ANATOMY AND PHYSIOLOGY OF DIARTHRODIALJOINTS

BY

D. V. DAVIES

School of Anatomy, Cambridge

It is proposed to deal only with the freely movingdiarthrodial joint with a synovial cavity, omittingmany points of interest about the gross anatomyand the mechanics of joints.The several components of the diarthrodial joint

are derived from mesoderm. The joint cavity isdeveloped by liquefaction of the mesenchymal tissuewhich at first separates the chondrified skeletalelements. The perichondrium (later periosteum)is continued over this cavity as the capsule, whilethe lining of the cavity presents, on the ends of thebones, the articular cartilage and, on the capsule,the synovial membrane. In some cases, such asthe knee and temporo-mandibular joint, lique-faction occurs in two regions in close proximity, theintervening mesenchyme forming the joint disk ormeniscus. The capsule, like the periosteum or peri-chondrium, is composed of tough, inelastic, whitefibrous tissue with marked powers of resistance todisease and relatively low powers of repair. Thesynovial membrane, on the other hand, is a thin,soft, freely movable, elastic membrane, richly sup-plied with pain fibres, with an abundant bloodsupply, good powers of repair and well-markedphagocytic properties.

Articular CartilageThe articular cartilage is admirably adapted to

its purpose. It is so smooth that when lubricatedwith synovia the friction between the surfaces isreduced to a minimum; it is so elastic that it pre-vents any jarring of the bones upon one another.It is devoid of sensation so that the movements ofthe joint, during which the cartilages are subjectedto a good deal of pressure by the contraction ofmuscle and the tension of the ligaments, take placewithout pain. Finally, it is avascular. " Through-out the life cycle cartilage lives, grows, and decayswithout a blood supply, without a demonstrablenerve supply " (Harris, 1933). Subjacent to thearticular cartilage and separating it from the under-lying vascular bone is a zone of calcified cartilage.This increases in depth with age. The articularcartilage when traced towards the joint cavity showsa series of graduated senescent changes likened byHarris (1933) to those seen in the epidermis. Bor-dering on the calcified cartilage is a zone ofprolifera-tion where the cells are frequently arranged ingroups of twos and fours, though cells in the act ofD

division are rarely seen. In young bone this zoneis continuous at the articular margins with the zoneof proliferating cartilage in the epiphyseal disk,these together constituting the mitotic annulus ofHarris (1933) (Fig. 1).

FIG. 1.-Diagram showing the zone of mitoses in arti-cular and epiphyseal cartilages. The cells in thiszone have their nuclei shown in the form of asters.(Harris: Bone Growth in Health and Disease, 1933,Fig. 121.)

In the immature animal the normal mechanism ofdivision in the articular cartilage is by mitosi s,possibly giving way with age to amitotic division(Elliott, 1936). When traced towards the articularsurface the cells derived from this zone of prolifera-tion become flattened and arranged tangentially,gradually losing their staining reaction and shrink-ing. They reach the articular surface as a sheet ofsenescent and dead cells to be removed by theconstant wear and tear which accompanies jointmovements. This senescence and death of cartilagecells has been disregarded in all studies concerning

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the metabolism of articular cartilage. The cellcontent of articular cartilage decreases with agewhile the proportion of matrix is correspondinglyincreased. Rosenthal and his colleagues (1941)estimate that the cell content of bovine articularcartilage decreases by two-thirds from youth toadolescence and. by three-quarters from youth toold age.Even in its deepest layers the metabolic require-

ments of articular cartilage can be met from sub-stances in the synovial fluid. The respiratory rateof articular cartilage per unit of weight is low ascompared with other tissues. The high propor-tion of matrix accounts for this, so that the glyco-lytic power per cell is said to be the same as thatof other tissues (Bywaters, 1937). The metabolicactivity of the cartilage cell is further said todecrease appreciably with age, according to Rosen-thal and his colleagues (1941) by 60% from young-to adult in cattle. Whether this is explicable onthe basis of the changing proportions of prolifera-ting to senescent cells has not been considered.The reparative powers of articular cartilage are

feeble. Lesions confined to the articular cartilageover the weight-bearing surfaces show little or noevidence of repair. At the articular margins repair

FIG. 2.-Section through the articular cartilage on thehead of the radius showing the repair of a six weeks'old fracture. The proliferation of the connectivetissue from the marrow spaces to fill the defect inthe articular cartilage is seen. x 41.

may be effected by the ingrowth of synovial tissue,which may become transformed into an imperfectfibrous cartilage. Lesions extending through the

articular cartilage into the underlying vascular boneand marrow spaces are repaired by proliferation of

the connective tissue from the latter, the deeperlayers of which may ossify while the most superficiallayers may yield a mixture of fibrous tissue andfibrocartilage at the articular surface (Gravilleet al., 1932; Fisher, 1923). This is also the responsein fractures involving articular surfaces (Fig. 2).

Apposition of articular surfaces is essential forthe preservation of the normal structure and func-tion of the articular cartilage. Loss of apposition

FIG. 3.-Section of the detached fragment from a typicalcase of osteochondritis dissecans, showing the trans-formation of the articular cartilage into fibroustissue. x 66.

leads to a degeneration of the cartilage and its trans-formation into fibrous tissue. Such a transforma-tion into fibrous tissue is seen in the acetabulum incases of congenital dislocation of the hip, and inthe cartilage covering loose bodies detached fromthe articular surfaces, as in osteochondritis dis-secans (Fig. 3).

Articular cartilage possesses considerable resist-ance to autolysis both in vivo and in vitro. It alsoresists the action of various enzymes and toxinswhich occur in normal and abnormal synovial fluids.

Intra-articular disksThere has been much speculation in the past as

to the morphological and functional value of intra-articular disks or menisci. The constancy of theiroccurrence and distribution in the mammals isworthy of note. A washer-like action, minimizingthe incongruity at the joint surfaces, is hardlyjustified by their distribution. They are found inthe temporo-mandibular joints of practically allmammals, irrespective of the configuration of thejoint surfaces. They are even present in the carni-vores, in which the articular surfaces are transverseconcentric cylinders, accurately adapted one to theother and allowing uni-axial movement. A bufferor shock-absorbing function fails to receive supportfrom their distribution. MacConaill (1932) sup-poses that they act in the nature of Michel pads,helping to maintain a film of lubricant between thearticular surfaces. In most positions of the jointthey maintain rather than decrease the incongruitybetween the articular surfaces, thus creating a wedge-shaped interval, pointing in the direction of motion,through which synovial fluid is continuously fed onto the articulating surfaces.

Structurally they consist not of pure fibro-cartilage but of a mixture of cartilage embedded ina much larger mass of dense fibrous tissue. Theirremoval is said to be followed by a regeneration offibrous pads from the synovial membrane (Bruce andWalmsley, 1937). Whether this regeneration con-stantly occurs in all animals and irrespective of ageremains unsettled.

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ANATOMY AND PHYSIOLOGY OF DIARTHRODIAL JOINTS

Synovial MembraneThe synovial membrane covers all the joint

surfaces except the cartilage-covered weight-bearingareas. It affords an investment to any tendons orligaments passing through the joint, covers theedges of the articular cartilage, with which it isfirmly adherent, and terminates gradually by shadingoff into the superficial layer of the cartilage withoutshare demarcation. Normally it is very thin andits internal surface is furnished with a variablenumber of delicate villi and folds, such as the alarfolds in the knee-joint, and at the articular margindelicate, frequently overlapping, vascular fringes(Figs. 7 and 8). Occasional cord-like bridges areseen at its reflections from bone to capsule.The surface of the membrane is relatively cellular,

recently Key (1928), ascribed much importance tothe connective tissue underlying the synovialmembrane. Its quantity and quality determines themobility of the synovial membrane. At thearticular margins and lining the ligaments it con-sists of dense collagenous tissue, restricting inde-pendent mobility of the synovial membrane. Else-where the underlying tissue is of a loose areolartexture, allowing free movement of the synovialmembrane. In most joints there are localizedaccumulations of fat in various regions in thistissue forming distinct pads, erroneously regardedby Clopton Havers (Todd, 1836) as mucilaginousglands. They fill up the spaces between the bonesand between the bones and ligaments in the move-ments of the joints. According to MacConaill

AokI~ofR;Z'v^s' X i~-

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FIG. 4.-Section of the synovial membrane from theankle-joint of an ox to show the perivascular distri-bution of the metachromatically stained cells (x).Stained with polychrome methylene blue. x 375.

some portions being richer in cells than others.The cells vary much in size and are generally irregu-larly branched, the surface of the membranebetween the cells, and sometimes over them, beingformed by a homogeneous eosinophilic ground sub-stance or by collagenous fibres. No cells with themorphological characters and staining reactions ofgoblet cells are seen in the synovial membrane.Cells staining metachromatically with toluidine blueor thionin do occur in the deeper layers. Thesehave been characterized as mucin-secreting but mustbe considered as mast cells on account of theircharacters and perivascular distribution. They arenever seen actually at the synovial surface (Davies,1943) (Fig. 4).Velpeau (quoted by Todd, 1836), and more

FIG. 5.-Section of the synovial membrane from ahuman knee-joint, showing the incorporation intothe membrane of a deposit of fibrin from the joint.x 300.

(1932) they affect the distribution of the synovialfluid within the joint. The more mobile areas ofsynovial membrane are provided with an abundanceof elastic fibres, often forming one or more distinctelastic laminae. This prevents the membrane beingnipped between the articular surface during move-ment. Vili are devoid of elastic fibres except thosein association with the blood vessels. Elasticfibres are sparse where the synovial membrane isfirmly adherent to the underlying tissues (Davies,1946b).The extent of the synovial membrane in the

various joints, and its relation to the area of articularcartilage, is clearly important in connexion with thenutritive functions of the joint. Despite this, com-parative figures are not available. Isolated esti-mates show that whereas in the human knee-jointthe area of the synovial membrane is almost doublethat of the cartilage, in the ankle the proportion isnearer unity (Davies, 1946b).Much confusion exists as to the nature of the

synovial lining. A distinction is frequently drawnbetween the synovial membrane and the lining ofother serous cavities, such as the pleura or peri-toneum. Recently the synovial lining has beendefined as modified connective tissue. There islittle morphological resemblance between the flat-tened connective tissue lining of false joints and the

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lining layer of true joints (Davies, 1946b). Thecells lining the synovial membrane differ fromfibroblasts on culture in their ability to lyse fibrinand to secrete a mucin-like substance (Vaubel,1933a and b). Nor is there much morphologicalresemblance between the lining of the synovialmembrane and that of serous cavities like the pleuraor peritoneum. Both, however, exhibit similarreactions to irritation. Key (1928) claims that,following synoviectomy, the lining is reformed bymetaplasia of the underlying connective tissue, butthere is nothing to show that the newly formedlining preserves the peculiar characters of synovialcells or that the fluid within the joint is not modifiedby this procedure. In contrast with this is theprocess of incorporation of fibrin deposits into thesynovial membrane by an overgrowth of synovialcells from the periphery of such a deposit, and itssubsequent organization (Fig. 5).

Blood Supply and Lymphatic DrainageThe joint, like the periosteum, is supplied by

numerous freely anastomosing blood vessels fromthe main arteries in the neighbourhood. Free

FIG. 6.-Photograph of a cleared specimen of synovialmembrane and articular cartilage from the meta-carpo-phalangeal joint of an ox, showing the vascularloops at the articular margin. The blood vesselshave been injected with India ink. x 271.

communication exists between the articular bloodvessels and those of the adjoining epiphyses. Inthe synovial membrane the capillaries form a finelymeshed network so close to the joint surface thatsome observers have thought that some of thecapillaries may be naked at this surface. The bloodvessels advance a short distance upon the non-weight-bearing regions of the articular cartilage,forming the looped anastomoses of the circulusarticuli vasculosus of William Hunter (1743) (Fig. 6).In the villi and fringes they form delicate tufts sup-

FIG. 7.-Photograph of the synovial folds and fringesfrom the metacarpo-phalangeal joint of an ox. Theblood vessels have been injected with India ink andthe specimen cleared. x 22.

plied by one or more central arterioles (Figs. 7 and8). The richness and superficial position of thesynovial capillaries account for the frequency ofhaemorrhages into the synovial cavity. Simplepuncture is almost invariably followed by someextravasation of red blood cells into the synovialfluid, while small extravasations of red cells, notrecognizable clinically, accompany almost alltraumatic lesions in the vicinity of the joint. Simplebruising over a joint is frequently associated withsome extravasation of blood into the fluid. Nocomparison of the blood supply of joint tissues pergramme per minute with that of other tissues and

FIG. 8.-High-power view of one of the fringes from thepreceding specimen. x 84.

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organs is available. The early liberation ofnitrogen into the joint in decompression sicknesssuggests that the capillaries are near the surface andpermeable.The lymphatic vessels of the synovial membrane

are neither as numerous nor as superficial as theblood vessels. Unlike other serous cavities it hasgenerally been considered that no direct communica-tion exists between the cavity of the joint and thelumen of its lymphatics. A single widely-meshedlymphatic plexus lies in the synovial membrane.Within the meshes are numerous blindly endingvessels often exhibiting terminal lacuniform enlarge-

FIG. 9.-Cleared specimen of the synovial membranefrom the metatarso-phalangeal joint of an ox,showing the lymphatic vessels. The lymphaticshave been injected with India ink. x 10.

ments directed towards the surface of the membrane.From this plexus the main vessels pass in groups oftwos and threes along the blood vessels towards theflexor aspect of the limb, and communicate freelywith the lymphatic plexus on the periosteum (Fig. 9).Few valves occur in the synovial lymphatics butthese are frequent in the periosteal vessels (Davies,1 946a).

Synovial Membrane Nerve SupplyThe nerves to the synovial membrane, both medul-

lated and non-medullated, are partly destined forthe blood vessels and for the most part accompanythem in their course. The vasomotor nerves, pre-sumably of sympathetic origin, continue along theblood vessels. Many of the remainder terminate asfree nerve endings in the more superficial parts ofthe membrane. These presumably subserve painsensations to which the synovial membrane is

markedly susceptible. Other nerve endings, vari-ously described as of the Ruffini, Golgi-Mazzoni,looped or knotted type, are possibly concerned withsensations such as tension, pressure, or proprio-ception. Gardner (1942) states that in the mousethe encapsulated nerve endings in the synovialmembrane show a distribution which would indicatethat they may be stimulated by pressure producedby flexion of the joint. Little is known concerningthe extent to which the synovial membrane respondsto such stimuli. In an isolated case where thesynovial membrane of the knee-joint was exposedand opened under local anaesthesia no evidence ofany sensation of pressure touch was found in thesynovial membrane away from the area of theinfiltration and incision. Tension applied fromthe edge of the incision of the membrane was appre-ciated, but its localization was particularly inaccu-rate. Sensitivity to pain was tested by a sharpneedle. This sensation was acute but its localiza-tion again failed to be very accurate, except thatthe side of the joint stimulated was always appre-ciated correctly.The physiological co-ordination which exists

between the joint and the overlying tissues is summedup in Hilton's Law (John Hilton, 1863). " Thesame trunks of nerves whose branches supply thegroups of muscles moving a joint also furnish adistribution of nerves to the skin over the insertionsof the same muscles and-what at this momentmore especially merits our attention-the interior ofthe joint receives its nerves from the same source."

Exchange of Substances across the Synovial BarrierSolutions and particles of small molecular dimen-

sions pass rapidly in either direction across thesynovial barrier, mainly through the blood capillarybed. The lymphatic system plays a minor part intheir absorption; the role of the synovial membranein this process is probably purely passive thoughthere are some anomalies as yet unexplained by theknown laws of osmosis and diffusion. Colloidalparticles, including proteins, reach the joint moreslowly and their removal is proportionately slow.The synovial cells and phagocytes play an important

a ;. . . --...* ... 4

FIG. 10.-Section of the synovial membrane from ahuman knee-joint, to show the accumtulation ofhaemosiderin. The joint contained a single osteo-cartilaginous loose body. x 55.

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part in their transfer across the synovial barrier.Particulate matter of 100,u upwards passes slowlyinto the subsynovial tissues and is deposited there.Haemosiderin collects there in large quantitiesfollowing haemorrhage into the joint cavity (Fig. 10).In the removal of particulate matter from the jointthe synovial cells and phagocytes of the synovialmembrane play an important part. Movementaffects the rate of absorption, particularly of smallcolloidal material. Its effect on the rate of absorp-tion of solutions is less noticeable on account oftheir very rapid removal even when the joint isimmobilized (Adkins and Davies, 1940). Acuteinflammation increases the rate of exchange acrossthe synovial barrier; the effects of chronic inflam-mation seem variable.

Investigations of the absorption from the jointcavity have been mainly confined to animals and,almost without exception, to the knee-joint. It ispossible that the rate of transference across thesynovial barrier differs from joint to joint in thesame animal.

Synovial FluidThe synovial fluid differs in its physical, chemical,

and cytological characters from joint to joint. Thesynovial fluid is a viscid, yellowish or colourless,glairy fluid. Its physical characters vary fromanimal to animal and from joint to joint in the sameanimal. Its volume bears no close relation to thesize or capacity of the joint, though it is moderatelyconstant for any particular joint. The human knee-joint generally contains a small volume (0-2 to0-3 c.cm.) of pale straw-coloured fluid, while thesmaller knee-joint of sheep or dog contains anequal volume of highly viscous colourless fluid. Inthe ox the knee-joint contains about 10 c.cm. offluid, while the smaller ankle-joint has on anaverage 25 c.cm., and sometimes considerably more.In the ankle of the ox the fluid is pale yellow andhas a relative viscosity of about 5 at 200 C., whilein the atlanto-occipital or atlanto-axial joints of thesame animal the fluid is generally of a deep yellowcolour and sets to a gel at 200 C. In any particularjoint there is no constant relation between thevolume and the viscosity of the fluid. Serialchanges in its physical characters are seen in thecosto-vertebral joints of the ox, the volume de-creasing and the viscosity increasing on passingbackwards (Davies, 1944).Knowledge of the composition of the synovial

fluid is essential to the understanding of its nature.The most reliable and complete analyses are pro-vided by Bauer and his colleagues (1940);. allprevious examinations have been fragmentary. Ofthe blood proteins, albumin and globulin occur inthe fluid in considerably lower concentration thanin the serum, with a preponderance of albumin inthe proportion of about 4 :1, in contrast to the1 : 1 ratio found in the serum. There is no fibro-nogen in the synovial fluid, hence it does not cloton standing, but the mucin may be precipitated ina sac-like fashion, presumably due to pH changes.

Non-electrolytes-urea, uric and amino acids-are in approximately the same concentration in thesynovial fluid as in the serum, suggesting freediffusion of these substances across the barrier ofsynovial membrane. The concentration of glucosein the fluid is generally less than that in the blood,possibly connected with a consumption of glucosewithin the joint. Any appreciable increase in thecell content of the synovial fluid is associated witha fall in the glucose content.The distribution of electrolytes in the synovial

fluid is in accord with the Donnan equilibriumtheory. There is a higher concentration of chloridesand bicarbonates, a similar concentration ofinorganic phosphate, and a lower concentration ofsodium, potassium, and magnesium. Comparedwith blood the calcium concentration is high. Partof this calcium is bound to the mucin and the con-centration of calcium in the fluid is proportionateto the mucin content and viscosity (Davies, 1946b).A typical example from an ox is:

Relative Calcium inSource of fluid viscosity Mgm./il0 c.cm

at 200 C. 0 .m

Atlanto-occipital joint Set into a gel. 10-6Radio-carpal joint Approximately 7-8

2,000.Ankle 7 6-9

Any condition causing increased permeability ofthe synovial membrane leads to increases in thecalcium content of the synovial fluid without neces-sarily yielding parallel increase in the mucin andthe viscosity. Such disproportionate increase inthe calcium is seen following aspiration of the fluidfrom the normal joint in the ox. More data con-cerning the variations in composition of synovialfluid from joint to joint are required.The viscous polysaccharide of the synovial mucin

has recently been isolated by Meyer, Smyth andDawson (1939). It is a sulphate-free compoundtermed hyaluronic acid and differs in respect to itssulphate-free nature from the sulphate-containingmucins found in cartilage and gastric secretions(chondritin and mucoitin sulphuric acids). Hyalu-ronic acid is also found in the viscous componentsof Wharton's jelly of the umbilical cord, in thevitreous of the eye, and in connective tissue. It isa substance of high-molecular weight, does notdialyse through colloidin membranes, and is non-antigenic. It does not stain metachromaticallywith such dyes as toluidine blue or thionin, as dothe sulphate-containing mucins. Its viscosity isdestroyed or decreased by many agents. Amongthese are vitamin C, which -is consequently notfound in synovial fluid (Mann, 1940), and an

enzyme, hyaluronidase, which is found in testicularextracts and in semen. A similar enzyme is alsoproduced by certain bacteria.

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Cell Content and CytologyObservations on the cell content and cytology of

normal human synovial fluid are few and confinedto the knee-joint. The largest series is that ofCoggeshall, Warren, and Bauer (1940), involving29 observations from a series of 16 cases, rangingfrom 32 to 80 years of age. The average nucleatedcell count was 63 per c.mm. with a range of 13 to180 cells per c.mm. The fluid was obtained imme-diately post mortem; the subjects mainly sufferedfrom chronic diseases and had been confined to bedfor some time before death. The following limitedobservations from cases suffering sudden death byaccident and having no evidence of articular diseaseindicate that these values are probably too low forsubjects maintaining full use of their joints:

Case Nucleated Red bloodNo. Sex Age Joint cells per cells per

c.mm. c.mm.

I M .25 Right elbow 420 01 M 25 ,, knee 481 1862 M 56 Left elbow 234 2,0003 F 12 ,, knee 286 4,0004 M 23 Right ankle 80 214 M 23 Left knee 152 264 M 23 Right knee 146 39

In this connexion studies on the cell content ofanimal synovial fluid reveal several points of interest.The content of nucleated cells varies significantlyfrom joint to joint in the same animal and fromspecies to species. It decreases with age within thespecies. In cattle, fluids with a high nucleated-cellcontent were obtained from joints with highlyviscous synovia and a marked freedom from dis-eases and degenerative changes. Thus the nucleated-cell content in the highly viscous fluids in theatlanto-occipital and atlanto-axial joints was of theorder of 1,200 per c.mm., while that in the muchless viscous ankle-joint fluid was about 200 perc.mm. Functional differences, though obviouslyinfluencing the nucleated-cell content markedly,are not adequate to explain all the differences(Davies, 1945a). Marked degenerative changes ina human joint are often associated with raisedcontents of nucleated cells but not invariably so.Increased wear and tear within the human knee-joint, such as that resulting from torn semilunarcartilages, is associated with a rise in cell contentgenerally, ranging from 300 to 700 per c.mm.,rarely exceeding 1,000 per c.mm.Red cells do not normally occur in synovial fluid

though, as already indicated, even simple punctureis followed by some extravasation. Of nucleatedcells, monocytes and clasmatocytes preponderatein normal synovial fluid. Lymphocytes are presentin small numbers, while only very occasional poly-morphonuclear leucocytes are seen. Certain cellsdesignated as synovial cells also occur in smallnumbers.

Theories of Origin of Synovial FluidThe work of Bauer and his colleagues (1940) on

the composition of synovial fluid indicates that itis probably an ultrafiltrate, or dialysate, of bloodplasma with the addition of mucin. The mucincannot be a product of the articular cartilage as thetwo mucins are different. It is not a product ofthe metachromatically staining cells of the synovialmembrane as these are mast cells and sulphate-freernucins do not stain metachromatically. It mayarise as a secretion of the synovial cells, a viewwhich is strongly supported by Vaubel's observa-tions (1933a and b) that these cells in tissue culturedo secrete a mucin-like substance. The identityof the latter is still awaited. Finally, the mucinmay be the matrix of the surrounding connectivetissue washed into the joint cavity. In this casethe mucin in the joint should be of the sulphate-freeand sulphate-containing types, both of which existin the connective tissue, and should occur in thesame proportion as in the connective tissue. Thereis no evidence that this is the case. Mucin pro-duction is probably a slow process. Aspiration ofa normal joint in cattle is rapidly followed byrestoration of the volume of the fluid but with con-siderably diminished mucin content and viscosity(Davies, 1946b).

Lubrication of the joint is obviously one of thefunctions of the synovial fluid. The mucin hashigh osmotic properties and maintains the fluid inthe joint. Further, the mucin may act as a buffermaintaining the alkalinity of the fluid, in associationwith the calcium. Nutrition of the articularcartilage, particularly in its central portions, isalmost exclusively done by the synovial fluid. Allother functions must take second place to this.

I wish to thank Professor H. A. Harris for permissionto reproduce Fig. 1.

REFERENCESAdkins, E. W. O., and Davies, D. V. (1940). Quart. J. exp. Physiol.,

30, 147.Bauer, W., Ropes, M. W., and Waine, H. (1940). Phys. Rev., 20,

272.Bennett, G. A., and Bauer, W. (1932). Amer. J. Path., 8, 499.Bruce, J., and Walmsley, R. (1937). Brit. J. Surg., 25, 17.Bywaters, E. G. L. (1937). J. Path. Bact., 44, 247.Coggeshall, H. C., Warren, C. F., and Bauer, W. (1940). Anat. Rec.,

77, 129.Davies, D. V. (1943). J. Anat., 77, 160.

(1944). Ibid., 78, 68.(1945). Ibid., 79, 66.

-(1946a). Ibid., 89, 21.(1946b). Lancet, in the Press.

Elliott, H. C. (1936). Amer. J. Anat., 58, 127.Fisher, A. G. T. (1923). Lancet, 2, 541.Gardner, E. D. (1942). Anat. Rec., 83, 401.Graville, A. B., and Bauer, W. (1932). Am. Jour. Path., 7, 499.Harris, H. A. (1933). Bone Growth in Health and Disease. Oxford

University Press. London.Hilton, J. (1863). Rest and Pain. London.Hunter, W. (1743). Phil. Trans. Roy. Soc., 42, 514.Key, J. A. (1928). Special Cytology. Edited by E. V. Cowdry,

Vol. II, p. 735. New York.Lutwak-Mann, C. (1940). Biochem. J., 34, 517.MacConail, M. A. (1932). J. Anat., 66, 210.Meyer, K., Smyth, E. M., and Dawson, M. H. (1939). J. biol. Chem.,

128, 319.Rosenthal, O., Bowie, M. A., and Wagoner, G. (1941). J. Cell.

Comp. Physiol., 17, 221.Todd, R. B. (1836). Cyclopaedia of Anatomy & Physiology, Vol. I.

London.Vaubel, E. (1933a). J. exp. Med., 58, 63.- (1933b). Ibid., 58, 85.

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