Introduction to central nervous system

THE NERVOUS SYSTEM is probably the most difficult and extremely organized of the assorted methods which make up the human physique. It’s the mechanism involved with the correlation and integration of varied bodily processes and the reactions and changes of the organism to its surroundings. As well as the cerebral cortex is anxious with acutely aware life. It might be divided into two components, central and peripheral.

The central nervous system consists of the encephalon or mind, contained inside the skull, and the medulla spinalis or spinal twine

lodged within the vertebral canal; the 2 parts are steady with each other on the stage of the higher border of the atlas vertebra.

Neuron Idea

The nerve cell and its processes collectively represent what’s termed a neuron, and Waldeyer formulated the idea that the nervous system is constructed up of quite a few neurons, “anatomically and genetically unbiased of each other.” Based on this idea (neuron idea) the processes of 1 neuron solely come into contact, and are by no means in direct continuity, with these of different neurons; whereas impulses are transmitted from one nerve cell to a different by means of these factors of contact, the synapses.

The synapse or synaptic membrane appears to permit nervous impulses to go in a single route solely, particularly, from the terminals of the axis-cylinder to the dendrons. This idea is predicated on the next details, viz.: (1) embryonic nerve cells or neuroblasts are totally distinct from each other; (2) when nervous tissues are stained by the Golgi technique no continuity is seen even between neighboring neurons; and (3) when degenerative modifications happen in nervous tissue, both as the results of illness or experiment, they by no means unfold from one neuron to a different, however are restricted to the person neurons, or teams of neurons, primarily affected. It should, nonetheless, be added that inside the previous few years the validity of the neuron idea has been referred to as in query by sure eminent histologists, who keep that by the employment of extra delicate histological strategies, minute fibrils may be adopted from one nerve cell into one other. Their existence, nonetheless, within the dwelling is open to query. Mott and Marinesco made cautious examinations of dwelling cells, utilizing even the ultramicroscope and agree that neither Nissl our bodies nor neurofibrils are current within the dwelling state.

For the current we could look upon the neurons because the items or structural components of the nervous system. All of the neurons are current at delivery that are current within the grownup, their division ceases earlier than delivery; they don’t seem to be all functionally energetic at delivery, however steadily assume purposeful exercise. There is no such thing as a indication of any regeneration after the destruction of the cell-body of any particular person neuron.

The peripheral nervous system consists of a collection of nerves by which the central nervous system is linked with the assorted tissues of the physique. For descriptive functions these nerves could also be organized in two teams, cerebrospinal and sympathetic, the association, nonetheless, being an arbitrary one, because the two teams are intimately linked and carefully intermingled. Each the cerebrospinal and sympathetic nerves have nuclei of origin (the somatic efferent and sympathetic efferent) in addition to nuclei of termination (somatic afferent and sympathetic afferent) within the central nervous system. The cerebrospinal nerves are forty-three in quantity on both facet—twelve cranial, connected to the mind, and thirty-one spinal, to the medulla spinalis. They’re related to the features of the particular and basic senses and with the voluntary actions of the physique. The sympathetic nerves transmit the impulses which regulate the actions of the viscera, decide the caliber of the bloodvessels, and management the phenomena of secretion. In relation with them are two rows of central ganglia, located one on both facet of the center line in entrance of the vertebral column; these ganglia are intimately linked with the medulla spinalis and the spinal nerves, and are additionally joined to one another by vertical strands of nerve fibers in order to represent a pair of knotted cords, the sympathetic trunks, which attain from the bottom of the cranium to the coccyx. The sympathetic nerves issuing from the ganglia type three nice prevertebral plexuses which provide the thoracic, belly, and pelvic viscera; in relation to the partitions of those viscera intricate nerve plexuses and quite a few peripheral ganglia are discovered.

1. Construction of the Nervous System

The nervous tissues are composed of nerve cells and their numerous processes, along with a supporting tissue referred to as neuroglia, which, nonetheless, is discovered solely within the mind and medulla spinalis. Sure lengthy processes of the nerve cells are of particular significance, and it’s handy to think about them aside from the cells; they’re referred to as nerve fibers.

To the bare eye a distinction is clear between sure parts of the mind and medulla spinalis, viz., the grey substance and the white substance. The grey substance is basically composed of nerve cells, whereas the white substance incorporates solely their lengthy processes, the nerve fibers. It’s within the former that nervous impressions are obtained, saved, and reworked into efferent impulses, and by the latter that they’re performed. Therefore the grey substance kinds the important constituent of all of the ganglionic facilities, each these within the remoted ganglia and people aggregated within the mind and medulla spinalis; whereas the white substance kinds the majority of the commissural parts of the nerve facilities and the peripheral nerves.

Neuroglia.—Neuroglia, the peculiar floor substance wherein are imbedded the true nervous constituents of the mind and medulla spinalis, consists of cells and fibers. A number of the cells are stellate in form, with ill-defined cell physique, and their advantageous processes turn out to be neuroglia fibers, which lengthen radially and unbranched  among the many nerve cells and fibers which they support in supporting. Different cells give off fibers which department repeatedly. A number of the fibers begin from the epithelial cells lining the ventricles of the mind and central canal of the medulla spinalis, and go by means of the nervous tissue, branching repeatedly to finish in slight enlargements on the pia mater. Thus, neuroglia is evidently a connective tissue in perform however will not be so in improvement; it’s ectodermal in origin, whereas all connective tissues are mesodermal.

Nerve Cells are largely aggregated within the grey substance of the mind and medulla spinalis, however smaller collections of those cells additionally type the swellings, referred to as ganglia, seen on many nerves. These latter are discovered mainly upon the spinal and cranial nerve roots and in reference to the sympathetic nerves.

The nerve cells fluctuate in form and dimension, and have a number of processes. They might be divided for functions of description into three teams, in accordance with the variety of processes which they possess: (1) Unipolar cells, that are discovered within the spinal ganglia; the only course of, after a brief course, divides in a T-shaped method. (2) Bipolar cells, additionally discovered within the spinal ganglia when the cells are in an embryonic situation. They’re greatest demonstrated within the spinal ganglia of fish. Typically the processes come off from reverse poles of the cell, and the cell then assumes a spindle form; in different cells each processes emerge on the identical level. In some circumstances the place two fibers are apparently linked with a cell, one of many fibers is basically derived from an adjoining nerve cell and is passing to finish in a ramification across the ganglion cell, or, once more, it might be coiled spirally across the nerve course of which is issuing from the cell. (3) Multipolar cells, that are pyramidal or stellate in form, and characterised by their giant dimension and by the quite a few processes which situation from them. The processes are of two varieties: certainly one of them is termed the axis-cylinder course of or axon as a result of it turns into the axis-cylinder of a nerve fiber. The others are termed the protoplasmic processes or dendrons; they start to divide and subdivide quickly after they emerge from the cell, and at last finish in minute twigs and turn out to be misplaced among the many different components of the nervous tissue.

The physique of the nerve cell, referred to as the cyton, consists of a finely fibrillated protoplasmic materials, of a reddish or yellowishbrown shade, which sometimes presents patches of a deeper tint, attributable to the aggregation of pigment granules at one facet of the nucleus, as within the substantia nigra and locus cжruleus of the mind. The protoplasm additionally incorporates peculiar angular granules, which stain deeply with fundamental dyes, akin to methylene blue; these are referred to as Nissl’s granules. They lengthen into the dendritic processes however not into the axis-cylinder; the small clear space on the level of exit of the axon in some cell varieties is termed the cone of origin. These granules disappear (chromatolysis) throughout fatigue or after extended stimulation of the nerve fibers linked with the cells. They’re purported to symbolize a retailer of nervous power, and in numerous psychological ailments are poor or absent. The nucleus is, as a rule, a big, well-defined, spherical physique, typically presenting an intranuclear community, and containing a well-marked nucleolus.

Along with the protoplasmic community described above, every nerve cell could also be proven to have delicate neurofibrils operating by means of its substance; these fibrils are steady with the fibrils of the axon, and are believed to convey nerve impulses. Golgi has additionally described an extracellular community, which might be a supporting construction.

Nerve Fibers.—Nerve fibers are discovered universally within the peripheral nerves and within the white substance of the mind and medulla spinalis. They’re of two varieties—viz., medullated or white fibers, and non-medullated or grey fibers.

The medullated fibers type the white a part of the mind and medulla spinalis, and in addition the higher a part of each cranial and spinal nerve, and provides to those constructions their opaque, white side. When completely recent they look like homogeneous; however quickly after removing from the physique every fiber presents, when examined by transmitted gentle, a double define or contour, as if consisting of two components . The central portion is called the axis-cylinder; round it is a sheath of fatty materials, staining black with osmic acid, named the white substance of Schwann or medullary sheath, which supplies to the fiber its double contour, and the entire is enclosed in a fragile membrane, the neurolemma, primitive sheath, or nucleated sheath of Schwann.

The axis-cylinder is the important a part of the nerve fiber, and is at all times current; the medullary sheath and the neurolemma are sometimes absent, expecially on the origin and termination of the nerve fiber. The axis-cylinder undergoes no interruption from its origin within the nerve heart to its peripheral termination, and should be thought to be a direct prolongation of a nerve cell. It constitutes about one-half or one-third of the nerve fiber, being higher in proportion within the fibers of the central organs than in these of the nerves. It’s fairly clear, and is subsequently indistinguishable in a superbly recent and pure state of the nerve. It’s made up of exceedingly advantageous fibrils, which stain darkly with gold chloride and at its termination could also be seen to interrupt up into these fibrillж. The fibrillж have been termed the primitive fibrillж of Schultze. The axis-cylinder is claimed by some to be enveloped in a particular reticular sheath, which separates it from the medullary sheath, and consists of a substance referred to as neurokeratin. The extra frequent opinion is that this community or reticulum is contained within the white matter of Schwann, and by some it’s believed to be produced by the motion of the reagents employed to indicate it.

The medullary sheath, or white matter of Schwann is thought to be being a fatty matter in a fluid state, which insulates and protects the important a part of the nerve—the axis-cylinder. It varies in thickness, in some forming a layer of maximum thinness, in order to be scarcely distinguishable, in others forming about one-half the nerve fiber. The variation in diameter of the nerve fibers (from 2 to 16μ) relies upon primarily upon the quantity of the white substance, although the axis cylinder additionally varies inside sure limits. The medullary sheath undergoes interruptions in its continuity at common intervals, giving to the fiber the looks of constriction at these factors: these are referred to as the nodes of Ranvier. The portion of nerve fiber between two nodes known as an internodal phase. The neurolemma or primitive sheath will not be interrupted on the nodes, however passes over them as a steady membrane. If the fiber be handled with silver nitrate the reagent penetrates the neurolemma on the nodes, and on publicity to gentle discount takes place, giving rise to the looks of black crosses, Ranvier’s crosses, on the axis-cylinder. There can also be seen transverse strains past the nodes termed Frommann’s strains the importance of those will not be understood. Along with these interruptions indirect clefts could also be seen within the medullary sheath, subdividing it into irregular parts, that are termed medullary segments, or segments of Lantermann, there may be purpose to consider that these clefts are artificially produced within the preparation of the specimens. Medullated nerve fibers, when examined within the recent situation, ceaselessly current a beaded or varicose look: this is because of manipulation and stress inflicting the oily matter to gather into drops, and in consequence of the intense delicacy of the primitive sheath, even slight stress will trigger the transudation of the fatty matter, which collects as drops of oil exterior the membrane.

The neurolemma or primitive sheath presents the looks of a fragile, structureless membrane. Right here and there beneath it, and located in depressions within the white matter of Schwann, are nuclei surrounded by a small quantity of protoplasm. The nuclei are oval and considerably flattened, and bear a particular relation to the nodes of Ranvier, one nucleus typically mendacity within the heart of every internode. The primitive sheath will not be current in all medullated nerve fibers, being absent in these fibers that are discovered within the mind and medulla spinalis.

Wallerian Degeneration.—When nerve fibers are reduce throughout, the central ends of the fibers degenerate so far as the primary node of Ranvier; however the peripheral ends degenerate concurrently all through their entire size. The axons break up into fragments and turn out to be surrounded by drops of fatty substance that are shaped from the breaking down of the medullary sheath. The nuclei of the primitive sheath proliferate, and at last absorption of the axons and fatty substance happens. If the reduce ends of the nerve be sutured collectively regeneration of the nerve fibers takes place by the downgrowth of axons from the central finish of the nerve. At one time it was believed that the regeneration was peripheral in origin, however this has been disproved, the proliferated nuclei within the peripheral parts collaborating merely within the formation of the so-called scaffolding alongside which the brand new axons go.

Non-medullated Fibers.—Many of the fibers of the sympathetic system, and among the cerebrospinal, include the grey or gelatinous nerve fibers (fibers of Remak). Every of those consists of an axis-cylinder to which nuclei are utilized at intervals. These nuclei are believed to be in reference to a fragile sheath corresponding with the neurolemma of the medullated nerve fiber. In exterior look the non-medullated nerve fibers are semitransparent and grey or yellowish grey. The person fibers fluctuate in dimension, typically averaging about half the scale of the medullated fibers.

Construction of the Peripheral Nerves and Ganglia.—The cerebrospinal nerves include quite a few nerve fibers collected collectively and enclosed in membranous sheaths. A small bundle of fibers, enclosed in a tubular sheath, known as a funiculus; if the nerve is of small dimension, it might consist solely of a single funiculus; but when giant, the funiculi are collected collectively into bigger bundles or fasciculi, that are certain collectively in a typical membranous funding. In construction the frequent membranous funding, or sheath of the entire nerve (epineurium), in addition to the septa given off from it to separate the fasciculi, include connective tissue, composed of white and yellow elastic fibers, the latter present in nice abundance. The tubular sheath of the funiculi (perineurium) is a advantageous, clean, clear membrane, which can be simply separated, within the type of a tube, from the fibers it encloses; in construction it’s made up of connective tissue, which has a distinctly lamellar association. The nerve fibers are held collectively and supported inside the funiculus by delicate connective tissue, referred to as the endoneurium. It’s steady with septa which go inward from the innermost layer of the perineurium, and exhibits a floor substance wherein are imbedded advantageous bundles of fibrous connective tissue operating for probably the most half longitudinally. It serves to help capillary vessels, organized in order to type a net-work with elongated meshes. The cerebrospinal nerves consist virtually completely of medullated nerve fibers, solely a really small proportion of non-medullated being current.

The bloodvessels supplying a nerve finish in a minute capillary plexus, the vessels composing which pierce the perineurium, and run, for probably the most half, parallel with the fibers; they’re linked collectively by quick, transverse vessels, forming slender, rectangular meshes, just like the capillary system of muscle. Wonderful non-medullated nerve fibers, vasomotor fibers, accompany these capillary vessels, and break up into elementary fibrils, which type a community across the vessels. Horsley has demonstrated sure medullated fibers operating within the epineurium and terminating in small spheroidal tactile corpuscles or finish bulbs of Krause. These nerve fibers, which Marshall believes to be sensory, and which he has termed nervi nervorum, are thought-about by him to have an necessary bearing upon sure neuralgic pains.

The nerve fibers, as far as is at current recognized, don’t coalesce, however pursue an uninterrupted course from the middle to the periphery. In separating a nerve, nonetheless, into its part funiculi, it might be seen that these don’t pursue a superbly insulated course, however sometimes be a part of at a really acute angle with different funiculi continuing in the identical route; from this, branches are given off, to joint once more in like method with different funiculi. It should be distinctly understood, nonetheless, that in these communications the person nerve fibers don’t coalesce, however merely go into the sheath of the adjoining nerve, turn out to be intermixed with its nerve fibers, and once more go on to intermingle with the nerve fibers in some adjoining funiculus.

Nerves, of their course, subdivide into branches, and these ceaselessly talk with branches of a neighboring nerve. The communications which thus happen type what known as a plexus. Typically a plexus is shaped by the first branches of the trunks of the nerves—because the cervical, brachial, lumbar, and sacral plexuses—and infrequently by the terminal funiculi, as within the plexuses shaped on the periphery of the physique. Within the formation of a plexus, the part nerves divide, then be a part of, and once more subdivide in such a fancy method that the person funiculi turn out to be interlaced most intricate’y; so that every department leaving a plexus could comprise filaments from all the first nervous trunks which type the plexus. Within the formation additionally of smaller plexuses on the periphery of the physique there’s a free interchange of the funiculi and primitive fibers. In every case, nonetheless, the person fibers stay separate and distinct.

It’s possible that by means of this interchange of fibers, each department passing off from a plexus has a extra in depth reference to the spinal twine than if it had proceeded to its distribution with out forming connections with different nerves. Consequently the components provided by these nerves have extra prolonged relations with the nervous facilities; by this implies, additionally, teams of muscle tissues could also be related for mixed motion.

The sympathetic nerves are constructed in the identical method because the cerebrospinal nerves, however consist primarily of non-medullated fibers, collected in funiculi and enclosed in sheaths of connective tissue. There may be, nonetheless, in these nerves a sure admixture of medullated fibers. The variety of the latter varies in several nerves, and could also be estimated by the colour of the nerve. These branches of the sympathetic, which current a well-marked grey shade, are composed mainly of non-medullated nerve fibers, intermixed with just a few medullated fibers; whereas these of a white shade comprise lots of the latter fibers, and few of the previous.

The cerebrospinal and sympathetic nerve fibers convey numerous impressions. The sensory nerves, referred to as additionally centripetal or afferent nerves, transmit to the nervous facilities impressions made upon the peripheral extremities of the nerves, and on this manner the thoughts, by means of the medium of the mind, turns into acutely aware of exterior objects. The centrifugal or efferent nerves transmit impressions from the nervous facilities to the components to which the nerves are distributed, these impressions both thrilling muscular contraction or influencing the processes of diet, progress, and secretion.

Origins and Terminations of Nerves.—By the expression “the terminations of nerve fibers” is signified their connections with the nerve facilities and with the components they provide. The previous are generally referred to as their origins or central terminations; the latter their peripheral terminations.

Origins of Nerves.—The origin in some circumstances is single—that’s to say, the entire nerve emerges from the nervous heart by a single root; in different cases the nerve arises by two or extra roots which come off from completely different components of the nerve heart, generally extensively aside from one another, and it typically occurs, when a nerve arises on this manner by two roots, that the features of those two roots are completely different; as, for instance, within the spinal nerves, every of which arises by two roots, the anterior of which is motor, and the posterior sensory. The purpose the place the nerve root or roots emerge from the floor of the nervous heart is called the superficial or obvious origin, however the fibers of the nerve may be traced for a sure distance into the substance of the nervous heart to some portion of the grey matter, which constitutes the deep or actual origin of the nerve. The centrifugal or efferent nerve fibers originate within the nerve cells of the grey substance, the axis-cylinder processes of those cells being extended to type the fibers. Within the case of the centripetal or afferent nerves the fibers develop inward both from nerve cells within the organs of particular sense, e. g., the retina, or from nerve cells within the ganglia. Having entered the nerve heart they department and ship their final twigs among the many cells, with out, nonetheless, uniting with them.

Peripheral Terminations of Nerves.—Nerve fibers terminate peripherally in numerous methods, and these could also be conveniently studied within the sensory and motor nerves respectively. The terminations of the sensory nerves are handled within the part on Sense Organs.

Motor nerves may be traced into both unstriped or striped muscular fibers. Within the unstriped or involuntary muscle tissues the nerves are derived from the sympathetic, and are composed primarily of non-medullated fibers. Close to their terminations they divide into quite a few branches, which talk and type intimate plexuses. On the junction of the branches small triangular nuclear our bodies (ganglion cells) are located. From these plexuses minute branches are given off which divide and break up into the final word fibrillж of which the nerves are composed. These fibrillж course between the involuntary muscle cells, and, in accordance with Elischer, terminate on the surfaces of the cells, reverse the nuclei, in minute swellings.

Within the striped or voluntary muscle the nerves supplying the muscular fibers are derived from the cerebrospinal nerves, and are composed primarily of medullated fibers. The nerve, after coming into the sheath of the muscle, breaks up into fibers or bundles of fibers, which type plexuses, and steadily divide till, as a rule, a single nerve fiber enters a single muscular fiber. Typically, nonetheless, if the muscular fiber be lengthy, multiple nerve fiber enters it. Throughout the muscular fiber the nerve terminates in a particular enlargement, referred to as by Kьhne, who first precisely described it, a motor end-plate. The nerve fiber, on approaching the muscular fiber, all of the sudden loses its medullary sheath, the neurolemma turns into steady with the sarcolemma of the muscle, and solely the axis-cylinder enters the muscular fiber. There it without delay spreads out, ramifying just like the roots of a tree, instantly beneath the sarcolemma, and turns into imbedded in a layer of granular matter, containing various clear, rectangular nuclei, the entire constituting an end-plate from which the contractile wave of the muscular fiber is claimed to begin.

Ganglia are small aggregations of nerve cells. They’re discovered on the posterior roots of the spinal nerves; on the sensory roots of the trigeminal, facial, glossopharyngeal, and vagus nerves, and on the acoustic nerves. They’re additionally present in reference to the sympathetic nerves. On part they’re seen to include a reddish-gray substance, traversed by quite a few white nerve fibers; they fluctuate significantly in type and dimension; the biggest are discovered within the cavity of the stomach; the smallest, not seen to the bare eye, exist in appreciable numbers upon the nerves distributed to the completely different viscera. Every ganglion is invested by a clean and agency, carefully adhering, membranous envelope, consisting of dense areolar tissue; this sheath is steady with the perineurium of the nerves, and sends quite a few processes into the inside to help the bloodvessels supplying the substance of the ganglion.

In construction all ganglia are basically comparable, consisting of the identical structural components—viz., nerve cells and nerve fibers. Every nerve cell has a nucleated sheath which is steady with the neurolemma of the nerve fiber with which the cell is linked. The nerve cells within the ganglia of the spinal nerves are pyriform in form, and have every a single course of. A brief distance from the cell and whereas nonetheless inside the ganglion this course of divides in a T-shaped method, one limb of the cross-bar turning into the medulla spinalis, the opposite limb passing outward to the periphery. Within the sympathetic ganglia the nerve cells are multipolar and every has one axis-cylinder course of and several other dendrons; the axon emerges from the ganglion as a non-medullated nerve fiber. Comparable cells are discovered within the ganglia linked with the trigeminal nerve, and these ganglia are subsequently thought to be the cranial parts of the sympathetic system. The sympathetic nervous system contains these parts of the nervous mechanism wherein a medullated nerve fiber from the central system passes to a ganglion, sympathetic or peripheral, from which fibers, normally non-medullated, are distributed to such constructions, e. g., bloodvessels, as usually are not beneath voluntary management. The spinal and sympathetic ganglia differ considerably within the dimension and disposition of the cells and within the variety of nerve fibers coming into and leaving them. Within the spinal ganglia the nerve cells are a lot bigger and for probably the most half collected in teams close to the periphery, whereas the fibers, that are principally medullated, traverse the central portion of the ganglion; whereas within the sympathetic ganglia the cells are smaller and distributed in irregular teams all through the entire ganglion; the fibers are also irregularly scattered; among the coming into ones are medullated, whereas lots of these leaving the ganglion are non-medullated.

Fasciculi, tracts or fiber methods are teams of axons having homologous origin and homologous distribution (as regards their collaterals, subdivisions and terminals) and are sometimes named in accordance with their origin and termination, the title of the nucleus or the situation of the cell physique from which the axon or fiber arises previous that of the nucleus or location of its termination. A given topographical space seldom represents a pure tract, as usually fibers of various methods are combined.

The Spinal Twine or Medulla Spinalis

The medulla spinalis or spinal twine kinds the elongated, practically cylindrical, a part of the central nervous system which occupies the higher two-thirds of the vertebral canal. Its common size within the male is about 45 cm., within the feminine from 42 to 43 cm., whereas its weight quantities to about 30 gms. It extends from the extent of the higher border of the atlas to that of the decrease border of the primary, or higher border of the second, lumbar vertebra. Above, it’s steady with the mind; under, it ends in a conical extremity, the conus medullaris, from the apex of which a fragile filament, the filum terminale, descends so far as the primary phase of the coccyx .

The place of the medulla spinalis varies with the actions of the vertebral column, its decrease extremity being drawn barely upward when the column is flexed. It additionally varies at completely different durations of life; as much as the third month of fetal life the medulla spinalis is so long as the vertebral canal, however from this stage onward the vertebral column elongates extra quickly than the medulla spinalis, in order that by the top of the fifth month the medulla spinalis terminates on the base of the sacrum, and at delivery in regards to the third lumbar vertebra.

The medulla spinalis doesn’t fill the a part of the vertebral canal wherein it lies; it’s ensheathed by three protecting membranes, separated from one another by two concentric areas. The three membranes are named from with out inward, the dura mater, the arachnoid, and the pia mater. The dura mater is a robust, fibrous membrane which kinds a large, tubular sheath; this sheath extends under the termination of the medulla spinalis and ends in a pointed cul-de-sac on the stage of the decrease border of the second sacral vertebra. The dura mater is separated from the wall of the vertebral canal by the epidural cavity, which incorporates a amount of free areolar tissue and a plexus of veins; between the dura mater and the subjacent arachnoid is a capillary interval, the subdural cavity, which incorporates a small amount of fluid, in all probability of the character of lymph. The arachnoid is a skinny, clear sheath, separated from the pia mater by a relatively extensive interval, the subarachnoid cavity, which is full of cerebrospinal fluid. The pia mater carefully invests the medulla spinalis and sends delicate septa into its substance; a slender band, the ligamentum denticulatum, extends alongside every of its lateral surfaces and is connected by a collection of pointed processes to the interior floor of the dura mater.

Thirty-one pairs of spinal nerves spring from the medulla spinalis, every nerve having an anterior or ventral, and a posterior or dorsal root, the latter being distinguished by the presence of an oval swelling, the spinal ganglion, which incorporates quite a few nerve cells. Every root consists of a number of bundles of nerve fibers, and at its attachment extends for a ways alongside the facet of the medulla spinalis. The pairs of spinal nerves are grouped as follows: cervical 8, thoracic 12, lumbar 5, sacral 5, coccygeal 1, and, for comfort of description, the medulla spinalis is split into cervical, thoracic, lumbar and sacral areas, corresponding with the attachments of the completely different teams of nerves.

Enlargements.—The medulla spinalis will not be fairly cylindrical, being barely flattened from earlier than backward; it additionally presents two swellings or enlargements, an higher or cervical, and a decrease or lumbar.

The cervical enlargement is the extra pronounced, and corresponds with the attachments of the big nerves which provide the higher limbs. It extends from in regards to the third cervical to the second thoracic vertebra, its most circumference (about 38 mm.) being on a stage with the attachment of the sixth pair of cervical nerves.

The lumbar enlargement offers attachment to the nerves which provide the decrease limbs. It commences in regards to the stage of the ninth thoracic vertebra, and reaches its most circumference, of about 33 mm., reverse the final thoracic vertebra, under which it tapers quickly into the conus medullaris.

The Anterior Median Fissure (fissura mediana anterior) has a mean depth of about 3 mm., however that is elevated within the decrease a part of the medulla spinalis. It incorporates a double fold of pia mater, and its flooring is shaped by a transverse band of white substance, the anterior white commissure, which is perforated by bloodvessels on their solution to or from the central a part of the medulla spinalis.

The Posterior Median Sulcus (sulcus medianus posterior) may be very shallow; from it a septum of neuroglia reaches fairly greater than half-way into the substance of the medulla spinalis; this septum varies in depth from 4 to six mm., however diminishes significantly within the decrease a part of the medulla spinalis.

On both facet of the posterior median sulcus, and at a brief distance from it, the posterior nerve roots are connected alongside a vertical furrow named the posterolateral sulcus. The portion of the medulla spinalis which lies between this and the posterior median sulcus is called the posterior funiculus. Within the cervical and higher thoracic areas this funiculus presents a longitudinal furrow, the postero-intermediate sulcus; this marks the place of a septum which extends into the posterior funiculus and subdivides it into two fasciculi—a medial, named the fasciculus gracilis (tract of Goll); and a lateral, the fasciculus cuneatus (tract of Burdach). The portion of the medulla spinalis which lies in entrance of the posterolateral sulcus is termed the antero-lateral area. The anterior nerve roots, not like the posterior, usually are not connected in linear collection, and their place of exit will not be marked by a sulcus. They come up by separate bundles which spring from the anterior column of grey substance and, passing ahead by means of the white substance, emerge over an space of some slight width. Probably the most lateral of those bundles is mostly taken as a dividing line which separates the antero-lateral area into two components, viz., an anterior funiculus, between the anterior median fissure and probably the most lateral of the anterior nerve roots; and a lateral funiculus, between the exit of those roots and the postero-lateral sulcus. Within the higher a part of the cervical area a collection of nerve roots passes outward by means of the lateral funiculus of the medulla spinalis; these unite to type the spinal portion of the accent nerve, which runs upward and enters the cranial cavity by means of the foramen magnum.

Grey Substance (substantia grisea centralis).—The grey substance consists of two symmetrical parts, one in every half of the medulla spinalis: these are joined throughout the center line by a transverse commissure of grey substance, by means of which runs a minute canal, the central canal, simply seen to the bare eye. In a transverse part every half of the grey substance is formed like a comma or crescent, the concavity of which is directed laterally; and these, along with the intervening grey commissure, current the looks of the letter H. An imaginary coronal airplane by means of the central canal serves to divide every crescent into an anterior or ventral, and a posterior or dorsal column.

The Anterior Column (columna anterior; anterior cornu), directed ahead, is broad and of a rounded or quadrangular form. Its posterior half is termed the bottom, and its anterior half the head, however these usually are not differentiated from one another by any well-defined constriction. It’s separated from the floor of the medulla spinalis by a layer of white substance which is traversed by the bundles of the anterior nerve roots. Within the thoracic area, the postero-lateral a part of the anterior column initiatives lateralward as a triangular area, which is called the lateral column (columna lateralis; lateral cornu).

The Posterior Column (columna posterior; posterior cornu) is lengthy and slender, and is directed backward and lateralward: it reaches virtually so far as the posterolateral sulcus, from which it’s separated by a skinny layer of white substance, the tract of Lissauer. It consists of a base, immediately steady with the bottom of the anterior horn, and a neck or barely constricted portion, which is succeeded by an oval or fusiform space, termed the head, of which the apex approaches the posterolateral sulcus. The apex is capped by a V-shaped or crescentic mass of translucent, gelatinous neuroglia, termed the substantia gelatinosa of Rolando, which incorporates each neuroglia cells, and small nerve cells. Between the anterior and posterior columns the grey substance extends as a collection of processes into the lateral funiculus, to type a net-work referred to as the formatio reticularis.