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Arachnoid

sinuses, named lateral, which curve forwards and downwards to the jugular foramina to terminate in the internal jugular veins. In its course each lateral sinus receives two petrosal sinuses, which pass from the cavernous sinus backwards along the upper and lower borders of the petrous part of the temporal bone.

The spinal part of the dura mater hangs loosely in the spinal canal. It does not form a periosteum for the vertebræ, but is separated from their bony rings by loose fat and a plexus of veins. It gives off no bands from its inner surface, and it does not split into two layers for the lodgment of venous blood sinuses. The spinal dura mater forms a tubular envelope for the spinal cord and the origins of the spinal nerves. It extends from the foramen magnum, where it is continuous with the cranial dura mater, to the lower end of the sacral canal, ends below in a funnel-shaped prolongation, and is pierced laterally by the roots of the several spinal nerves in their passage outwards to the intervertebral foramina.

Both the cranial and the spinal parts of the dura mater consist of a tough, fibrous membrane; somewhat flocculent externally, but smooth, glistening, and free on its inner surface. The inner surface has the appearance of a serous membrane, and when examined microscopically is seen to consist of a layer of squamous endothelial cells, similar to those drawn in fig. 34. Hence the dura mater is sometimes called a fibro-serous membrane. The dura mater is well provided with lymph vessels, which in all probability open by stomata on the free inner surface. Between the dura mater and the subjacent arachnoid membrane is a fine space containing a minute quantity of limpid serum, which moistens the smooth inner surface of the dura and the corresponding smooth outer surface of the arachnoid. It is regarded as equivalent to the cavity of a serous membrane, and is named the arachnoid cavity, or, more appropriately, the sub-dural space.

Arachnoid mater. The arachnoid is a membrane of membrane. great delicacy and transparency, which loosely envelopes both the brain and spinal cord. It is separated from these organs by the pia mater; but between it and the latter membrane is a distinct space, called sub-arachnoid. The sub-arachnoid space is more distinctly marked beneath the spinal than beneath the cerebral parts of the membrane, which forms a looser investment for the cord than for the brain. At the base of the brain, and opposite the fissures between the convolutions of the cerebrum, the interval between the arachnoid and the pia matter can, however, always be seen, for the arachnoid does not, like the pia mater, clothe the sides of the fissures, but passes directly across between the summits of adjacent convolutions. The sub-arachnoid space is subdivided into numerous freely-communicating loculi by bundles of delicate areolar tissue, which bundles are invested, as Key and Retzius have shown, by a layer of squamous endothelium. The space contains a limpid cerebro-spinal fluid, which varies in quantity from 2 drachms to 2 ounces. The fluid is alkaline, of sp. gr. 1.005, contains a little albumen, and a substance which, as Turner pointed out, reduces blue oxide of copper to the state of yellow sub-oxide. The arachnoid membrane is made up of delicate connective tissue. The free surface next the sub-dural space is smooth, like a serous membrane, and covered by a layer of squamous endothelium. This layer is reflected on to the roots of the spinal and cranial nerves, and, when they pierce the dura mater, it becomes continuous with the endothelial lining of that membrane. As the arrangement and structure so closely correspond with what is seen in the serous membranes, many anatomists regard the arachnoid as the visceral layer of a serous membrane, and the endothelial lining of the dura mater as the parietal layer, whilst the sub-dural space is the intermediate cavity.

bodies.

When the skull cap is removed, clusters of granular Pacchio bodies are usually to be seen imbedded in the dura mater nian on each side of the superior longitudinal sinus; these are named the Pacchionian bodies. When traced through the dura mater they are found to spring from the visceral or proper cerebral arachnoid. The observations of Luschka and Cleland have proved that villous processes invariably grow from the free surface of that membrane, and that when these villi greatly increase in size they form the bodies in question. Sometimes the Pacchionian bodies greatly hypertrophy, occasion absorption of the bones of the cranial vault, and depressions on the upper surface of the brain.

Pia mater. This membrane closely invests the whole Pia mater. outer surface of the brain. It dips into the fissures between the convolutions, and a wide prolongation, named velum interpositum, lies in the interior of the cerebrum. With a little care it can be stripped off the brain without causing injury to its substance. The pia mater invests the spinal cord, and is more intimately attached to it than to the brain, for not only does it send prolongations into the anterior and posterior fissures of the cord, but slender bands pass repeatedly from its inner surface into the columns of the cord. Hence it cannot be stripped off the cord without causing injury to its substance. The pia matter is prolonged on to the roots both of the cranial and spinal nerves, and on to the filum terminale. This membrane consists of a delicate connective tissue, in which the arteries of the brain and spinal cord ramify and subdivide into small branches before they penetrate the nervous substance, and in which the veins conveying the blood from the nerve centres lie before they open into the blood sinuses of the cranial dura mater and the extradural venous plexus of the spinal canal. The arteries which pass from the pia mater into the brain and spinal cord are invested by a loose sheath, which has been described as forming the wall of a peri-vascular lymphatic vessel; but Key and Retzius have shown that the space between the blood-vessel and the sheath opens into the subarachnoid space, and contains cerebro-spinal fluid. A network of lymph vessels ramifies freely in the pia mater. It is also well provided with nerves, which arise from the posterior roots of the spinal nerves, from some of the cranial nerves, and from the carotid and vertebral plexuses of the sympathetic. The epi-cerebral and epi-spinal spaces described by His as existing between this membrane and the brain and spinal cord are in all probability artificial productions.

culatum.

In the spinal canal a slender fibrous band projects from Ligamenthe pia mater covering the side of the cord, and, pushing tum denti the arachnoid membrane in front of it, is attached by about twenty-two pairs of denticulated processes to the inner surface of the dura mater. It is named ligamentun denticulatum, and its teeth alternate with the successive pairs of spinal nerves.

SPINAL CORD.-The MEDULLA SPINALIS, or SPINAL Spinal CORD, occupies the spinal canal, and extends from the cord foramen magnum to opposite the body of the first lumbar vertebra. In the early foetus it equals in length the canal itself; but as the spinal column grows at a greater proportional rate than the cord, the latter, when growth has ceased, is several inches shorter than the column. The cord is continuous above with the medulla oblongata, whilst it tapers off below into a slender thread, the filum terminale, which lies in the axis of the sacral canal, and is attached below to the back of the coccyx, or to the fibrous membrane which closes in below the sacral canal. The length of the cord is from 15 to 18 inches. It approaches a cylinder in shape, but is flattened on its anterior and posterior surfaces, and presents two enlargements which have a greater girth than the rest of the cord. The upper,

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diameter, but in all the axial cylinder and medullary sheath can be distinctly seen. Wherever the nerve roots enter into the cord, the fibres of these roots pass transversely or obliquely in their course inwards to the grey matter. Horizontal fibres are also found in the white anterior commissure, and a similar appearance can be seen in the posterior commissure. Horizontal fibres have also been traced from the lateral columns into the adjacent part of the grey matter.

The grey crescentic portion of the cord contains connective tissue, blood-vessels, nerve fibres, and nerve cells. The nerve fibres in the grey matter are numerous; and whilst some possess a medullary sheath, others consist only of the axial cylinder; they divide and subdivide, and, as Gerlach has shown, form a narrow-meshed network of extremely minute fibres. The nerve cells are multipolar, and are chiefly collected in the anterior and posterior horns of each crescent. The cells of the anterior cornu are large, distinct, and stellate, and form a well-defined group of nerve cells. Those of the posterior cornu are smaller in size, more elongated in shape, but with stellate branched processes. They are not so distinct as in the anterior horn, owing to the connective tissue with its corpuscles being so abundant. This tissue is best marked at the tip of the posterior horn, where it forms the substantia gelatinosa of Rolando. Lockhart Clarke has described an intermedio-lateral group of nerve cells situated at the outer side of the grey matter, about midway between the anterior and posterior horns, in the upper part of the cervical portion of the cord, and in the thoracic part between the brachial and crural enlargements.

called the cervical or brachial enlargement, extends from | transversely divided. The individual fibres vary much in opposite the third cervical to the first dorsal vertebra, and from it arise the nerves which supply the upper limbs; the lower, called the crural or lumbar enlargement, is opposite the last dorsal vertebra, and supplies with nerves the lower limbs. The cord is almost completely divided into right and left lateral halves by two fissures, named respectively anterior and posterior median fissures, which do not quite reach the centre of the cord, for at the bottom of the anterior fissure are the transverse fibres of the anterior white commissure, and at the bottom of the posterior fissure he fibres of the posterior grey commissure. By these commissures the two halves of the cord are united together. The fibres of the posterior commissure surround a canal, called the central canal, which extends along the whole length of the cord, and even passes into the upper end of the filum terminale. This canal is lined by a ciliated columnar endothelium, and expands superiorly into the cavity of the fourth ventricle. Each lateral half of the cord is subdivided into three columns by two depressions, which mark the points of emergence of the roots of the spinal nerves. The anterior nerve roots pass through the antero-lateral depression or fissure, and between it and the antero-median fissure is the anterior column of the cord. The posterior nerve roots pass through the postero-lateral fissure, and between it and the postero-median fissure is the posterior column, whilst between the anterior and posterior nerve roots lies the lateral column. In the cervical region, the part of the posterior column which lies next tue postero-median fissure is marked off by a fissure into a small internal or postero-median column. The subdivision of each lateral half of the cord into the columns, and the arrangement of its nervous tissues, are well seen in transverse sections through its substance. The cord is composed of white and grey matter. The white matter is external, and forms the columns of the cord. The grey matter is surrounded by the white, and has in each lateral

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FIG. 64.-Transverse section through the spinal cord.
AF, antero-median, and PF, postero-median fissures;
PC, posterior, LC, lateral, and AC, anterior columns;
AR, anterior, and PR, posterior nerve roots; C,
central canal of cord, with its columnar endothelial

lining. The pia mater is shown investing the cord,
sending processes into the anterior and posterior

fissures, as well as delicate prolongations into the
columns. The crescentic arrangement of the grey
matter is shown by the darker shaded portion.
half of the cord a crescentic shape. The horns of the
crescent are directed towards the fissures of emergence of the
nerve roots; the anterior horn is rounded; the posterior
long and narrow. The proportion of grey matter to the
white varies in different parts of the cord. At the com-
mencement of the filum terminale there is scarcely any
white matter; but the white matter increases in amount
from below upwards, so that its absolute quantity is greatest
in the cervical part of the cord. The grey crescents are
thicker in the upper and lower enlargements than in the
intermediate part.

The cord contains both nerve fibres and nerve cells. The external, columnar, white part of the cord consists of nerve fibres, with a supporting reticular framework of connective tissue and blood-vessels derived from the pia mater. Wellformed stellate connective tissue corpuscles lie in this supporting framework. The nerve fibres of the various columns extend longitudinally, and lie parallel to each other, so that in transverse sections through the columns the fibres are

The course of the fibres in the cord and their relations to the nerve cells should now be considered. There can be no doubt that of the longitudinal fibres some ascend from below upwards, and conduct either excito-motory impulses to the regions of the spinal cord itself, or sensory impulses to the brain. Other longitudinal fibres again descend from the brain and higher regions of the cord to the lower, and conduct motor and vaso-motor impulses from above downwards. The horizontal and oblique fibres of an anterior or motor nerve root enter the grey matter of the anterior cornu, and seem to have the following arrangement: some become directly continuous with the axial cylindrical processes of the nerve cells; others pass into the anterior commissure; others extend as far as the grey matter of the posterior horn. The nerve cells of the anterior cornu give origin, therefore, directly to nerve fibres by their unbranched processes. Gerlach's observations show that the branched processes of these cells become continuous with the network of extremely minute fibres already described in the grey matter; from this network medullated fibres appear to arise which leave the grey matter; some enter the lateral column, and ascend as the fibres of this structure; others pass as fibres of the anterior commissure to the opposite side of the cord, and ascend as the anterior column of that side. The anterior and lateral columns, therefore, are constantly receiving accessions of fibres from the enclosed grey matter.

The fibres of a posterior or sensory nerve root on entering the cord subdivide into two bundles; one does not enter the grey matter, but applies itself to the posterior column, of which it forms some of the vertical fibres. These fibres may ascend to the brain, or they may at some higher point in the cord enter the grey matter of the posterior horn. The other bundle of posterior root fibres at once enters the posterior horn of grey matter. The connections and ultimate arrangement of these fibres in the grey matter have not been satisfactorily made out. Gerlach states that, as they frequently subdivide on entering the grey matter, it

Spinal

erves.

is possible they may form the fine nerve fibre plexus of the grey substance; but a direct continuity between them and the axial-cylinder processes of the cells of the posterior horn does not seem to have been observed. From the plexus, formed by the much subdivided processes of these cells, fibres arise, which, forming the fibres of the posterior commissure, pass both in front of and behind the central canal to the opposite side, where they ascend towards the brain, "partly in the vertical fasciculi of the posterior cornua and partly in the posterior columns."

The structure of the spinal cord shows it to be both a nerve centre and a conductor of nervous impulses. The nerve cells in its grey matter give rise either directly, or through the delicate plexus formed by their branching processes, to nerve fibres, which may either pass out of the cord as the anterior and posterior roots of the spinal nerves, or may ascend to the brain as the columns of the cord. Hence the cord is anatomically continuous, on the one hand, through the nerves which arise from it, with the peripheral end-organs in the skin, and muscular system in which those nerves terminate; and, on the other hand, it is continuous with the brain. It serves, therefore, to conduct the impulses of touch-sensation from the skin upwards to the brain, and the motor impulses from the brain downwards to the muscles. But further, the cord is the great nerve centre concerned in reflex excito-motory actions. It must, also, be remembered that the two halves of the cord are anatomically continuous with each other through the nerve fibres of the commissures, so that it acts as a single organ, and not as two organs. Experiments have shown that sensory impulses are conducted upwards through the cord, not by that half from which the nerves arise that have been excited, but by the opposite half of the cord, which is obviously due to the crossing of the fibres of the posterior commissure. Motor impressions are, however, conducted downwards by that half of the cord from which the nerves arise that pass to supply the muscles to be moved.

The spinal cord is well supplied with blood by numerous arteries, which terminate in a diffused capillary network. The capillaries are much more numerous in the grey matter of the cord than in the white columns.

ORIGIN, ARRANGEMENT, AND DISTRIBUTION OF THE SPINAL NERVES.-The spinal cord gives origin to thirtyone pairs of SPINAL nerves, which pass out of the spinal canal through the intervertebral foramina. These nerves are arranged in groups, according to the region of the spine through the foramina in which they proceed. There are eight pairs of cervical nerves; the first or sub-occipital emerges between the occipital bone and the atlas, the eighth between the seventh cervical and first dorsal vertebræ. Twelve dorsal or thoracic nerves pass out on each side in relation to the dorsal vertebræ: five pairs of lumbar nerves in the region of the loins; five pairs of sacral nerves through the sacral foramina; and one pair of coccygeal nerves through the lowest openings in the spinal canal. Each spinal nerve arises by two roots, an anterior and a posterior, from the side of the cord. These roots are distinguished from each other both anatomically and physiologically. The posterior root has a swelling or ganglion on it, whilst no ganglion exists on the anterior root. The posterior root consists of sensory nerve fibres, i.e., of fibres which conduct impulses from the periphery into the nerve centre; whilst the anterior root is composed of motor nerve fibres, i.e., of fibres which conduct impulses from the centre to the periphery. The ganglion is situated on the posterior root, as a rule, in the intervertebral foramen; but the lower sacral nerves have the ganglia on their posterior roots in the spinal canal. These ganglia contain bipolar nerve cells, and the nerve fibres, as they

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pass through each ganglion, are apparently connected with the poles of the cells. The roots of the spinal nerves vary in direction and length. Those of the cervical nerves are short, and run almost horizontally outwards to their respective intervertebral foramina; those of the dorsal are longer and more oblique; whilst the roots of the lumbar and sacral nerves, owing to the cord ending much above the foramina through which the nerves proceed, are very long, and form a leash of nerves in the lower part of the spinal canal, which surrounds the filum terminale, and, from its general resemblance in arrangement to the hairs of a horse's tail, has been named cauda equina.

The anterior nerve root joins the posterior immediately outside the ganglion, and by their junction a spinal nerve is formed. This nerve contains a mixture of both motor and sensory fibres, and is compound therefore in function. Almost immediately after its formation the nerve separates into two divisions, an anterior and a posterior, and each division, like the nerve itself, contains both motor and sensory fibres.

The Posterior Primary Divisions of the spinal nerves, smaller than the anterior, are distributed both to the muscles and skin on the back of the axial part of the body. Their general arrangement is as follows: each division, with some three or four exceptions, subdivides into an internal and an external branch. In the back of the neck and the back of the upper part of the chest, the external branches of these nerves supply the deep muscles; the internal branches pierce the muscles close to the spines of the vertebræ, and end in the skin; the internal branch of the second nerve, called great occipital, and that of the third cervical, pass to the skin over the occipital bone. In the back of the lower part of the chest and of the loins, the internal branches supply the deep muscles, the external branches pass to the skin, those of some of the lumbar nerves extending as far as the skin of the buttock.

The Anterior Primary Divisions are not so uniform either in arrangement or distribution as are the pos terior. They supply the

The

PD

M

AR

SC

AD

AC

front and sides of the axial part of the neck and trunk, and the extremities. anterior divisions of the twelve thoracic nerves have the most simple arrangement. Each nerve, called from its position an intercostal nerve, runs outwards, immediately below the lower border of a rib, FIG. 65.-Diagram of the arrangement of a and gives origin to three series of branches, named communicating, muscular, and cutaneous. By the Communicating branch each intercostal nerve is connected with an adjacent ganglion on the thoracic portion of the sympathetic system. By the Muscular or motor branches these nerves supply the intercostal muscles, the levatores costarum, and the triangularis sterni, whilst the lower intercostal nerves run forwards and downwards into the wall of the abdomen, and supply the two oblique, the transverse, rectus, and pyramidalis muscles. The skin of the sides of the thorax and abdomen receives its nervous supply from the Lateral Cutaneous branches, whilst the skin on the front of the trunk is supplied by the Anterior Cutaneous terminations of these nerves. The lateral cutaneous branches of the second and third intercostal nerves are comparatively large in size, and assist in the supply of the skin of the inner side of

pair of thoracic spinal nerves. SC, spinal cord; AR, anterior nerve root; PR, posterior root, with its ganglion; PD, posterior primary division; AD, anterior

primary division, or intercostal nerve; SG, sympathetic ganglion, with the com

municating branchies between it and the anterior division; M, muscles, with the motor branches entering them; LC, lateral cutaneous, and AC, anterior cutaneous nerves,

Nervous plexuses.

the upper arm; hence they are called intercosto-humeral | colli, rhomboid, and subclavius muscles; the supra and

nerves.

In the regions of the neck, loins, and pelvis, the anterior divisions of the spinal nerves do not pass simply outwards to their distribution. In each region adjacent nerves interlace with each other, and form what is technically called a nervous plexus. When a branch arises from a thoracic nerve, it contains fibres derived from that nerve only; but when a branch arises from a plexus, it may contain fibres, not of one only, but of two or more of the nerves which, by their interlacement, form the plexus. Hence the parts which are supplied by these branches are brought into connection with a greater number of nerves, and consequently with a greater extent of the spinal cord or nerve centre, than are the parts which receive branches from a single nerve only. These plexuses are especially found in connection with the nerves which supply the extremities, where, owing to the complexity of the muscular movements, the co-ordination of these movements through the nervous system is rendered necessary.

The anterior divisions of the eight cervical nerves are arranged in two plexuses, named cervical and brachial. The Cervical plexus (Pl. XVII.) is formed of the four upper cervical nerves, which make, by interlacement with each other, a series of loops in front of the transverse processes of the cervical vertebræ. Arising either directly from these nerves, or from the plexus which they form, are communicating, muscular, and cutaneous branches. The Communicating branches connect these nerves with the large superior cervical ganglion of the sympathetic system, also with the vagus, accessory, and hypoglossal cranial nerves, and with the descending branch of the hypoglossal. The Muscular branches supply the anterior recti muscles of the neck, the levator scapula, the posterior scalenus, the diaphragm, and in part the sterno-mastoid and trapezius. The branch to the diaphragm, or the phrenic nerve, is the most important (Pl. XVII. ); it springs from the third, fourth, and fifth cervical, and passes down the lower part of the neck, and through the thorax, to supply its own half of the diaphragm. The Cutaneous branches are as follows: the occipitalis minor, to the skin of the occiput; the auriculo-parotidean, to the skin over the parotid gland and the adjacent part of the auricle; the transversalis colli, to the skin of the front of the side of the neck; the supra-clavicular nerves, to the skin of the lower part of the side of the neck, and upper part of the chest.

infra-spinatus muscles, through a branch called suprascapular; the serratus magnus, through the posterior thoracic branch; the greater and lesser pectorals, through the two anterior thoracic branches; and the subscapularis, teres major, and latissimus dorsi, through the three subscapular branches. The Cutaneous branches arise from the inner cord, and are the lesser internal cutaneous, which ends in the skin of the inner side of the upper arm, and joins the intercosto-humeral; and the internal cutaneous, which not only sends branches to the skin of the upper arm, but supplies the skin of the inner side of the forearm, both on its anterior and posterior surfaces. The Mixed branches are large and very important:a, The Circumflex, from the posterior cord, supplies the deltoid and teres minor muscles, the skin over the deltoid, and the shoulder joint. b, The Musculo-Spiral, also from the posterior cord, supplies the triceps and anconeus, the supinator longus and extensor carpi radialis longior muscles; and by its external cutaneous branch, the skin of the outer side of the back of the forearm. It then divides into the radial and posterior interosseous branches. The radial passes through the forearm to the hand, and supplies the skin on the back of the thumb, index and middle digits, and radial side of the ring digit. The posterior interosseous supplies the muscles on the back of the forearm and the articulations of the carpal joints. c, The Musculo-Cutaneous branch of the outer cord of the plexus supplies the biceps, brachialis anticus, and coraco-brachialis muscles, and ends in an external cutaneous branch, which supplies the skin of the outer side of the forearm, both in front and behind. d, The Ulnar nerve arises from the inner cord, passes through the upper arm, and enters the forearm between the inner condyle and olecranon, where it supplies the elbow joint. Here it may easily be compressed, when a pricking sensation is experienced in the course of its distribution. In this spot it is popularly called the "funny bone." In the forearm the ulnar nerve supplies the flexor carpi ulnaris and inner part of the flexor profundus digitorum muscles. In the hand it supplies the muscles of the ball of the little finger, the two inner lumbricales, the interossei muscles, and the adductor and deep part of the short flexor of the thumb. It also supplies a dorsal cutaneous branch to the back of the hand, and the back of the little and of the ulnar side of the ring digits. Palmar cutaneous branches are also given to the palm and the palmar aspects of the same digits. e, The Median nerve arises by two roots, one from the inner, the other from the outer cord of the plexus. It enters the forearm in front of the elbow joint, supplies, either directly or through its anterior interosseous branch, all the flexors and pronators, except those supplied by the ulnar; is continued to the hand, where it supplies the abductor, opponens, superficial part of the short flexor of the thumb, and two outer lumbrical muscles. It also supplies a palmer branch to the skin of the palm, and gives digital cutaneous branches to the thumb, index and middle digits, and radial side of the ring digit.

The Brachial plexus (Pl. XVII. 1, 2, 3, 4) is formed of the four lower cervical nerves, and of the larger portion of the first intercostal, called also first dorsal nerve. It is of large size, and is principally for the supply of the upper limb. Its exact mode of arrangement presents many variations, but the following is not unfrequently found:-The fifth and sixth nerves join to form a large nerve, which, after a short course, is joined by the seventh; in this manner the upper cord of the plexus is formed. The eighth cervical and the first dorsal then join, to form the lower cord of the plexus. The Lumbar plexus, of large size, is situated at the back These cords then pass behind the clavicle and subclavius of the abdominal cavity in the region of the loins, and is muscle into the axilla, where they become modified in formed by the four upper lumbar nerves, which form a arrangement. From each a large branch arises, and these series of loop-like interlacements in front of the transverse two branches then join to form a third cord. These three processes of the lumbar vertebræ. It gives origin to comcords have special relations to the axillary artery: the one municating, muscular, cutaneous, and mixed branches. which lies to its outer side is named the outer cord; that to The Communicating branches join the four upper lumbar the inner, the inner cord; that behind, the posterior cord. ganglia of the sympathetic system. The Muscular These nerves and the cords formed by them give origin to branches supply the quadratus lumborum muscle, and communicating, muscular, cutaneous, and mixed branches. give branches to the psoas. The Cutaneous branches are The Communicating branches join the middle and in-named-a, Ilio-hypogastric, which gives an iliac branch to ferior cervical and first thoracic ganglia of the sympathetic the skin of the buttock, and a hypogastric branch to the system. The Muscular branches supply the scaleni, longus skin of the abdomen above the pubic symphysis; b,

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