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In anatomy, that part of the central nervous system in man which lies in the spinal canal formed by the vertebrae, and reaches from the foramen magnum to the lower margin of the first lumbar vertebra. It is about 18 in. long, and only occupies the upper two-thirds of the spinal canal. The cord is protected by the same three membranes which surround the brain. Outside is the dura mater, which differs from that of the brain in FIG. I. - Transverse Section of the forming a periosteum Spinal Cord and its Membranes. not to the bones, in sending no processes inward, and in having no blood sinuses enclosed within its walls. In other words the spinal dura mater is the continuation of only the inner or cerebral layer of the dura mater of the skull. Inside the dura mater is the arachnoid, which is delicate and transparent, while between the two lies the sub-dural space, which reaches down to the second or third sacral vertebra. The pia mater is the innermost covering, and is closely applied to the surface of the cord into the substance of which it sends processes. Between it and the arachnoid is the sub-arachnoid space, which is much larger than the sub-dural and contains the cerebro-spinal fluid. Across this space, on each side of the cord, run a series of processes of the pia mater arranged like the teeth of a saw; by their apices they are attached to the dura mater, while their bases are continuous with the pia mater surrounding the cord. These ligaments, each consisting of twenty-one teeth, are the ligaments denticulata, and by them the spinal cord is moored in the middle of the cerebro-spinal fluid.

The spinal cord itself is a cylinder slightly flattened from before backward. In the cervical region it is enlarged where the nerves forming the brachial plexus come off, while opposite the lower thoracic vertebrae the lumbar enlargement marks the region whence the lumbo-sacral nerves are derived. (See fig. 2.) Opposite the second lumbar vertebra the cylindrical cord becomes pointed and forms the conus medullaris, from the apex of which a glistening membranous thread runs down among the nerves which form the cauda equina, and, after blending with the termination of the dural sheath, is attached to the back of the coccyx.

In a transverse section of the cord two median fissures are seen; the antero-median (see fig. 3, A) is wide, and reaches about a third of the way along the antero-posterior diameter of the cord; it is lined by the pia mater, which, at its orifice, is thickened to form a `` 't10. FP (From Gray's Anatomy, Descriptive and Surgical.) glistening band, known as the linea splendens; in front of this lies the single anterior spinal artery.

The postero-median fissure (fig. 3, P.) is much deeper and narrower, and has no reflection of the pia mater into it. Where the posterior nerve roots emerge (fig. 3, P.R.) is a depression which is called the postero- lateral fissure, while between this and the postero-median a slight groove is seen in the cervical region, the para median fissure (fig. 3, P.M; see also fig. 2). On looking at fig. 3 it will be seen CVi that the anterior nerve roots (A.R.) do not emerge from a definite fissure.

Postero-median _ The spinal cord, like the brain, consists of grey and white matter, but, as there is here no representative of the cortical grey matter of the brain, the white matter entirely surrounds the grey. In section the grey matter has the form of an H, the cross bar forming the grey commissure. In the middle of this the central canal can just be made DVn out by the naked eye (see fig. 4). The anterior limbs of the H form the anterior cornua, while the posterior, which in the greater part of the cord are longer and thinner, are the posterior cornua. At the tips of these is a lighter-coloured cap (fig. 3, S.G.) which is known as the substantia gelatinosa Rolandi. On each side of the H is a slighter projection, the lateral cornu, which is best marked in the thoracic region (see fig. 4).

On referring to fig. 4 it will be seen that the grey matter has different and characteristic appearances in different regions of the cord, and it will be noticed that in the cervical and lumbar enlargements, where the nerve to the limbs comes off, the anterior horns are broadened.

Lumbar swelling D P T A. R.

FIG. 3. - Diagram to show the Tracts in the Spinal Cord.

A. Antero-median fissure.

P. Postero-median fissure. A.R. Anterior nerve roots.

(From D. J. Cunningham, in CunningP.R. Posterior nerve roots.

Spinal Cord as seen from behind.

FIG. 2. - Diagram of the ham's Text-Book of Anatomy.) S.G. Substantia gelatinosa. P.M. Paramedian fissure.

G.T. Tract of Goll.

B.T. Tract of Burdach.

C.T. Comma tract.

CVI shows the level of the O.A. Oval area.

I st cervical vertebra; CVv of L.T. Lissauer's tract.

the 5th cervical vertebra; D.C.T. Direct cerebellar tract. Dvii of the 2nd dorsal T.G. Gowers' tract.

vertebra; DVx of the 10th C.P.T. Crossed pyramidal tract. dorsal vertebra; DVxii of L.B.B. Lateral basis bundle.

the 12th dorsal vertebra; A.B.B. Anterior basis bundle. LVii of the 2nd lumbar D.P.T. Direct pyramidal tract. vertebra.

Histologically the grey matter is made up of neuroglia, medullated and non-medullated nerve fibres, and nerve cells (for details see Nervous System). The nerve cells are arranged in three main columns, ventral, intermedio-lateral and posterior vesicular. The ventral cell column has the longest cells, and these are again subdivided into antero-mesial, antero lateral, postero-lateral and central groups. The intermedio lateral cell column is found in the lateral horn of the thoracic region.

The posterior vesicular or Clarke's column is also largely confined to the thoracic region, and lies in the mesial part of the posterior cornu. It is the place to which the sensory fibres of the sympathetic system (visceral afferents) run. The white matter, as has been shown, surrounds the grey. and passes across the middle line to form the white cornmissure, which lies in front of the grey. It is composed of neuroglia and medullated nerve fibres, which are arranged in definite tracts, although in a section of a healthy cord these tracts cannot be distinguished even with the microscope. They have been and are still being gradually mapped out by pathologists, physiologists and embryologists.

On tracing a sensory nerve into the cord (fig. 3, P.R.) through the posterior nerve root it will be seen to lie quite close to the mesial side of the posterior horn of grey matter, where most of it runs upward. The next root higher up takes the same position and pushes the former one toward the middle line, so that the lower nerve fibres occupy an area close to the postero-median fissure known as the tract of Goll (fig. 3, G.T.), while the higher lie more externally in the tract of Burdach (B.T.). The greater part of each nerve sooner or later enters the grey matter and comes into close relation with the cells of Clarke's column, but some fibres run right up to the nucleus gracilis and cuneatus in the medulla (see Brain), while a few turn down and form a descending tract, which, in the upper part of the cord, is situated in the inner part of the tract of Burdach and is known as the comma tract (fig. 3, C.T.), but lower down gradually shifts quite close to the postero-median fissure and forms the oval area of Flechsig (fig. 3, O.A.). It will be obvious that both these tracts could not be seen in the same section, and that fig. 3 is only a diagrammatic outline of their position.

A few fibres of each sensory nerve ascend in a small area known as Lissauer's tract (fig. 3, L.T.) on the outer side of the posterior nerve roots, and eventually enter the substantia gelatinosa.

To the outer side of Lissauer's tract and lying close to the lateral surface of the cord is the direct cerebellar tract (fig. 3, D.C.T.), the fibres of which ascend from the cells of Clarke's column to the cerebellum. As Clarke's column is only well developed in the thoracic region this tract obviously cannot go much lower.

In front of the last and also close to the lateral surface of the cord is another ascending tract, the tract of Gowers (fig. 3, T.G.), or, as it is sometimes called, the lateral sensory fasciculus. It probably begins in the cells of the posterior horn, and runs up to join the fillet and also to reach the cerebellum through the superior cerebellar peduncle. The crossed pyramidal tract (fig. 3, C.P.T.) lies internal to the direct cerebellar tract, between it and the posterior cornu. It is the great motor tract by which the fibres coming from the Rolandic area of the cerebral cortex are brought into touch with the motor cells in the anterior cornu of the opposite side. This tract extends right down to the fourth sacral nerve.

In front of the crossed pyramidal tract is the lateral basis bundle (fig. 3, L.B.B), which probably consists of association fibres linking up different segments of the cord.

The anterior basis bundle (fig. 3, A.B.B.) lies in front and on the mesial side of the anterior cornu, and through it pass the anterior nerve roots. Like the lateral bundle it consists chiefly of association fibres, but it is continued up into the medulla as the posterior longitudinal bundle to the optic nuclei.

The direct pyramidal tract (fig. 3, D.P.T.) is a small bundle of the motor fibres from the Rolandic area, which, instead of crossing to the other side at the decussation of the pyramids in the medulla, runs down by the side of the antero-median fissure. Its fibres, however, keep on gradually crossing to the opposite side through the anterior white commissure of the cord, and by the time the midthoracic region is reached it has usually disappeared.

The roots of the spinal nerves in the upper part of the canal rise from the cord nearly opposite the points at which they emerge between the vertebrae, but the farther one passes down the higher the origin of each root becomes above its point of emergence. Consequently the lumbar and sacral nerves run a long way down from the lumbar enlargement to their spinal foramina and are enclosed in the dural and arachnoid sheaths to form a mass like a horse's tail, which is therefore known as the cauda equina. The relation between the origin of each nerve and the spinous processes of the vertebrae has been worked out by R. W. Reid ( Journ. Anat. and Phys., xxiii.

341).

Embryology

The early development of the neural tube from the ectoderm is outlined in the article on the Brain. When the neural groove becomes a tube it is oval in section with a very large laterally Cervical swelling_ Posterior paramedian fissure Postero-Iateral fissure fissure DVxn LVn DVx CVv FIG. 4. - Sections of Spinal Cord, twice scale of nature.

i. Cervical enlargement.

2. Thoracic region.

3. Lumbar enlargement.

4. Sacral region.

compressed central canal (see fig. 5). The original ectodermal cells elongate and, radiating outward from the canal, are now known as spongioblasts, while the inner ends of some of them bear cilia and so the canal becomes ciliated. A number of round cells, known as germinal cells, now appear close to the central canal, except at the thin mid-dorsal and mid-ventral laminae (roof-plate and floorplate). From the division of these the primitive nerve cells or neuroblasts are formed and these later on migrate from the region (From D. J. Cunningham in Cunningham's FIG. 5. - Schema of a Transverse Section through the Early Neural Tube (Young). The left side of the section shows an earlier stage than the right side.

of the canal and shoot out long processes - the axons. The permanent central canal of the cord was formerly said only to represent the ventral end of the large embryonic canal, the dorsal part being converted into a slit by the gradual closing in of its lateral walls, thus forming the postero-median fissure. A. Robinson, however, does not believe that the posterior fissure is any remnant of the central canal, and there are many points which bear out his contention (Studies in Anatomy, Owens College, 1891). The most modern view (1908) is that the fissure is formed partly by an infolding and partly from the original central canal. The antero-median fissure is caused by the ventral part of the cord growing on each side, but not in the mid-line where no germinal cells are.

The anterior nerve roots are formed by the axons of the neuroblasts in the developing anterior cornua, but the posterior grow into the cord from the posterior root ganglia (see Nerve: Spinal ), and, as they grow, form the columns of Goll and Burdach. That part of the grey matter from which the ventral, anterior or motor nerve roots rise is known as the basal lamina of the cord, while the more dorsal part into which the posterior nerve roots enter is the alar lamina. These parts are important in comparing the morphology of the spinal cord with that of the brain.

In the embryo up to the fifth month there is little difference in the appearance of the grey and white matter of the cord, but at that time the fibres in the columns of Burdach acquire their medullary sheaths or white substance of Schwann, the fatty matter of which is probably abstracted from the blood. Very soon after these the basis bundles myelenate and then, in the sixth month, the columns of Goll. Next follow the direct cerebellar tracts and, in the latter half of the eign Lh month the tracts of Gowers, while the fibres of the pyramidal and Lissauer's tracts do not gain their medullary sheaths until just before or after birth. At first the spinal cord exends as far as the last mesodermal somite, but neuroblasts are only formed as far as the first coccygeal somite, so that behind that the cord is non-nervous and degenerates later into the filum terminale. After the fourth month the nervous portion grows more slowly than the rest of the body and so the long cauda equina and filum terminale are produced. At birth the lower limit of the cord is opposite the third lumbar vertebra, but in post-natal development it recedes still farther to the lower level of the first.

For further details see Quain's Anatomy, vol. i. (London, 1908); J. P. McMurrich, Development of the Human Body (1906). Most modern descriptions are founded on the writings of W. His, references to which and to other literature will be found on p. 463 of McMurrich's book.

Comparative Anatomy

In the Amphioxus there is little difference between the spinal cord and the brain; the former reaches the whole length of the body and is of uniform calibre. It encloses a central canal from which a dorsal fissure extends to the surface of the cord and it is composed of nerve fibres and nerve cells; most of the latter being grouped round the central canal or neurocoele, as they are in the human embryo. Some very large multipolar ganglion cells are present, and there are also large fibres known as giant fibres, the function of which is not clear.

When the reptiles are reached the cord shows slight enlargements in the regions of the limbs and these become more marked in birds and mammals.

In the lumbar region of birds the dorsal columns diverge and open up the central canal, converting it into a diamond-shaped space which is only roofed over by the membranes of the cord, and is known as the sinus rhomboidalis. In all these lower vertebrates except the Anura (frogs and toads), the cord fills the whole length of the spinal canal, but in the higher mammals (Primates, Chiroptera and Insectivora) it grows less rapidly, and so the posterior part of the canal contains the cauda equina within its sheath of dura mater. In mammals below the anthropoid apes there are no direct pyramidal tracts in the cord, since the decussation of the pyramids in the medulla is complete. Moreover, the crossed tracts vary very much in their proportional size to the rest of the cord in different animals. In man, for example, they form 11 . 87% of the total cross area of the cord, in the cat 7.7 6%, in the rabbit 5.3%, in the guinea-pig 3%, and in the mouse 1.14 / o. In the frog no pyramidal tract is found. It is obvious, therefore, that in the lower vertebrates the motor fibres of the cord are not so completely gathered into definite tracts as they are in man.

A good deal of interest has lately been taken in a nerve bundle which in the lower vertebrates runs through the centre of the central canal of the cord, and takes its origin in the optic reflex cells in close relation to the posterior commissure of the brain. More posteriorly (caudad) it probably acquires a connexion with the motor cells of the cord and is looked upon as a means by which the muscles can be made to actively respond to the stimulus of light. It is known as Reissner's fibre, and its morphology and physiology have been studied most carefully in cyclostomes and fishes. It is said to be present in the mouse, but hitherto no trace of it has been found in man. It was discovered in 1860, but for forty years has been looked upon as an artifact.

See P. E. Sargent, " Optic Reflex Apparatus of Vertebrates," Bull. Mus. Comp. Zool. Harvard, vol. xlv. No. 3 (July, 1904); also for general details R. Wiedersheim, Comparative Anatomy of Vertebrates (London, 1907); Lenhossek, Bau des Nervensystems (1895). (F. G. P.) Surgery Of The Spine And Spinal Cord Fracture of the spine may occur from indirect violence, as when a man falls from a height upon his head, or in a sitting position; or it may result from direct violence, as when he is hanged, or as when he is run over by a loaded van, or in a fall from a height across a beam. The vertebrae above the fracture being displaced from those below it, the spinal cord is generally torn across, and the parts of the trunk, or the limbs, which are supplied by the spinal nerves passing out from the cord below the seat of injury are of necessity cut off from their connexion with the brain, and at once deprived of sensation and of the power of voluntary movement. In some cases of fracture of the spine there is at the time marvellously little constitutional disturbance. The higher up the column that the fracture occurs the more quickly does death ensue. If .the fracture is in the middle of the back the patient may linger for several weeks, but even if he is lying upon a water-bed, and even if every care is taken of him, inflammation of the bladder and intractable bed-sores are apt to make their appearance, and his existence becomes truly miserable. Operative surgery is unable to effect much in these cases on account of the spinal cord being generally torn across or hopelessly crushed.

Curvature of the spine may be due to deformity of the bodies of the vertebrae caused by irregular pressure, or to the disintegration of their anterior parts by tuberculous ulceration, known as Pott's disease or spinal caries. Thus the causes of spinal curvature are very different, and it is necessary that the actual condition be clearly recognized or treatment may prove harmful. Briefly, the curvature which is due to tuberculous disease requires absolute and continuous rest; the other calls for well-regulated exercises.

Lateral or rotatory curvature of the spine is a deformity which comes on during the developing period of life, before the bodies of the vertebrae are solidly formed. In young people who are growing rapidly, and whose muscular system is weak, the bad habit of standing, and throwing the weight of the body constantly on one leg, gives rise to a serious tilting of the trunk; or, if, when writing at a desk, they sit habitually in a twisted position, a lateral curvature of the spine is apt to take place. By constant indulgence in these bad habits the spinal column gets permanently set in a faulty position. Sometimes the tilting of the base of the trunk is due to a congenital or acquired difference in the length of the legs. In the concavity of the curve there is increased pressure, and, necessarily, diminished growth; in the convexity of the curve there is diminished pressure with increased growth. The patient's friends probably notice that one shoulder is higher than the other, or that " the hip is growing out," and unless means are taken to alter the abnormal distribution of pressure, the condition becomes worse, until complete ossification checks the further progress of the deformity. The growth of the subject being completed, the deformity ceases to increase. And when the growth is completed and the bones are solid and misshapen the condition is quite incapable of improvement. The usual curvature is one in which there is a convexity of the spine in the chest-region towards the right, with the right shoulder higher than the left. Compensatory curves in the opposite direction form in the loins and neck. Along with the lateral bending of the spine a rotation of the bodies of the vertebrae towards the convexity of the curve takes place, the spinous processes turning towards the concavity of the curve. Since the line of the spinous processes of the vertebrae can be easily traced through the skin, their deviation may mislead the superficial observer as to the actual amount of the curvature.

To counteract this deformity in the earliest stages (and it is in the early stage that treatment effects most), the patient (generally a girl) should be encouraged to walk perfectly erect. Systematic exercises, to strengthen the muscles of the back, ought to be strictly and persistently carried out under the direction of a surgeon with the assistance of a skilled instructor of gymnastics. During the intervals of rest the child should lie upon her back on a firm board, and should avoid taking exercise which gives rise to weariness of the muscles; for whenever the muscles become wearied she will attempt to take up a position which throws the strain on to her ligamentous and bony structures. One of the best exercises is to lay the patient on her face, fix her feet, and encourage her to raise herself by using the muscles of the back. Whilst she hangs from a trapeze the weight of the lower limbs and pelvis will help to straighten the spine as a whole, necessarily diminishing the increased pressure upon the cartilaginous bodies of the vertebrae towards the concavity, and increasing the pressure between the sides of the bodies towards the convexity. It is often a good thing to remove a girl with commencing lateral curvature from the sedentary life of school or town and to let her run wild in the country, exercising her muscles to the full.

If the deformity is due to inequality in the length of the legs, a high boot on the short leg may correct it. In some cases of lateral curvature a tilted seat is useful. Mechanical " spinal supports " are as expensive as they are inefficient. As a rule, indeed, they are positively harmful, in that they add to the weight of the trunk and hinder needful muscular development.

By kyphosis is meant an exaggerated degree of roundness of the shoulders. It can be effaced only by constant drillings and exercises whilst the spinal column is still plastic. When once the bones are solid no great improvement is possible. The deformity is sometimes due to short sight. It is well, therefore, to have the child's vision duly tested.

Lordosis is an exaggeration of the normal concavity of the loin-region of the spine. It is most often met with in those cases in which from congenital displacement of the head of the thigh bone, or from old disease of the hip-joint, the subject has acquired the habit of throwing the shoulders back in order to preserve the balance.

Tuberculous disease of the spine (Pole s disease), is the result of a deposit of tubercle-germs in the body of the vertebra.

Inflammation having thus been set up, ulceration (caries) of the vertebra, or of several vertebrae, occurs, and if the case runs on unchecked extensive abscesses may form in the thigh, loin or groin. The trouble is often begun by a blow or by a sprain of the spine, which, by lowering the power of resistance of the delicate bone, prepares it for the bacillary invasion. The earliest symptoms are likely to be a dull aching in the back with stiffness of the spine. The child complains of being tired, and is anxious to lie down and be left quiet whilst his little companions are running about. If the disease is in the middle part of the spine, pains are complained of in the front of the chest or at the pit of the stomach. Unfortunately such pains are often ascribed to indigestion. If the disease is in the upper part of the spine the pains may be in the head, the shoulders or the arms. If in the loin-region of the spine they are in the lower part of the trunk, the thighs or the legs. (These obscure peripheral pains are often misunderstood and are apt to be attributed to rheumatism). The back is stiff so that the child cannot stoop. In trying to pick up anything from the floor he keeps his back straight and bends his knees. If the disease is in the neck-region he cannot easily look upwards, and, instead of turning his head to look sideways, he wheels round his whole body. In some cases, though the disease is far advanced, there have been no complaints of pain in the back. As the bodies of the vertebrae crumble away, the spine bends forwards under the influence of the weight of the head and of the upper part of the trunk, and a projection may appear in the middle, line of the back. In the neck, and in the loinregion, the projection is rarely well marked, but in the chest-region a conspicuous boss may make its appearance - the " hump-back." The projection is often spoken of as an angular curvature - a contradiction in terms, for a thing which is angular is not curved. When the deformity is great there may be pressure upon the spinal cord with more or less paralysis in the parts below.

The treatment of tuberculous disease of the spine demands absolute and uninterrupted rest. The best thing is to put the patient flat on his back for as many months as may be found necessary, but not in a close bedroom. If he is compelled to lie in a bedroom the windows should be open night and day. If the patient is a child, he should be laid flat in a box-splint, or upon a thin horsehair mattress, and should be carried out of doors every day - but always lying flat. When the pressure-symptoms, such as the pains in the legs, thighs or arms, the " belly-ache," or the pains in the chest or neck have passed away, a firm leather splint may be moulded on to keep the parts quiet until consolidation has taken place, or a cuirass of poroplastic felt or of plaster of Paris may be applied. The danger in these cases is of leaving off treatment too soon: they must not be hurried, or the trouble will be likely to come back again with, perhaps, increased deformity. If the disease is in the upper part of the dorsal spine, or in the neck-region, a cervical collar of leather, or a double Thomas's hip-splint may be found useful.

In cases of advanced tuberculous disease of the spine, in which the spinal cord is compressed within its bony canal either by the posterior parts of the vertebral bodies or by inflammatory products, or in which, after severe injury, the cord is pressed upon by a displaced piece of bone, the surgeon may think it expedient to open the spinal canal from behind, removing in the procedure the posterior arches (laminae) of the vertebrae. The operation is called by the hybrid word laminectomy. Sometimes in the case of tuberculous disease, where the propriety of resorting to the operation is being discussed, the symptoms of the compression begin to clear off and the child makes a complete recovery without being operated on; the moral is that we should wait patiently and give Nature a full chance of doing her work in her own way. The operative treatment of these cases is not highly'satisfactory. Still, there are a certain small number of cases in which it may be given a trial.

The treatment of spinal abscess has been greatly influenced by the Listerian method. The collection of broken-down tuberculous material or fluid is not an abscess in the usual sense, for it does not contain " pus " or " matter," being, as a rule, destitute of septic micro-organisms. A spinal abscess is therefore no longer drained: it is incised, scraped, washed out, and swabbed dry, the opening being carefully and permanently sewn up. In this way septic germs are effectually excluded from the cavity, and the patient is spared the depressing and tedious discharging of the cavity which so often followed the old methods of treatment. It must be clearly understood, however, that every spinal abscess does not undergo cure after being subjected to the evacuation and closure treatment mentioned above, but that the surgeon is sometimes compelled to use irrigation and drainage.

In 1897 Dr Calot of Berk-sur-Mer reintroduced the method of straightening out the hump of the back, so often left after disease of the spine, by stretching the child on a flat table and dealing with the hump, under chloroform, with what is commonly known as " brute force." A considerable number of hump-backed children on the Continent as well as in England and America were thus dealt with, but it is doubtful whether the records of those cases, could they all be collected and published, would be found to justify the enthusiasm and publicity with which the method was inaugurated and its details were spread abroad. It is scarcely necessary to say that the forcible straightening of a spine which has developed a hump because tuberculous disease has wrecked the front of the vertebral segments is in no sense a curative operation. Diminishing the size of the projection does not cure the tuberculous ulceration of the bones; indeed, it may increase the ulcerative process or determine a scattering of the germs of tubercle throughout the body. The operation has not been accepted by British and American surgeons. In the practice of the foreign surgeon death ensued in three cases out of thirteen that were operated on, and an English surgeon reported fourteen cases " in all of which the deformity had recurred although the spines had been fixed in plaster of Paris after the straightening." Being deeply placed in the mass of the muscles of the back, and, moreover, being jealously locked within the bony canal of the vertebral column, the spinal marrow or spinal cord was, until the last few years, generally considered to be beyond the reach even of the most enterprising surgeon. Still, like other tissues, it was liable to diseases and injuries. The exact situation of a tumour pressing upon the spinal cord can now be located with great precision by noting the areas of pain and numbness, and the height in the limbs or trunk to which loss of power of voluntary movement ascends, and by noting also whether these effects are symmetrical upon the two sides or appear more upon one side than on the other. By cutting away the posterior parts of certain segments of the vertebral column, tumours of various sorts have been successfully removed from the interior of the canal. Displaced fragments of bone in tuberculous affection of the spine, abscess-contents and inflammatory tissue have also been similarly dealt with. Sir William Macewen of Glasgow and Sir Victor Horsley of London have been pioneers in this development of surgery. In cases of fracture of the spine, with displacement of the vertebrae and compression of the spinal cord, surgeons have also been trying what relief can be afforded by the adoption of bold operative measures, but as in most of these cases of fracture-dislocation the spinal cord is torn right across or crushed beyond hope of repair, active measures cannot be undertaken with much prospect of success.

" Concussion of the Spine." - Occasionally one hears persons, whose professional education should have taught them better, speaking or writing of concussion of the spine as if that were in itself a disease. It is an expression which is not infrequently used in an equally comprehensive and incorrect way when the ill-informed person is speaking of the injuries, real or imaginary, of which an individual makes complaint after having met with a severe shake when travelling on a railway. One might as well speak of concussion of the skull as of concussion of the spine, for the spine is but the bony envelope of the spinal cord, as the skull is of the brain. The violent shaking of the spinal cord and the spinal nerves in a serious accident may, however, be followed by some functional disturbance, which may be .associated with pains in the back, by numbness and tingling in the limbs, or with muscular weakness. In some cases the disturbance is due to slight haemorrhages into the nerve sheaths, which may clear up with rest and quiet. But when the presence of these obscure symptoms, after a railway accident for instance, becomes the subject of an action-at-law, there is a great chance that they will not pass off until the case is settled in one way or the other. Not, perhaps, that the individual concerned is dishonest in his estimation of them, but because the anxiety of the overhanging lawsuit has so grievously disturbed his mind and altered his perspective that his sense of proportion is for a time in abeyance. After the action-at-law the symptoms may clear up with a rapidity which to some people appears surprising. (E. O.*) Physiology Of The Spinal Cord The name spinal cord, given by early morphologists to the nervous mass lying in the tubular chamber enclosed by the vertebral column, was doubtless given under the supposition that the organ so named could be treated as an entity. Scientifically, however, it cannot be so treated, either as regards its structure or its function. It is merely a part of that great nervous structure which throughout the length of the body forms the central meeting-place of the nerve-paths arriving from and issuing to all regions with which nerve fibres are in touch. To separate from the rest of this system the part which lies within the spine is an artificial and in many ways misleading provision. This artificial treatment is the outcome of crude ideas drawn from the study of merely the gross form of the bodily parts. But crude as the distinction is, its historic priority has influenced the study of the vertebrate nervous system, not only in regard to morphological description but also in regard to exposition of the functional reactions of the nervous system and even up to the present day. Hence it is still customary arbitrarily to detach certain of the reactions of the nervous system into a separate group and describe that group by itself, simply because they occur in nervous arcs whose central courses in the great central nervous organ lie within that part of it extending along the spine. An additional inconvenience attaching to the mode of description of the nervous system customary in works on human anatomy, is that in such works the parts of the nervous arcs outside the central organ are described apart from it under the term peripheral nerves. This severs artificially structures which are functionally indissolubly united. The study and description of the working of the nervous system is hampered by this unphilosophic subdivision of its structural parts.

To gain a broader and truer point of view as starting-point for understanding the working of the spinal cord one must prepare the exposition by a short reference to the general function of the nervous system in the bodily economy.

Relation to General Nervous System

An animal of microscopic size may continue throughout its life to be constituted entirely by one single cell. Animals of larger bulk, although each begins its existence as a single cell, attain their development by the multiplication of the original single cell, so that from it there comes to be formed a coherent mass of cells very many millions in number. In these multicellular animals each of the constituent cells is a minute self-centred organism, individually born, leading its own life and destined for individual death. The corporate power of the complex animal is the sum of the powers of those manifold individual existences, its cells. In the complex animal the several organs, even the most homogeneous, such as muscles or glands, are each composed of many thousands of cells similarly specialized but living each per se. The solidarity of action which a complex animal thus built up exhibits is the result of the binding together of the units which compose the compleisorganism. Of the agencies which integrate the complex animal, one of the most potent is nervous action. A certain number of the unit cells composing the animal are specially differentiated from the rest to bind the whole together by nervous action. These specially differentiated cells are called " neurones." They constitute living threads along which waves of physico-chemical disturbance are transmitted to act as releasing forces for the energy in distant cells, where they finally impinge.

It is characteristic of this nervous system, the system of neurones, that, although ramifying far and wide through the body, it is a continuum from end to end. The peripheral nerves are formed of bundles of neurones lying side by side, but these, although packed close together, are strictly isolated one from another as conductors and remain isolated throughout the whole length of the nerve. The points of functional nexus of the neurones one with another are confined to one region only of the whole system. All their conductive connexions one with another take place solely in the central nervous mass which constitutes the so-called central nervous system, a nervous organ extending axially along the length of the body midway between the body's lateral halves. Thither the neurones converge in vast numbers, those of each body segment converging to that fraction of the central organ which belongs to their body segment. The central nervous organ thus receiving these neurones is, where it lies in the head, called the brain, the rest

of it is called in vertebrates the " spinal cord," in vermes and arthropods the " nerve-cord." The central organ not only receives neurones which converge to it from outside, but many of its own neurones thrust out their conductive arms from it as nerve fibres carrying nervous influence outwards to regulate the activity of glands and muscles. In the vertebrata the ingoing neurones for each segment and similarly the out-going neurone fibres are collected into a segmental nerve. To the spinal cord these are each attached by two roots, one dorsal, consisting of the afferent fibres, the other ventral, consisting of the efferent fibres.

The Reflex. - Analysis of function of this nervous system leads to what is termed " the reflex " as the unit of its action. The simplest complete reaction of the system is a reflex. There are many reflexes which are extremely complex, being built up of a number of simpler reflexes combined together. A reflex is a reaction started by the environment acting as a stimulus upon some nerve which communicates the excitement thus started in itself to other nerves by means of its connexions with these in the central nervous organ. The excitement so generated and transmitted finally travels outward from the central organ by one or more of the efferent nerves and through these reaches muscles or glands producing in them its final effect. The muscles and glands are from this point of view termed effector organs. The reaction is therefore " reflected " from the central organ. The nerve structures along which it runs in its trajectory are spoken of as a nervous arc. The whole purpose of the central nervous organ is therefore to bring afferent neurones into touch with efferent neurones. The whole purpose of reflex arcs is to bind one part of the organism to another part in such a way that what the environment is doing to the organism at one place may appropriately call forth or restrain movement or secretion in the muscles or glands possessed by the organism.

Receptor Cells. - There is one condition for the due performance of these reactions which is not provided by the nervous system itself. The afferent neurones are not in most cases so constituted as to be excitable themselves directly by the environment - for instance, they cannot be stimulated by light. Their amenability to the environment, their sensitization to environmental agencies, is effected by special cells adjunct to their peripheral ends. These cells from organs are called receptors. They are delicately adapted to be stimulated by this or that particular agent and are classifiable into various species, so that each species is easily excited by a particular agent which is " adequate " for it, and is quite inexcitable or only excitable with difficulty by agencies of other kinds. Thus in the skin some receptors are adapted for mechanical stimuli (touch) and not for thermal stimuli, while others (cold spots, warm spots) are adapted for thermal stimuli and not for mechanical. As far as it is known each afferent neurone is connected with receptors of one species only. The receptors thus confer upon the reflex arcs selective excitability. Each arc is thus tuned to respond to certain stimuli, while other arcs not having that kind of receptor do not respond. The receptors, therefore, while increasing the responsiveness of the organism to the environment, prevent confusion of reactions (inco-ordination) by limiting to particular stimuli a particular reaction.

Proprioceptors

The system of neurones is thus made accessible to the play of the external world acting on the body. And in addition to those receptors which are stimulated directly by the external world, are others lying within the mass of the organism itself, which are excitable by actions occurring in the organism itself. These are called proprioceptors. They are distributed preponderantly in the muscles and structures functionally adjunct to muscle, such as joints, ligaments, fasciae, &c. The reactions induced in such motor structures reflexly in response to environmental stimuli tend therefore secondarily to be followed and accompanied by reflex reactions initiated from proprioceptors.

Conduction

The process by which the excitement generated in the afferent neurone travels along the reflex arc is known as conduction. Conduction along afferent and efferent nerves xxv. 22 differs in some important respects from that obtaining in the nerve centre, i.e. in the piece of the central nervous system connecting the afferent nerve with the efferent nerve. In a nervetrunk the excited state set up in it by a stimulus travels along its fibres as wave-like disturbance at a speed of about thirtythree metres per second, and does not alter in intensity or speed in its travel. A nerve-trunk when excited (artificially) at some point along its length transmits the " impulse," i.e. the wavelike excited state in both directions, i.e. both up and down each fibre, from the point stimulated. This is true whether the fibre is afferent or efferent. The speed of travel of the nervous impulse along the nerve-trunk is practically the same whether the state of excitement, i.e. nervous impulse, is weak or intense. The nerve-trunk shows practically no delay in its response to an effective even though weak stimulus and its response ceases practically at once on cessation of the exciting stimulus. When excited by repeated brief stimuli the rhythm of the response corresponds closely with that of the stimuli, even when the frequency of the latter is as high as Ioo per second. With momentary stimuli a response even so brief as 20" can be given by the nerve-trunk. Finally, nerve-trunk conduction is singularly resistant to fatigue, to impoverished blood supply, and to many drugs which powerfully affect reflex actions.

In conduction through the central nervous organ the travel of the nervous impulse exhibits departure from these features. Its intensity is liable to be altered in transit. Its time of transit, especially if it be weak, is much longer than for a similar length of nerve-trunk. Its direction of transmission becomes polarized, that is, confined to one direction along the nervous path. The state of excitement engendered does not subside immediately on cessation of the stimulus, and may outlast the stimulus by many seconds. The rhythm of response to a rhythmic stimulus does not change in correspondence with change in the stimulus-rhythm. A response, however brief the stimulus, is probably never shorter than 50 in duration.

These are striking differences, and morphological study of the structural features of the central organ does not at present suggest how they for the most part arise. It seems certain, however, that in the central organ it is that part which consists of so-called grey matter which forms the place of their occurrence. There the spread of the impulse from one nerve-fibre to others seems clearly due to the fact that each afferent fibre breaks up into branching threadlets which ramify in various directions and terminate in close apposition with other neurones. There has been much dispute as to whether the termination is one of contiguity with the next neurone or of actual continuity with it. The result of recent investigation seems to show that in the vast majority of cases contiguity and not actual homogeneous continuity is the rule in the spinal cord. The point of nexus of one neurone with another is termed the synapse. If synapsis occurs by contiguity and not homogeneous continuity, it is fair to suppose that at it the transmission of nervous impulses must be different from that observable in the homogeneous conducting threads of nerve fibres. The conduction must traverse something of the nature of a membrane. To the properties of synaptic membranes many of the features peculiar to conduction in the grey matter may be due, for instance, the feature of irreversible direction of conduction.

Reflex Reactions

When the spinal cord is severed at any point the reflex arcs of the portion of the body behind the transection are quite cut off from the rest of the nervous system in front, including the brain. The reflex reactions elicited from the thus isolated region cannot therefore be modified by the action of the higher nervous centres. It is important to see what character these reflexes possess. The higher centres in the brain exercise powers over the motor machinery of the body and in doing so make use of the simpler nervous centres that belong to the segments severally, that is the local nervous centres existing for and in each body segment itself. In the head the local centres are overlaid by higher centres which cannot by any simple severance be separated from them. By studying, therefore, the powers of the cord behind a complete spinal transection we can obtain in a comparatively simple way information as to the powers of the purely local or segmental reflex mechanisms.

The so-called " flexion-reflex " of the limb is one of the most accessible of the local reflex reactions which can thus be studied with an isolated portion of the spinal cord as its centre.

Let it be supposed that the limb observed is the hind limb. The three main joints of the limb are the hip, the knee and the ankle. Each of these joints is provided with muscles which flex or bend it, and others which extend or straighten it. It is found that the reflex throws into contraction the flexor muscles of each of these joints. It matters little which of all the various afferent nerves of the limb is stimulated, whichever of these the afferent nerve may be, the centrifuged discharge from the cord goes to practically the same muscles, namely, always to the flexors of the joints.

The centrifuged discharge does not go to the extensor muscles of the limb. However strong the stimulus and however powerful the afferent nerve chosen the spinal centre does not discharge impulses into the extensor muscles, though these muscles receive motor nerves issuing from the very same region of the cord as that supplying motor nerves to the flexor muscles. Not only does the reflex action not discharge motor impulses into the nerves of the extensor muscles, but if the spinal cord happens to be discharging impulses into these nerves when the reflex is evoked this discharge is suppressed or diminished (inhibited) . The result is that when the reflex occurs not only are the flexor muscles made to contract, but their antagonists, the extensors, are, if in contraction at the same time, thrown out of contraction, that is, relaxed. In this way the latter muscles are prevented from impeding the action of the contracting flexors. This inhibition occurs at the beginning of the reflex action which excites the muscles and continues so long as the flexion-reflex itself continues. It thus prevents other reflexes from upsetting for the time being the due action of the flexionreflex, for it renders the muscles opposing that reflex less accessible to motor discharge through the spinal cord whatever the quarter whence incitation to that discharge may come.

A feature of this reflex is its graded intensity. A weak stimulus evokes in the flexor muscles a contraction which is weak and in the extensor muscles a relaxation which is slight. Not only is the contraction weak in the individual flexor muscles, but it is limited to fewer of them, and in large muscles seems to involve only limited portions of them.

The duration of the reflex similarly varies directly with the duration of the exciting stimulus applied to the afferent nerve. The time relations of electrical stimuli can be controlled by the experimenter with much precision. In the single induction shock he has at command a stimulus of extreme brevity, lasting only a few millionths of a second. With such stimulus a lower limit is soon found to the brevity of the reflex effect as expressed 1;,y muscles, It is found difficult to evoke with brief stimuli reflex contractions so brief as those evoked from the muscle by similar stimuli applied direct to the motor nerve of the muscle. There is reason to think that such stimuli applied to a nerve may evoke one single nerve-impulse. A single nerve impulse generated in a motor nerve causes in the muscle a brief contraction which is called a twitch, and lasts a tenth of a second. A single nerve impulse generated in an afferent nerve sometimes fails on arriving at the spinal centre to evoke any observable reflex effect at all, but if it is effective the muscle contraction tends to be longer than a " twitch," often much longer. It is therefore questioned whether the spinal centre when excited even most briefly ever discharges one single centrifugal impulse only; it seems usually to discharge a short series of such impulses.

Allied to this character is the tendency which even simple spinal reflexes exhibit to continue discharging for a certain time after their exciting stimulus has ceased to be applied. This after-discharge succeeding a strong stimulus may persist even for several seconds.

Refractory Phase. - Besides characters common to all or many spinal reflexes certain spinal reflexes have features peculiar to themselves or exhibited by them in degrees not obvious in other reflexes. One of these features is refractory phase. The scratchreflex exemplifies this. In the dog, cat, and many other animals the hind limb often performs a rapid scratching movement, the foot being applied to the skin of the shoulder or neck as if to groom the hairy coat in that region. This movement is in the intact animal under control of the brain, and can be executed or desisted from at will. When certain of the higher centres in the brain have been destroyed, this scratching action occurs very readily and in, as it were, an uncontrolled way. When the spinal cord has been severed in the neck this scratching movement of the hind limb can be elicited with regularity as a spinal reflex by merely rubbing the skin of the side of the neck or shoulder, applying there a weak electric current to the skin. In this reflex the stimulus excites afferent nerves connected with the hairs in the skin and these convey impulses to the spinal centres in the neck or shoulder segments, and these in turn discharge impulses into nerve fibres entirely intraspinal passing backward along the cord to reach motor centres in the hind limb region. These motor centres in turn discharge centrifugal impulses into the muscles of the hind limb of the same side of the body as the shoulder which is the seat of irritation. The motor discharge is peculiar in that it causes the muscles of the hind limb to contract rhythmically at a rate of about four contractions per second, and the discharge is peculiar further in that it excites the flexor and extensor muscles of the joints alternately so that at the hip for instance the limb is alternately flexed and extended, each single phase of the movement lasting about an eighth of a second. Now this rhythmic discharge remains the same in rate whether the exciting stimulus applied to the skin be continuous or one of many various rates of repetition. Evidently at some point in the reflex arc there is a mechanism which after reacting to the impulses reaching it remains for a certain brief part of a second unresponsive, and then becomes once more for a brief period responsive, and so on. And this phasic alternation of excitability and inexcitability repeats itself through the continuance of the reflex even when that endures for minutes. The phase of inexcitability is termed the refractory phase. It is important as an essential element in the co-ordination; without it the scratching movement would obviously not be obtained for alternation of flexion and extension is essential to the act. A similar element almost certainly forms part of the co-ordinating mechanism for many other cyclic reflexes, including

those of the stepping of the limbs, the movement of the jaw in mastication, the action of the eyelids in blinking, and perhaps the respiratory movements of the chest and larynx.

Fatigue

Nerve trunks do not easily tire out under stimulation even most prolonged. Reflex actions on the other hand relatively soon tire. Some are more resistant, however, than are others. The flexion-reflex may be continued for ten minutes at a time and the scratch-reflex can be maintained so long. As a reflex tires, the muscular contraction which it causes tends to become less intense and less steady. The relatively rapid onset of fatigue in reflexes is counterbalanced by speedy recovery in repose. A long flexion-reflex, when from fatigue it has become weak, tremulous and irregular, will recommence after 30 seconds' repose with almost the same vigour and steadiness as if it had not recently been tired out.

This character of reflexes is in accordance with their executing movements which for the most part are not under natural circumstances required to last long. Such movements are the taking of a step by a limb, the movement of the jaw in mastication, the descent of the diaphragm in breathing, the withdrawal of the foot or the pinion from a noxious stimulus or the movement of the eyelids to wash off a particle touching the cornea, in all these no very prolonged reflex discharge is required. These natural movements to which the artificially provoked reflexes seem to correspond do not demand prolonged motor activity, or when they do, demand it in rhythmic repetition with intervening pauses which allow repose.

Reflex Postures

But there are certain reflexes which do persist for long periods at a stretch. These are reflex postures. The hind limbs of the " spinal "frog assume an attitude which is reflex, for it ceases on severance of the afferent spinal roots. This attitude is one of flexion at hip, knee and ankle, resembling the well-known natural posture of the frog as it squats when quiet in the tank. Similarly in the " spinal " dog or cat certain muscles exhibit a slight but persistent contraction. This is seen well in the extensor muscles of the knee. These tonic reflexes are related to attitudes. In the dog and cat they are exhibited by those muscles whose action antagonizes gravity in postures which are usual in the animal, thus the extensors of the knee and hip and shoulder and elbow are in tonic contraction during standing. The reflex arcs concerned in reflex maintenance of this tonic contraction of muscles have been shown in several cases to arise within those muscles, and in those very muscles which themselves exhibit the tonic contraction, It is not, however, certain that all muscles exhibit a reflex tonus: for instance, it is not certain that in the dog the tail muscles exhibit such a tonus. And in those muscles which do exhibit the spinal reflex tonus attempts to obtain a similar untiring slight steady reflex contraction by artificial stimuli applied to receptive organs or nerves have failed.

The Spinal Reflex Arcs of the Hind Limb

When the skin of the limb is stimulated the flexion-reflex already described is evoked. The reflex is excited by nocuous stimuli such as a prick or squeeze applied to the skin anywhere in the limb, but most easily when applied to the foot. Electrical stimuli wherever applied evoke the same reflex. Similarly electrical stimuli applied to any afferent nerve of the limb evoke this reflex, whether the afferent nerve be from skin or from the muscles. Since the reflex always provokes excitation of the flexor muscles and inhibition of the extensor muscles, the result is that central stimulation of the afferent nerve of a flexor muscle excites its own muscle and inhibits its antagonist (reciprocal innervation), while similar stimulation of the afferent nerve of an extensor muscle inhibits its own muscle and excites its antagonist (reciprocal innervation). The reflex flexion of the ipselateral hind limb is commonly accompanied by reflex extension of the opposite hind limb. If the reflex spreads to the fore limb, it produces extension of the same side fore limb with flexion of the crossed fore limb sometimes, but sometimes extension of both fore limbs.

In the dog and cat extension of the ipselateral hind limb can, however, be excited by stimulation of the skin in three limited regions. One of these is the sole of the foot; smooth pressure between the pads excites a strong brief extension. This is called the extensor thrust. It is accompanied by a similar sudden brief extension of all three other limbs. This reflex may be related to the action of galloping, and the pressure which excites resembles that which the weight of the body bears on the pads against the ground.

The two other regions are the skin of the front of the groin supplied by the crural branch of the genito-crural nerve, and the skin just below and mesial to the buttock. These always excite the extensor muscles, not the flexors. They may be concerned with sexual acts.

Reflexes of the Fore Limb. - These resemble those obtainable from the hind limb. The ipselateral reflex is flexion at shoulder, elbow and wrist. The contra lateral fore limb at the same time is extended at shoulder, elbow and wrist. When the reflex spreads to the hind limbs the hind limb of the same side is extended at hip, knee and ankle, that of the crossed side is sometimes flexed at hip, knee and ankle, but sometimes is instead extended at hip, knee and ankle. The reflex sometimes spreads to the neck, causing the head to be turned toward the fore limb, which is the seat of the stimulation.

The Scratch Reflex

This has already been partly described above. The area from which it can be excited by appropriate stimulation is a large one, namely, a field of skin which is somewhat saddle-shaped having its greatest width transversely across the shoulders. It extends from close behind the pinna back to the loin. The stimuli which are effective are rubbing the skin or lightly pricking it, or lightly pulling on the hairs: also faradisation by a needle electrode whose point is only just inserted among the hairs but not deeper than their roots. If the stimulus be applied to the right hand of the mid-line the right hind limb is flexed at hip and performs the rapid scratching movement described above, and the left hind limb is thrown into steady extension. And conversely, when the stimulus is to the left side of the mid-line.

Each of these reflexes is a co-ordinate reaction. It is seen, therefore, that through the medium of the spinal cord the body behind the head has at command a certain number of reflexes and that each of these manages the skeletal musculature in a co-ordinate way. It will also be clear from the facts mentioned above about these separate reflexes that the fields of muscles worked by these several reflexes is to a large extent common to them all. Thus the reflex excited from the skin of the right hind limb acts on the muscles of that limb and also on those of the three other limbs. So similarly the reflex excited from the left hind limb, and from each fore limb. Study of the inter-relatio