The adventitious osseous matter is sometimes so abundantly effused on the exterior of the articular surfaces, that a strong lamina of bone, forming a species of splint, connects the bones, and effectually renders them immovable. This kind of complete ankylosis is described by Cruveilhier under the term l'ankylose par invagination. The knee and elbow (see fig. 3) frequently illustrate this condition.

Ankylosis by invagination. Ankylosis of elbow effected by deposit of

osseous matter exterior to articulation.-From Cruveilhier's Anatomie Pathologique, livraison 9, pl. 4.

Destruction of cartilage, by a process (on the nature of which much discrepancy of opinion exists) similar to ulceration, but differing by an absence of the reproductive process, the attendant on the ulcerative process in other tissues, occasionally co-exists with ankylosis, and more particularly with compound ankylosis. The destruction of the ligaments, and the alteration in the form of the articular surfaces, as in the hip, permit the production of partial or complete luxation by the muscles, aided by external circumstances. As an instance of the operation of external circumstances in favouring displacement, I may adduce the increase, if not the entire production, of the occasional great rotation outwards of the tibia in false ankylosis of the knee (see fig. 4). A patient labouring under acute inflammation of the knee lies for days or weeks on the same side as the affected limb, the slightest change of position in the joint is not tolerated, but gradually the body is turned so that the sufferer reclines partly on the back. This is accomplished through the movement of the pelvis on the head of the femur: at length the patient lies entirely supine; but as complete abduction and rotation outwardly of the thigh cannot long be borne, the femur becomes rotated inwardly, the tibia remaining still; the insertion of the biceps flexor cruris is approximated to its origin, and gradually contracts; so that the ankylosis taking place in this position, outward rotation of the tibia remains. This change of relation of the tibia is also favoured by the weight of the foot, increased occasionally by the pressure of the bed-coverings on it, acting by means of the wedge-shaped articular process of the astragalus upon the malleoli, and keeping the tibia rotated outwardly, whilst the femur is rotated inwardly, in the manner above described.

I have sometimes been disposed to attribute the uniform tendency to the existence of this rotation of the tibia outwardly, partly to the action of the biceps being greater than

that of the other flexor muscles of the knee; but it is questionable whether the biceps be more powerful than the inner

Incomplete angular ankylosis of right knee, viewed from the external aspect : e c, e c, external surface of the condyle of the thigh bone, presenting naturally; f, the proper anterior surface of the fibula, and t, proper anterior surface of tibia, presenting outwardly; p, the patella situated diagonally between the front of the external condyle and the apophysis tibiæ; a, a strong fibrous band of adhesion between upper and outer edge of the patella and front of outer condyle. Beneath this a series of slender cellular adhesions between the patella and under portion of outer condyle is visible. From preparation F. c. 33 in London Hospital Museum.

ham-string muscles, aided in their action by the sartorius and gracilis.*

Attrition of the cartilages, by which is understood a wearing away, produced by the mechanical friction of the bones on each other, supposed by some pathologists to occur independently of any morbid process, occasionally co-exists with incomplete ankylosis. Many excellent observers are disposed to regard this removal of cartilage as identical with ulceration; and it should be remarked, that the advocates of the unorganised nature of cartilage describe its destruction by friction as a mechanical process, whilst those who assert the vitality of cartilage regard its removal as the effect of ulceration.† Friction, with or without previous "ulcerative absorption," is capable of the destruction of cartilage, and, after its entire disappearance, may grind away the bones. Continued pressure, as daily witnessed in the effects of aneurisms and tumors, compels, in like manner, the removal of the hardest tissues.

The effects of friction would be experienced in every individual at every movement but for certain counteracting influences. These are,-a healthy condition of the tissues exposed to its operation; the presence of a proper secretion; and the correct apposition of the articular surfaces. One or all of these conditions may be absent in an ankylosed joint. Inflammation being the invariable precursor of ankylosis, whether from accident or disease, it is not surprising that the organisation of the bones and other articular tissues, their capability of resisting external causes, should be diminished;

* Benjamin Bell (Treatise on Diseases of Bones, 1828, p. 287 et seq.) very unphilosophically attributes this distortion to the patient twisting his leg round the crutch, a habit frequently acquired by persons thus affected. In opposition to this explanation it is sufficient to state, that this distortion is frequently observed in those who have not quitted their bed since the commencement of disease of the joint.

†The importance of a due consideration of the existence of ulceration or attrition of cartilage will be shewn in the chapter on treatment of ankylosis.

the secretion of the synovial fluid is very commonly altered, if not arrested; and an alteration of the relation of the articulating surfaces we have described as a frequent concomitant of ankylosis. The occurrence of attrition of the cartilages and bones, after long-continued ankylosis in old persons, is not surprising, when we reflect on the diminished vitality of those structures at an advanced period of life (see fig. 5).

False angular ankylosis of right knee, with luxation, from disease within the articulation, viewed from the external surface of the articulation, exhibiting the peculiar rotation described page 20, erosion of cartilages and bone, "formation of new joint," membranous adhesions, eburnation, &c. a, the upper extremity of tibia, which, instead of presenting two slight concavities for the reception of the two condyles,