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sinuses, named lateral, which curve forwards and down When the skull cap is removed, clusters of granular Pacchio. wards to the jugular foramina to terminate in the internal bodies are usually to be seen imbedded in the dura mater nian
bodies. jugular veins. In its course each lateral sinus receives two on each side of the superior longitudinal sinus; these are petrosal sinuses, which pass from the cavernous sinus back- named the Pacchionian bodies. When traced through the wards along the upper and lower borders of the petrous dura mater they are found to spring from the visceral or part of the temporal bone.
proper cerebral arachnoid. The observations of Luschka The spinal part of the dura mater hangs loosely in the and Cleland have proved that villous processes invariably spinal canal. It does not form a periosteum for the vertebræ, grow from the free surface of that membrane, and that when but is separated from their bony rings by loose fat and a these villi greatly increase in size they form the bodies plexus of veins. It gives off no bands from its inner surface, in question. Sometimes the Pacchionian bodies greatly and it does not split into two layers for the lodgment of hypertrophy, occasion absorption of the bones of the cranial venous blood sinuses. The spinal dura mater forms a tubu- vault, and depressions on the upper surface of the brain. lar envelope for the spinal cord and the origins of the spinal Pia mater.—This membrane closely invests the whole Pia mater. nerves. It extends from the foramen magnum, where it is outer surface of the brain. It dips into the fissures continuous with the cranialdura mater, to the lowerend of the between the convolutions, and a wide prolongation, sacral canal, ends below in a funnel-shaped prolongation, and named velum interpositum, lies in the interior of the is pierced laterally by the roots of the several spinal nerves cerebrum. With a little care it can be stripped off the in their passage outwards to the intervertebral foramina. brain without causing injury to its substance. The pia
Both the cranial and the spinal parts of the dura mater mater invests the spinal cord, and is more intimately consist of a tough, fibrous membrane; somewhat flocculent attached to it than to the brain, for not only does it send externally, but smooth, glistening, and free on its inner prolongations into the anterior and posterior fissures of the surface. The inner surface has the appearance of a serous cord, but slender bands pass repeatedly from its inner membrane, and when examined microscopically is seen to surface into the columns of the cord. Hence it cannot be consist of a layer of squamous endothelial cells, similar to stripped off the cord without causing injury to its subthose drawn in fig. 34.
Hence the dura mater is some stance. The pia matter is prolonged on to the roots both of tiines called a fibro-serous membrane. The dura mater is the cranial and spinal nerves, and on to the filum terminale. well provided with lymph vessels, which in all probability This membrane consists of a delicate connective tissue, in open by stomata on the free inner surface. Between the which the arteries of the brain and spinal cord ramify and dura mater and the subjacent arachnoid membrane is a fine subdivide into small branches before they penetrate the space containing a minute quantity of limpid serum, which nervous substance, and in which the veins conveying the moistens the smooth inner surface of the dura and the blood from the nerve centres lie before they open into the corresponding smooth outer surface of the arachnoid. It blood sinuses of the cranial dura mater and the extrais regarded as equivalent to the cavity of a serous mem dural venous plexus of the spinal canal. The arteries brane, and is named the arachnoid cavity, or, more which pass from the pia mater into the brain and spinal appropriately, the sub-dural space.
cord are invested by a loose sheath, which has been deArachnoid Arachnoid mater.—The arachnoid is a membrane of scribed as forming the wall of a peri-vascular lymphatic membrane, great delicacy and transparency, which loosely envelopes vessel ; but Key and Retzius have shown that the space
both the brain and spinal cord. It is separated from these between the blood-vessel and the sheath opens into the sub-
space is more distinctly marked beneath the is also well provided with nerves, which arise from the
culatum. freely-communicating loculi by bundles of delicate areolar about twenty-two pairs of denticulated processes to the tissue, which bundles are invested, as Key and Retzius have inner surface of the dura mater. It is named ligament un shown, by a layer of squamous endothelium. The space denticulatum, and its teeth alternate with the successive contains a limpid cerebro-spinal fluid, which varies in quan- pairs of spinal nerves. tity from 2 drachms to 2 ounces. The fluid is alkaline, of SPINAL CORD.—The MEDULLA SPINALIS, or SPINAL Spinal sp. gr. 1.005, contains a little albumen, and a substance CORD, occupies the spinal canal, and extends from the cord which, as Turner pointed out, reduces blue oxide of foramen magnum to opposite the body of the first lumbar copper to the state of yellow sub-oxide. The arachnoid vertebra. In the early fætus it equals in length the canal membrane is made up of delicate connective tissue. itself ; but as the spinal column grows at a greater proThe free surface next the sub-dural space is smooth, portional rate than the cord, the latter, when growth has like a serous membrane, and covered by a layer of squam- ceased, is several inches shorter than the column. The cord ous endothelium. This layer is reflected on to the roots is continuous above with the medulla oblongata, whilst it of the spinal and cranial nerves, and, when they pierce the tapers off below into a slender thread, the filum terminale, dura mater, it becomes continuous with the endothelial which lies in the axis of the sacral canal, and is attached lining of that membrane. As the arrangement and struc- below to the back of the coccyx, or to the fibrous memture so closely correspond with what is seen in the serous brane which closes in below the sacral canal. The length membranes, many anatomists regard the arachnoid as the of the cord is from 15 to 18 inches. It approaches a visceral layer of a serous membrane, and the endothelial cylinder in shape, but is flattened on its anterior and poslining of the dura mater as the parietal layer, whilst the terior surfaces, and presents two enlargements which have sub-dural space is the intermediate cavity.
a greater girth than the rest of the cord.
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 diameter, but in all the axial cylinder and medullary sheath from it arise the nerves which supply the upper limbs; the can be distinctly seen. Wherever the nerve roots enter lower, called the crural or lumbar enlargement, is opposite into the cord, the fibres of these roots pass transversely or the last dorsal vertebra, and supplies with nerves the obliquely in their course inwards to the grey matter. lower limbs. The cord is almost completely divided into Horizontal fibres are also found in the white anterior comright and left lateral halves by two fissures, named re- missure, and a similar appearance can be seen in the spectively anterior and posterior median fissures, which do posterior commissure. Horizontal fibres have also been not quite reach the centre of the cord, for at the bottom of traced from the lateral columns into the adjacent part of the anterior fissure are the transverse fibres of the anterior the grey matter. white commissure, and at the bottom of the posterior fissure The grey crescentic portion of the cord contains con
he fibres of the posterior grey commissure. By these com nective tissue, blood-vessels, nerve fibres, and nerve cells. missures the two halves of the cord are united together. The nerve fibres in the grey matter are numerous; and The fibres of the posterior commissure surround a canal, whilst some possess a medullary sheath, others consist called the central canal, which extends along the whole only of the axial cylinder; they divide and subdivide, length of the cord, and even passes into the upper end of and, as Gerlach has shown, form a narrow - meshed the filuin terminale. This canal is lined by a ciliated network of extremely minute fibres. The nerve cells are columnar endothelium, and expands superiorly into the multipolar, and are chiefly collected in the anterior and cavity of the fourth ventricle. Each lateral half of the cord posterior horns of each crescent. The cells of the anis subdivided into three columns by two depressions, which terior cornu are large, distinct, and stellate, and form mark the points of emergence of the roots of the spinal a well-defined group of nerve cells. Those of the posuerves. The anterior nerve roots pass through the terior cornu are smaller in size, more elongated in shape, antero-lateral depression or fissure, and between it and the but with stellate branched processes. They are not so antero-median fissure is the anterior column of the cord. distinct as in the anterior horn, owing to the connective The posterior nerve roots pass through the postero-lateral tissue with its corpuscles being so abundant. This tissue fissure, and between it and the postero-median fissure is is best marked at the tip of the posterior horn, where it the posterior column, whilst between the anterior and forms the substantia gelatinosa of Rolando. Lockhart posterior nerve roots lies the lateral column. In the Clarke has described an intermedio-lateral group of nerve cervical region, the part of the posterior column which lies cells situated at the outer side of the grey matter, about next tue postero-median fissure is marked off by a fissure midway between the anterior and posterior horns, in the into a small internal or postero-median column. The sub- upper part of the cervical portion of the cord, and in the division of each
thoracic part between the brachial and crurat enlargements. lateral half of the
The course of the fibres in the cord and their relations cord into the
to the nerve cells should now be considered. There can columns, and the
be no doubt that of the longitudinal fibres some ascend arrangement of its
from below upwards, and conduct either excito-motory nervous tissues, are
impulses to the regions of the spinal cord itself, or senwell seen in trans
sory impulses to the brain. Other longitudinal fibres again verse sections
descend from the brain and higher regions of the cord to through its sub
the lower, and conduct motor and vaso-motor impulses stance. The cord
from above downwards. The horizontal and oblique fibres is composed of
of an anterior or motor nerve root enter the grey matter of white and grey
the anterior cornu, and seem to have the following arrangematter. The white
ment: some become directly continuous with the axial cylinmatter is external, Fla. 64.- Transverse section through the spinal cord. drical processes of the nerve cells; others pass into the anand forms the coPC, posterior, LC, lateral, and AC, anterior columns;
terior commissure; others extend as far as the grey matter lumns of the cord. AR, anterior, and PR, posterior nerve roots ; C. of the posterior horn. The nerve cells of the anterior cornu The grey matter is lining. The pia mater is shown investing the cord,
give origin, therefore, directly to nerve fibres by their unsurrounded by the sending processes into the anteriorand posterior branched processes. Gerlach's observations show that the white, and has columns. The crescentic arrangement of the grey branched processes of these cells become continuous with in each lateral matter is shown by the darker shaded portion.
the network of extremely minute fibres already described Lalf of the cord a crescentic shape. The horns of the in the grey matter; from this network medullated fibres crescent are directed towards the fissures of emergence of the appear to arise which leave the grey matter; some enter the nerve roots; the anterior horn is rounded; the posterior lateral column, and ascend as the fibres of this structure; long and narrow. The proportion of grey matter to the others pass as fibres of the anterior commissure to the white varies in different parts of the cord. At the com- opposite side of the cord, and ascend as the anterior column mencement of the filum terminale there is scarcely any of that side. The anterior and lateral columns, therefore, white matter; but the white matter increases in amount are constantly receiving accessions of fibres from the from below upwards, so that its absolute quantity is greatest enclosed grey matter. in the cervical part of the cord. The grey crescents are The fibres of a posterior or sensory nerve root on entering thicker in the upper and lower enlargements than in the the cord subdivide into two bundles; one does not enter the intermediate part.
grey matter, but applies itself to the posterior column, of The cord contains both nerve fibres and nerve cells. The which it forms some of the vertical fibres. These fibres external, columnar, white part of the cord consists of nerve may ascend to the brain, or they may at some higher point fibres, with a supporting reticular framework of connective in the cord enter the grey matter of the posterior horn. The tissue and blood-vessels derived from the pia mater. Well- other bundle of posterior root fibres at once enters the formed stellate connective tissue corpuscles lie in this sup- posterior horn of grey matter. The connections and ultiporting framework. The nerve fibres of the various columns mate arrangement of these fibres in the grey matter have extend longitudinally, and lie parallel to each other, so that not been satisfactorily made out. Gerlach states that, as in transverse sections through the columns the fibres are they frequently subdivide on entering the grey matter, it
is possible they may form the fine nerve fibre plexus of the į pass through each ganglion, are apparently connected with grey substance; but a direct continuity between them and the poles of the cells. The roots of the spinal nerves vary the axial-cylinder processes of the cells of the posterior | in direction and length. Those of the cervical nerves horn does not seem to have been observed. From the are short, and run almost horizontally outwards to their plexus, formed by the much subdivided processes of these respective intervertebral foramina; those of the dorsal are cells, fibres arise, which, forming the fibres of the pos- longer and more oblique; whilst the roots of the lumbar terior commissure, pass both in front of and behind the and sacral nerves, owing to the cord ending much above central canal to the opposite side, where they ascend towards the foramina through which the nerves proceed, are very the brain, "partly in the vertical fasciculi of the posterior long, and form a leash of nerves in the lower part of the cornua and partly in the posterior columns.”
spinal canal, which surrounds the filum terminale, and, from The structure of the spinal cord shows it to be both a its general resemblance in arrangement to the hairs of a nerve cells in its grey matter give rise either directly, | OfThe anterior nerve root joins the posterior immediately or through the delicate plexus formed by their branching outside the ganglion, and by their junction a spinal nerve processes, to nerve fibres, which may either pass out of is formed. This nerve contains a mixture of both motor the cord as the anterior and posterior roots of the spinal and sensory fibres, and is compound therefore in function. nerves, or may ascend to the brain as the columns of the Almost immediately after its formation the nerve separates cord. Hence the cord is anatomically continuous, on the into two divisions, an anterior and a posterior, and each one hand, through the nerves which arise from it, with the division, like the nerve itself, contains both motor and peripheral end-organs in the skin, and muscular system in sensory fibres. which those nerves terminate; and, on the other hand, it is The Posterior Primary Divisions of the spinal nerves, continuous with the brain. It serves, therefore, to conduct smaller than the anterior, are distributed both to the the impulses of touch-sensation from the skin upwards muscles and skin on the back of the axial part of the body. to the brain, and the motor impulses from the brain Their general arrangement is as follows: each division, downwards to the muscles. But further, the cord is the with some three or four exceptions, subdivides into an great nerve centre concerned in reflex excito-motory internal and an external branch. In the back of the neck actions. It must, also, be remembered that the two halves and the back of the upper part of the chest, the external of the cord are anatomically continuous with each other branches of these nerves supply the deep muscles; the through the nerve fibres of the commissures, so that it acts internal branches pierce the muscles close to the spines as a single organ, and not as two organs. Experiments of the vertebræ, and end in the skin; the internal branch have shown that sensory impulses are conducted upwards of the second nerve, called great occipital, and that of through the cord, not by that half from which the nerves the third cervical, pass to the skin over the occipital bone. arise that have been excited, but by the opposite half of In the back of the lower part of the chest and of the loins, the cord, which is obviously due to the crossing of the the internal branches supply the deep muscles, the external fibres of the posterior commissure. Motor impressions branches pass to the skin, those of some of the lumbar are, however, conducted downwards by that half of the nerves extending as far as the skin of the buttock. cord from which the nerves arise that pass to supply the The Anterior Primary Divisions are not so uniform muscles to be moved.
either in arrangement or distribution as are the posThe spinal cord is well supplied with blood by numerous terior. They supply the arteries, which terminate in a diffused capillary network. front and sides of the axial The capillaries are much more numerous in the grey matter part of the neck and trunk, of the cord than in the white columns.
and the extremities. The ORIGIN, ARRANGEMENT, AND DISTRIBUTION OF THE anterior divisions of the SPINAL NERVES.—The spinal cord gives origin to thirty-twelve thoracic nerves have one pairs of SPINAL nerves, which pass out of the spinal | the most simple arrangecanal through the intervertebral foramina.
ment. Each nerve, called are arranged in groups, according to the region of the from its position an interspine through the foramina in which they proceed. costal nerve, runs outThere are eight pairs of cervical nerves; the first or wards, immediately below sub-occipital emerges between the occipital bone and the the lower border of a rib, Fig. 65.-- Diagram of the arrangement of a
pair of thoracic spinal nerves. SC, spinal atlas, the eighth between the seventh cervical and first and gives origin to three cord; AR, anterior nerve root; PR, posdorsal vertebræ. Twelve dorsal or thoracic nerves pass out series of branches, named
terior root, with its ganglion; PD, pos
terior primary division; AD, anterior on each side in relation to the dorsal vertebræ: five pairs communicating, muscular, primary division, or intercostal nerve; of lumbar nerves in the region of the loins; five pairs of and cutaneous.
SG, sympathetic ganglion, with the com
municating branches between it and the sacral nerves through the sacral foramina; and one pair Communicating branch anterior division; M, muscles, with the
motor branches entering them; LC, of coccygeal nerves through the lowest openings in the each intercostal nerve is lateral cutaneous, and AC, anterior spinal canal.
Each spinal nerve arises by two roots, an connected with an adanterior and a posterior, from the side of the cord. These jacent ganglion on the thoracic portion of the sympathetic roots are distinguished from each other both anatomically system. By the Muscular or motor branches these nerves and physiologically. The posterior root has a swelling or supply the intercostal muscles, the levatores costarum, and ganglion on it, whilst no ganglion exists on the anterior the triangularis sterni, whilst the lower intercostal nerves root. The posterior root consists of sensory nerve fibres, run forwards and downwards into the wall of the abdomen, i.e., of fibres which conduct impulses from the periphery and supply the two oblique, the transverse, rectus, and into the nerve centre; whilst the anterior root is composed pyramidalis muscles. The skin of the sides of the thorax of motor nerve fibres, i.e., of fibres which conduct im- and abdomen receives its nervous supply from the Lateral pulses from the centre to the periphery. The ganglion is Cutaneous branches, whilst the skin on the front of the situated on the posterior root, as a rule, in the interverte- trunk is supplied by the Anterior Cutaneous terminations of bral foramen; but the lower sacral nerves have the ganglia these nerves. The lateral cutaneous branches of the second on their posterior roots in the spinal canal. These ganglia and third intercostal nerves are comparatively large in size, contain bipolar nerve cells, and the nerve fibres, as they and assist in the supply of the skin of the inner side of
the upper arm; hence they are called intercosto-humeral colli, rhomboid, and subclavius muscles; the supra and
infra-spinatus muscles, through a branch called supraNervous In the regions of the neck, loins, and pelvis, the anterior scapular ; the serratus magnus, through the posterior plexuses. divisions of the spinal nerves do not pass simply outwards thoracic branch; the greater and lesser pectorals, through
to their distribution. In each region adjacent nerves in the two anterior thoracic branches; and the subscaputerlace with each other, and form what is technically called laris, teres major, and latissimus dorsi, through the three a nervous plexus. When a branch arises from a thoracic subscapular branches. The Cutaneous branches arise from nerve, it contains fibres derived from that nerve only; but the inner cord, and are the lesser internal cutaneous, when a branch arises from a plexus, it may contain fibres, which ends in the skin of the inner side of the upper not of one only, but of two or more of the nerves which, arm, and joins the intercosto-humeral; and the internal by their interlacement, form the plexus. Hence the parts cutaneous, which not only sends branches to the skin of which are supplied by these branches are brought into con the upper arm, but supplies the skin of the inner side nection with a greater number of nerves, and consequently of the forcarm, both on its anterior and posterior surfaces. with a greater extent of the spinal cord or nerve centre, The Mixed branches are large and very important:than are the parts which receive branches from a single a, The Circumflex, from the posterior cord, supplies the nerve only. These plexuses are especially found in con deltoid and teres minor muscles, the skin over the delnection with the nerves which supply the extremities, toid, and the shoulder joint. b, The Musculo-Spiral, also where, owing to the complexity of the muscular move from the posterior cord, supplies the triceps and anconeus, ments, the co-ordination of these movements through the the supinator longus and extensor carpi radialis longior nervous system is rendered necessary.
muscles; and by its external cutaneous branch, the skin of The anterior divisions of the eight cervical nerves are the outer side of the back of the forearm. It then divides arranged in two plexuses, named cervical and brachial. into the radial and posterior interosseous branches. The
The Cervical plexus (Pl. XVII.) is formed of the four radial passes through the forearm to the hand, and supplies upper cervical nerves, which make, by interlacement with the skin on the back of the thumb, index and middle digits, each other, a series of loops in front of the transverse and radial side of the ring digit. The posterior interosseprocesses of the cervical vertebræ. Arising either directly ous supplies the muscles on the back of the forearm and the from these nerves, or from the plexus which they form, are articulations of the carpal joints. c, The Musculo-Cutanecommunicating, muscular, and cutaneous branches. The ous branch of the outer cord of the plexus supplies the Communicating branches connect these nerves with the biceps, brachialis anticus, and coraco-brachialis muscles, large superior cervical ganglion of the sympathetic system, and ends in an external cutaneous branch, which supplies also with the vagus, accessory, and hypoglossal cranial the skin of the outer side of the forearm, both in front and nerves, and with the descending branch of the hypoglossal. behind. d, The Ulnar nerve arises from the inner cord, The Muscular branches supply the anterior recti muscles passes through the upper arm, and enters the forearm beof the neck, the levator scapulæ, the posterior scalenus, tween the inner condyle and olecranon, where it supplies the diaphragm, and in part the sterno-mastoid and trape- the elbow joint. Here it may easily be compressed, when zius. The branch to the diaphragm, or the phrenic nerve, a pricking sensation is experienced in the course of its disis the most important (Pl. XVII. o); it springs from the tribution. In this spot it is popularly called the “funny third, fourth, and fifth cervical, and passes down the lower bone.” In the forearm the ulnar nerve supplies the flexor part of the neck, and through the thorax, to supply its carpi ulnaris and inner part of the flexor profundus digiown half of the diaphragm. The Cutaneous branches are torum muscles. In the hand it supplies the muscles of the as follows:-the occipitalis minor, to the skin of the ball of the little finger, the two inner lumbricales, the interocciput; the auriculo-parotidean, to the skin over the ossei muscles, and the adductor and deep part of the short parotid gland and the adjacent part of the auricle; the flexor of the thumb. It also supplies a dorsal cutaneous transversalis colli, to the skin of the front of the side of branch to the back of the hand, and the back of the little the neck; the supra-clavicular nerves, to the skin of the and of the ulnar side of the ring digits. Palmar cutaneous lower part of the side of the neck, and upper part of the branches are also given to the palm and the palmar aspects chest.
of the same digits. e, The Median nerve arises by two roots, The Brachial plexus (Pl. XVII. 1, 2, 3, 4) is formed one from the inner, the other from the outer cord of the of the four lower cervical nerves, and of the larger por- plexus. It enters the forearm in front of the elbow joint, tion of the first intercostal, called also first dorsal nerve. supplies, either directly or through its anterior interosseous It is of large size, and is principally for the supply branch, all the flexors and pronators, except those supplied of the upper limb. Its exact mode of arrangement by the ulnar; is continued to the hand, where it supplies presents many variations, but the following is not un the abductor, opponens, superficial part of the short flexor frequently found :-The fifth and sixth nerves join to of the thumb, and two outer lumbrical muscles. It also form a large nerve, which, after a short course, is joined supplies a palmer branch to the skin of the palm, and gives by the seventh ; in this manner the upper cord of the digital cutaneous branches to the thumb, index and middle plexus is formed. The cighth cervical and the first digits, and radial side of the ring digit. 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 ; 6,