one another to form the brachial plexus. As a general law underlies the composition of the limb plexuses it will be worth while to study the structure and distribution of this one with some little care. It will be seen from the accompanying diagram (fig. 8) that each component nerve with the exception of the first thoracic divides into an anterior (ventral) and a posterior (dorsal) division which are best spoken of as secondary divisions in order to prevent any confusion with the anterior and posterior primary divisions which all the spinal nerves undergo. In the diagram the anterior secondary divisions are white, while the posterior are shaded. It has been suggested by A. M. Paterson that the posterior secondary branches correspond with the lateral branches of the thoracic nerves already mentioned, but there are still certain difficulties to be explained before altogether accepting this.
|
| From Gray’s Anatomy, Descriptive and Surgical. |
| Fig. 9.—Plan of the Lumbar and Sacral Plexuses. |
Later on in the plexus three cords are formed of which the posterior is altogether made up of the posterior secondary divisions, while the anterior secondary divisions of the fifth, sixth and seventh cervical nerves form the external cord, and those of the eighth cervical and first thoracic the inner. As a general rule the nerves which rise from the ventral secondary divisions of the limb plexuses run only to that surface of the limb which was ventral in the embryo, while the dorsal secondary divisions are confined to the original dorsal area, but, in order to apply this to the human adult, it must be realized that the limbs are at one time flattened buds coming off at right angles from the side of the body and having dorsal and ventral surfaces, one (preaxial) border toward the head of the embryo, and one (postaxial) toward the tail. If a person lies prone upon the floor with the arms outstretched and the palms downward the embryological position of the forelimb is to some extent restored, and it will now be easily understood that the more preaxial part of the limb will be supplied by those nerves which enter it from nearer the head, while the postaxial part draws its nerve supply from lower down the spinal cord. To use Herringham’s words: “(A) Of two spots on the skin, that nearer the preaxial border tends to be supplied by the higher nerve. (B) Of two spots in the preaxial area the lower tends to be supplied by the lower nerve, and of two spots in the postaxial area the lower tends to be supplied by the higher nerve.” Other points of general importance in regard to cutaneous nerve supply are, firstly, that the area of skin supplied by one spinal nerve is not sharply marked off from that of the next, but the two are separated by an overlapping region; and, secondly, that the area supplied by any one spinal nerve is liable to variation in different individuals within moderate limits. This variation may affect the whole plexus, and the term “prefixed plexus” has been devised by C. S. Sherrington to indicate one in which the spinal nerves entering into its formation are rather higher than usual, while, when the opposite is the case, the plexus is spoken of as “postfixed.”
|
| From A. M. Paterson, in Cunningham’s Text-book of Anatomy. |
| Fig 7.—The Triangles of the Neck (Nerves). |
With regard to the muscular supply of a limb the general rule is that each muscle is supplied by fibres derived from more than one spinal nerve; this, of course, is made possible by the redistribution of fibres in the plexuses. Moreover, the muscular supply does not necessarily correspond to that of the overlying skin, because (see Muscular System) some of the primitive muscles have been suppressed, others have fused together, while others have shifted their position to a considerable distance. Bearing the foregoing facts in mind, the main distribution of the nerves of the brachial plexus may be surveyed, though the exact details must be sought in the human anatomy text-books. The outer cord of the plexus gives off the external anterior thoracic nerve (C. 5, 6, 7) to the pectoralis major, the musculo-cutaneous nerve (C. 5, 6) to the muscles on the front of the arm, and to the skin of the outer side of the forearm and the outer head of the median nerve (C. 5?, 6, 7), which joins the inner head (C. 8, Th. 1) and supplies most of the flexor muscles of the front of the forearm as well as those of the ball of the thumb, the outer two lumbricals and also the skin of the outer part of the palm including the outer three digits and half the fourth.
From the inner cord come the inner head of the median just mentioned, the ulnar nerve (C. 8, Th. 1), which passes down behind the internal condyle of the humerus, where it is popularly known as the “funny bone” and supplies the flexor carpi ulnaris, half the flexor profundus digitorum, and most of the muscles of the hand as well as the inner digit and a half on the palmar and dorsal aspects. Other branches of the inner cord are the internal cutaneous (C. 8, Th. 1) supplying the inner side of the forearm, the lesser internal cutaneous (Th. 1) which often joins the intercosto-humeral or lateral cutaneous branch of the second intercostal nerve to supply the skin on the inner side of the upper arm, and the internal anterior thoracic nerve (C. 8, Th. 1) to the pectoralis minor and major.
From the posterior cord are derived the three subscapular nerves (C. 5, 6, 7, 8) which supply the subscapularis, teres ma]or and latissimus dorsi muscles, the circumflex nerve (C. 5, 6) supplying the deltoid and teres minor muscles, and the skin over the lower part of the deltoid, and the musculo-spiral nerve (C. 5, 6, 7, 8) which is the largest branch of the brachial plexus and gives off cutaneous twigs to the outer side and back of the arm and to the back of the forearm, as well as muscular twigs to the triceps and adjacent muscles. At the elbow this nerve divides into the radial and posterior interosseous. The radial is entirely sensory and supplies the skin of the outer side of the back of the hand, including three digits and a half, while the posterior interosseous is wholly muscular, supplying the muscles on the back of the forearm. It will be seen that the posterior cord is derived altogether from posterior secondary divisions of the plexus, but there are three other nerves derived from these which should be mentioned.
The posterior thoracic or respiratory nerve of Bell comes off the back of the fifth, sixth and seventh cervical nerves before the anterior and posterior secondary divisions separate, and runs down to supply the serratus magnus muscle.
The posterior scapular or nerve to the rhomboid muscles runs to those muscles from the fifth cervical.
The suprascapular nerve (C, 5, 6) passes through the suprascapular notch to supply the supraspinatus and infraspinatus muscles.
The spinal nerves which are distributed to the lower limbs first intercommunicate in the lumbar and sacral plexuses, which, with the perineal nerves, are sometimes spoken of together as the lumbo sacral plexus. The lumbar plexus (see fig. 9) is formed as a rule of the first four lumbar nerves, though the greater part of the first number is segmental in its distribution and resembles one of the thoracic nerves. It early divides into an ilio-hypogastric and ilio-inguinal branch, which run round the abdominal wall in the substance of the muscles, and of which the former gives off an iliac branch, which is in series with the lateral cutaneous branches of the intercostal nerves and passes over the crest of the ilium to the gluteal region, while the hypogastric branch runs round to the skin of the pubic region. The ilio-inguinal, on the other hand, gives off no lateral cutaneous or iliac branch, but is prolonged down the inguinal canal to supply the skin of the scrotum as well as that of the thigh which touches it. In all probability the hypogastric branch of the ilio-hypogastric and the whole of the ilio-inguinal represent the anterior secondary division of the first lumbar nerve, while the posterior secondary division is the iliac branch of the ilio-hypogastric.
The other anterior secondary divisions of the lumbar plexus is the obturator (see fig. 8). The obturator nerve (L. 2, 3, 4) supplies the adductor group of muscles on the inner side of the thigh as well as the hip and knee joints; it occasionally has a cutaneous branch on the inner side of the thigh. The posterior secondary branches of the plexus are the genito-crural, the external cutaneous and the anterior crural. The genito-crural nerve (L. 1, 2) is partly anterior (ventral) and partly posterior (dorsal). It sends one anterior branch through the inguinal canal to supply the cremaster muscle, and another (posterior) to the skin of the thigh just below the groin.
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| From A. M. Paterson, in Cunningham’s Text-Book of Anatomy. | |||
| Fig. 8.—The Nerves of the Brachial Plexus. | |||
| Sy, | Sympathetic gangliated cord. | Cb, | Nerve to coraco-brachialis. |
| Phr, | Phrenic nerve. | M, | Median nerve. |
| C.4, 5, 6, 7, 8, T.1, 2, 3, Anterior primary divisions of the lower cervical and upper thoracic nerves. | Inner Cord. | ||
| I.A.T, | Internal anterior thoracic nerve. | ||
| U, | Ulnar nerve. | ||
| m1, m2, Muscular branches to axial muscles. | I.C, | Internal cutaneous nerve. | |
| P.T, | Long thoracic nerve. | L.I.C, | Lesser internal Cutaneous nerve. |
| Rh, | Nerve to rhomboids (posterior scapular). | Posterior Cord. | |
| Subcl, Nerve to subclavius muscle. | Circ, | Circumflex nerve. | |
| Int, | Intercostal nerves. | M.S, | Musculo-spiral nerve. |
| S.Sc, Supra-scapular nerve. The intercostal part of the first thoracic nerve is omitted. |
S.Sub, | Short subscapular nerve. | |
| M.Sub, Lower subscapular nerve. | |||
| Outer Cord. | L.Sub, | Long subscapular nerve. | |
| E.A.T, | External anterior thoracic nerve. | I.H, | Intercosto-humeral nerve. |
| M.C, | Muscular-cutaneous nerve. | Lat, | Lateral branch of third intercostal nerve. |
The external cutaneous nerve (L.2, 3) supplies the skin of the outer side of the thigh, while the anterior crural (L.2, 3, 4) innervates the muscles on the front of the thigh, the skin on the front and inner side of the thigh, through its middle and internal cutaneous branches, and the skin of the inner side of the leg and foot through the internal saphenous branch. At first sight it is difficult to understand how the anterior crural nerve, which supplies the skin of the front of the thigh, is a posterior secondary division of the lumbar plexus, but the explanation is that the front of the human thigh was originally the dorsal surface of the limb bud, and the distribution of the nerve is quite easily understood if the position of the hind limb of a lizard or crocodile is glanced at. The fourth lumbar nerve is sometimes called the nervus furcalis, because, dividing, it partly goes to the lumbar, and partly to the sacral plexus (fig. 8), though, when the plexus is prefixed, the third lumbar may be the nervus furcalis, or, when it is postfixed, the fifth lumbar. Under ordinary conditions the descending branch of the fourth lumbar nerve joins the fifth, and together they make the lumbo-sacral cord, which, with the first three sacral nerves, forms the sacral plexus. This plexus, like the others, contains anterior and posterior secondary divisions of its spinal nerves, and it resembles the brachial plexus in that the lowest nerve to enter it contributes no dorsal secondary division.
All the constituent nerves of the plexus run into one huge nerve, the great sciatic, which runs down the back of the thigh and, before reaching the knee, divides into external and internal popliteal nerves. These two nerves are sometimes separate from their first formation in the plexus, and may always be separated easily by the handle of a scalpel, since they are only bound together by loose connective tissue to form the great sciatic nerve. When they are separated in this way it is seen that the external popliteal is made up entirely of posterior (dorsal) secondary divisions (see fig. 9), and is derived from the fourth and fifth lumbar and first and second sacral nerves, while the internal popliteal is formed by the anterior (ventral) secondary divisions of the fourth and fifth lumbar and first, second and third sacral nerves. The external popliteal nerve supplies the short head of the biceps femoris (see Muscular System), and, just below the knee, divides into anterior tibial and musculo-cutaneous branches, which both supply the dorsal surface of the leg and foot. The anterior tibial nerve is chiefly muscular, innervating the muscles in front of the tibia and fibula as well as the extensor brevis digitorum pedis on the dorsum of the foot, though it gives one small cutaneous branch to the cleft between the first and second toes. The musculo-cutaneous nerve supplies the peroneus longus and brevis muscles, and the rest of the skin of the dorsum, of the foot, and lower part of the leg, while the skin of the upper part of the dorsum of the leg, below the knee, is supplied by the external popliteal before its division. The internal popliteal nerve, after supplying the hamstrings, is continued into the calf of the leg as the posterior tibial and innervates all the muscles on this, the ventral, surface. Behind the inner ankle it divides into the external and internal plantar nerves, from which the muscles and skin of the sole are supplied. A little above the knee each popliteal nerve gives off a contribution to help form the external or short saphenous nerve. That from the internal popliteal is called the communicans tibialis, while that from the external popliteal is the communicans fibularis. These join about the middle of the back of the calf, and the, now formed, short saphenous nerve runs down behind the outer ankle to supply the outer side of the foot. Sometimes it encroaches on the dorsum of the foot, replacing part of the musculo-cutaneous, though, when this is the case, its dorsal contribution from the external popliteal (communicans fibularis) is always larger than usual. To return to the sacral plexus: branches are given off from the anterior secondary divisions to the short external rotator muscles of the hip (pyriformis, quadratus femoris, &c.), while from the posterior secondary divisions come the superior gluteal (L. I.S. 4, 5) and the inferior gluteal (L.5, S. 1, 2) to the muscles of the buttocks.
In modern descriptions the lower branches of the lumbo-sacral plexus are grouped into a pudendal plexus, and the plan, though open to criticism on morphological grounds, has such descriptive advantages that it is followed here. Contributions from the first, second, third and fourth sacral, and the coccygeal nerve, form it, and these contributions are almost all anterior (ventral) secondary divisions. The branches of this plexus are the small sciatic, pudic, visceral, perforating cutaneous, muscular and sacro-coccygeal nerves. The small sciatic (S.1, 2, 3) is partly dorsal and partly ventral in its origin and distribution; it supplies the skin of the perineum, buttock and the back of the thigh. The pudic nerve (S.2, 3, 4) helps to supply the skin and muscles of the perineum and genital organs. The visceral branches form the pelvic stream of white rami communicantes (see Nervous System); they run from the second and third or third and fourth sacral nerves to the pelvic plexuses of the sympathetic system. The perforating cutaneous nerve (S.2, 3) pierces the great sacro-sciatic ligament and supplies the skin over the lower internal part of the buttock. The muscular branches (S.3, 4) supply the external sphincter, levator ani and coccygeus.
The sacro-coccygeal nerve (S.4, 5, Coc.1) runs down on each side of the coccyx to supply the adjacent skin, and represents the ventrolateral nerve of the tail of lower mammals. (F. G. P.)