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and laterally among the muscles, and become superficial at a greater distance from the median plane. In both regions the nerves pursue a sinuous course to the surface, and the lower series emerge and become superficial a considerable distance below the level of their spinal origin. There are considerable individual differences in the origin, course, and distribution of the several nerves.
First Cervical Nerve (N. sub-occipitalis). It has already been pointed out that the posterior root of this nerve may be very small, or even absent altogether. Its posterior ramus is larger than the anterior ramus; it does not divide into medial or lateral branches, and it does not directly supply any cutaneous branch.
FIG. 607.-THE DISTRIBUTION OF CUTANEOUS NERVES ON THE BACK OF THE TRUNK.
Posterior rami of first three lumbar nerves; S.1 et seq., Posterior'rami of sacral nerves; Acr, Posterior
On the other side a schematic representation is given of the areas supplied by the above nerves, the numerals indicating the spinal origin of the branches of distribution to each area.
capitis (O.T. complexus), rectus capitis posterior major and minor, and obliqui capitis, superior and inferior.
(b) A communicating branch descends to join the second cervical nerve.
The communicating branch may arise in common with the nerve to the obliquus inferior, and reach the second cervical nerve by piercing or passing superficial or deep to that muscle; or it may accompany the nerve to the semispinalis capitis and communicate with the greater occipital nerve, under or over that muscle.
Second Cervical Nerve.-The posterior ramus of this nerve is larger than the corresponding anterior ramus. It passes backwards between the atlas and epistropheus, and in the interval between the obliquus inferior and the semispinalis cervicis muscles, under cover of the semispinalis capitis muscle. In this situa
tion the nerve gives off several small muscular and communicating branches. The main trunk, after piercing the semispinalis capitis and trapezius muscles, accompanies the occipital artery to the scalp as the greater occipital nerve. This is the chief cutaneous nerve for the posterior part of the scalp. It enters the superficial fascia at the level of the superior nuchal line of the occipital bone and about an inch from the external occipital protuberance. Ramifying over the surface, it supplies the skin of the scalp as far as the vertex. It communicates on the scalp with the following nerves: great auricular, lesser occipital, posterior auricular, and third occipital.
The muscular branches of the second cervical nerve are destined for the semispinalis capitis, obliquus inferior, semispinalis cervicis, and multifidus.
Its communicating branches form the posterior cervical plexus. Descending over the posterior arch of the atlas is a branch from the sub-occipital nerve which forms a loop or network with a branch of the second nerve. From this loop twigs are supplied to the surrounding muscles. A similar loop is formed by a communication between branches of the second and third nerves, from which muscles are also supplied. Occasionally an additional loop is formed between branches of the third and fourth nerves.
Third Cervical Nerve. This is much smaller than the second nerve. Near its origin it forms a loop of communication with the second, and it may give off a similar communicating branch to the fourth nerve. The main trunk divides into medial cutaneous and lateral muscular branches. The lateral muscular branch enters contiguous muscles; the medial cutaneous branch passes backwards and medially, and becomes superficial as the third occipital nerve (O.T. n. occipitalis minimus), close to the median plane of the neck. It supplies fine branches to the neck and scalp, and communicates with the greater occipital nerve.
The fourth, fifth, and sixth cervical nerves are still smaller. Beneath the semispinalis capitis each divides into lateral muscular and medial cutaneous branches. The muscular branches supply neighbouring muscles; the cutaneous branches are small nerves, which, passing backwards, become superficial close to the median plane. They supply the skin of the back of the neck. The sixth is the smallest, and the cutaneous branches of the fifth and sixth nerves may be absent altogether. In certain cases the fourth nerve forms, with the third, a loop of communication from which muscles are supplied.
Seventh and Eighth Cervical Nerves.-These are the smallest of the posterior rami of the cervical nerves. They give off ordinarily no cutaneous branches, and end in the deep muscles of the back. There is occasionally a small cutaneous offset from the eighth nerve.
The posterior ramus of each thoracic nerve divides into a medial and a lateral branch. In the case of the upper six or seven thoracic nerves the medial branches are distributed chiefly as cutaneous nerves,-only giving off small muscular branches while the lateral branches are wholly muscular in their distribution; in the case of the lower five or six thoracic nerves the opposite is the case. In all cases the muscular branches serve to innervate the longitudinal muscles of the back. The distribution of the cutaneous branches is different in the upper and lower part of the back. The upper six or seven thoracic nerves innervate the skin of the scapular region. The medial cutaneous branches, after a sinuous backward course from their origin, among the dorsal muscles, reach the surface near the spines of the vertebræ and are directed almost horizontally laterally over the trapezius muscle. The first is small; the second is very large and reaches to the acromion. The rest diminish in size, from above downwards, and become more and more oblique in direction. The lateral cutaneous branches of the lower five or six thoracic nerves are directed from their origin obliquely downwards and laterally among the parts of the sacro-spinalis muscle. Becoming cutaneous by piercing the latissimus dorsi at some distance from the median plane, they supply the skin of the back in the lower part of the chest and loin, the lowest nerves (eleventh and twelfth) reaching over the iliac crest on to the buttock. The lower nerves often subdivide into two branches before or after their emergence from the latissimus dorsi muscle.
First three Lumbar Nerves.-The posterior rami of the first three lumbar nerves subdivide into medial and lateral branches, in the same way as the lower thoracic nerves. The medial branches are muscular and innervate the deep muscles of the back. The lateral branches are chiefly cutaneous. They are directed obliquely downwards and laterally among the fibres of the sacro-spinalis and become superficial by piercing the lumbo-dorsal fascia, just above the iliac crest and a short distance in front of the posterior superior iliac spine. They are then directed downwards in the superficial fascia of the buttock, and supply a lengthy strip of skin, extending from the median plane above the iliac crest to a point distal to and behind the greater trochanter of the femur. There may be only two cutaneous branches, derived from the first two lumbar nerves; in other cases the three nerves are the branches of the twelfth thoracic and first two lumbar
The fourth and fifth lumbar nerves (like the last two cervical nerves) usually supply only muscular branches to the longitudinal muscles of the back. The fifth nerve in many cases sends a branch to form a loop of connexion with the posterior ramus of the first sacral nerve (posterior sacral plexus).
SACRAL AND COCCYGEAL NERVES.
The posterior rami of the sacral nerves issue from the posterior sacral foramina. As in the case of the thoracic and lumbar nerves, the upper sacral nerves differ from the lower in their distribution.
The first three sacral nerves supply medial muscular branches for the multifidus, and lateral cutaneous branches which pierce the fibres of the sacrotuberous ligament and the gluteus maximus muscle, and supply the skin over the back of the sacrum and contiguous part of the buttock, giving rise to the posterior sacral plexus.
The posterior sacral plexus consists, like the posterior cervical plexus, of loops or plexiform communications over the back of the sacrum between the posterior rami of the first three sacral nerves, to which are frequently joined branches of the last lumbar nerve and fourth and even the fifth sacral nerve. From these loops branches proceed to supply the multifidus muscle; others, piercing the sacro-tuberous ligament, form secondary loops beneath the gluteus maximus muscle. From the secondary loops, two or more cutaneous branches arise, which, after traversing the muscle, supply the skin over the sacrum and medial part of the buttock.
Posterior Ano-coccygeal Nerve. The posterior rami of the fourth and fifth sacral nerves do not divide into medial and lateral branches. They unite together to form a loop which is joined by the minute posterior ramus of the coccygeal nerve. The union of the three nerves constitutes the posterior ano-coccygeal nerve, which, after perforating the sacro-tuberous ligament, is distributed to the skin in the neighbourhood of the coccyx. It supplies no muscles. This nerve is the representative of the superior caudal trunk of tailed animals.
MORPHOLOGY OF THE POSTERIOR RAMI.
There are several points of morphological interest in relation to the posterior rami of the spinal nerves.
1. Muscular Distribution.-In their muscular distribution they are strictly limited to the longitudinal muscles of the back: namely, those associated with the axial skeleton alone.
2. Cutaneous Distribution. Their cutaneous distribution represents two points of interest. A. In the first place, while the skin of the back is supplied in a regularly segmental manner by the several nerves, certain of them fail to reach the surface at all. The absence of a cutaneous branch from the sub-occipital nerve may be due either to the absence of a perfect posterior root, or to its communication with the second nerve. The other nerves which do not usually supply the skin are the last two, three, or four cervical, and the fourth and fifth lumbar nerves. These nerves are placed in the centre of regions in which the upper and lower limbs are developed. They are minute nerves, while the corresponding anterior rami are among the largest of the spinal nerves. Thus, opposite the centre of each limb, posteriorly, there is a hiatus in the segmental distribution of the posterior rami of the spinal nerves to the skin of the shoulder and buttock, attributable to the formation of the limbs, and the extension into them of the greater part of the nerves of the region. This gap, in the case of the upper limb, commences at the level of the vertebra prominens; in the case of the lower limb it commences opposite the level of the posterior superior iliac spine. It can be continued on to each limb as a hypothetical area (the dorsal axial line), which indicates the area of contact (and overlapping) of cutaneous nerves not in strictly numerical sequence. Thus, in the region of the shoulder, the sixth (or fifth) cervical nerve innervates an area of skin adjoining that supplied by the eighth cervical or first thoracic nerve; in the region of the buttock the third lumbar nerve supplies an area contiguous with that supplied by the fifth lumbar or first sacral nerve.
B. The cutaneous branches of the posterior rami of the spinal nerves differ from the muscular branches in respect of their penetration into regions beyond those supplied by their motor roots. The cutaneous branches, in regions where outgrowths or extensions from the trunk have occurred, follow these extensions; and, in consequence, supply skin covering parts which do not belong to segments represented by the nerves in question. Thus, the second and third cervical nerves (greater and third occipital) are drawn upwards so as to supply the posterior part of the scalp; the upper thoracic nerves are drawn laterally over the scapular region; the upper lumbar and sacral nerves supply the skin of the buttock; and the ano-coccygeal nerve forms a rudimentary caudal nerve.
3. Plexuses. The plexuses formed by the posterior rami of the upper cervical and upper sacral nerves are the simplest met with in the human body. The posterior cervical plexus is one from which muscular branches are supplied; the posterior sacral plexus is mainly concerned in producing cutaneous offsets. In the case of the posterior cervical plexus the loops of communication between the first three or four cervical nerves result in the formation of a series of nerves for the supply of the semispinales and other muscles, which bring into contact with these muscles, simultaneously, a considerable area of the spinal medulla, and provide a combined and simultaneous innervation for the several parts of each muscle. In the case of the posterior sacral plexus, the formation of loops between the nerves results in the innervation of any given spot in the cutaneous area supplied from these loops by more than one spinal nerve. As has been said already, the cutaneous nerves, even without the formation of plexuses, overlap in their cutaneous distribution. The formation of a plexus causes a more intimate union of neighbouring spinal nerves, so that stimulation of the surface affects a wider area in the spinal medulla than if the nerves passed separately to it from the surface. While segmentation becomes less obvious, increased co-ordination of both movement and sensation is effected.
ANTERIOR RAMI OF THE SPINAL NERVES.
The anterior rami (O.T. anterior primary divisions) of the spinal nerves, are, with the exception of the first two cervical nerves, much larger than the corresponding posterior rami. Composed of elements of both posterior and anterior roots, each nerve separates from the posterior ramus on emerging from the intervertebral foramen, and, proceeding laterally, is distributed to structures on the lateral and anterior aspects of the body,-including the limbs.
Each nerve is joined near its origin by a gray ramus communicans from the corresponding sympathetic gangliated trunk; and in the case of certain thoracic, lumbar, and sacral nerves, the anterior ramus gives off a delicate bundle of fibres. which forms the white ramus communicans to the sympathetic trunk. That part of the spinal nerve which is distributed to the body wall and limbs may be termed somatic; the small white ramus communicans, innervating structures in the splanchnic area, may be termed the visceral or splanchnic part of the spinal nerve.
The anterior rami of the spinal nerves are distributed in a regular segmental manner only in certain cases. Except in the case of the thoracic nerves, the anterior rami combine to form the three great plexuses-cervical, brachial, and lumbo-sacral-and their arrangement and distribution is rendered exceedingly complex.
À thoracic nerve, such as the fifth or sixth, may be regarded as a type to illustrate the mode of distribution of the anterior rami of the spinal nerves (Fig. 606, p. 686). It occupies an intercostal space; near its origin it possesses gray and white rami communicantes; it courses through the interval between the intercostal muscles; it supplies branches to those muscles and gives off, when it reaches the side of the chest, a lateral branch, which, after supplying small muscular branches, pierces the external intercostal muscle, and is distributed to an area of ¦ skin over the lateral part of the trunk, contiguous dorsally with a similar area, innervated by the cutaneous branches of the posterior ramus of the same nerve. The lateral branch generally subdivides into a smaller posterior and a larger anterior ramus, as it pierces the muscles clothing the wall of the chest. The main trunk of the nerve, having given off its lateral branch, then pursues its course obliquely forwards to the side of the sternum, where, after piercing the pectoral muscles, it appears superficially as the terminal anterior cutaneous branch. This supplies an area of skin continuous with that supplied by the anterior part of the lateral branch of the same nerve. Such a nerve thus supplies, by means of its lateral and anterior branches, an area of skin which (with the area supplied by the cutaneous branch of its posterior ramus) forms a continuous and uninterrupted belt, extending from the median plane behind to the median plane in front. The lateral and anterior branches of the nerve innervate in their course the intercostal and other muscles, to be afterwards mentioned in detail.
The anterior rami of the cervical nerves, together with parts of the first and second thoracic nerves, are distributed to the head, neck, and upper extremity.