248 



ARTICULATION. 



a. Diarthrodial cartilage. The general 

 characters of this class of articular cartilage 

 may be best examined on the articulating ex- 

 tremities of the long bones. Here we observe 

 it moulded exactly to the forms of those sur- 

 faces, insomuch that, after a little maceration, 

 the cartilage may, by careful dissection, be 

 removed from the bone, to which it adheres 

 with great firmness, and will be found to ex- 

 hibit an exact mould of the articular ex- 

 tremity ; hence these cartilages have been called 

 "cartilages of incrustation" This cartilage 

 is perfectly distinct at the early periods of life 

 from the temporary cartilage which forms the 

 nidus of the future bone, and cannot be re- 

 garded as a portion of that cartilage left un- 

 ossified ; this may easily be seen by examining 

 a vertical section of a femur or tibia at this 

 period ; and the peculiar arrangement of the 

 fibres of the articular cartilage, hereafter to be 

 noticed, constitutes an additional proof that it 

 is completely distinct from that which is after- 

 wards transformed into bone. 



The physical properties and general charac- 

 ters of this form of cartilage do not differ from 

 those of the others ; it possesses the same 

 pearly whiteness the same apparent homoge- 

 neousness of structure the same elasticity the 

 same absence of vessels carrying red blood. It 

 is not covered by a perichondrium ; the surface 

 towards the joint is peculiarly smooth and glis- 

 tening, and is generally supposed to owe these 

 properties to its being lined by a layer of the 

 synovial sac of the joint; this point, however, 

 has been controverted, as we shall notice in a 

 subsequent part of the article. The first and 

 the most complete investigation of the true 

 anatomical construction of articular cartilage 

 was that announced by Dr. William Hunter so 

 long ago as 1743.* His paper still deserves 

 the most attentive perusal, not only for the 

 actual information it affords on its professed 

 subject, but as a specimen of the careful and 

 original method of observation pursued by its 

 distinguished author. To examine the structure 

 of articular cartilages, it is necessary to subject 

 them to boiling or along-continued maceration .f 



" When an articulating cartilage is well pre- 

 pared," says Dr. Hunter, " it feels soft, yields 

 to the touch, but restores itself to its former 

 equality of surface when the pressure is taken 

 off. This surface, when viewed through a 

 glass, appears like a piece of velvet. If we 

 endeavour to peel the cartilage off in lamellae, 

 we find it impracticable, but if we use a certain 

 degree of force, it separates from the bone in 

 small parcels, and we never find the edge of 

 the remaining part oblique, but always perpen- 

 dicular to the subjacent surface of the bone. 

 If we view this edge through a glass, it appears 

 like the edge of velvet, a mass of short and 

 nearly parallel fibres rising from the bone, and 

 terminating at the external surface of the carti- 

 lage : and the bone itself is planned out into 



* Of the Structure and Diseases of Articular 

 Cartilage, Phil. Trans, vol. xlii. 



t The articular cartilage on the patella may be 

 selected as very favourable for this purpose. See 

 the plate annexed to W. Hunter's paper. 



small circular dimples where the little bundles 

 of the cartilaginous fibres were fixed. Thus 

 we may compare the texture of a cartilage to 

 the pile of velvet, its fibres rising up from the 

 bone, as the silky threads of that rise from the 

 woven cloth or basis. In both substances the 

 short threads sink, and bend in waves upon 

 being compressed, but by the power of elasti- 

 city recover their perpendicular bearing as 

 soon as they are no longer subjected to a 

 compressing force. If another comparison 

 was necessary, we might instance the flower 

 of any corymbiferous plant, where the fiosculi 

 and stamina represent the little bundles of 

 cartilaginous fibres, and the calyx, upon which 

 they are planted, bears analogy to the bone."* 



The total absence of vessels capable of car- 

 rying red blood in articular cartilage is proved 

 by the failure of even the minutest injections to 

 pass into the cartilage, and a further confirma- 

 tion of this opinion is derived from the fact 

 that madder taken into the system of a young 

 animal does not stain them. The attempts of 

 anatomists to trace lymphatics and nerves into 

 this structure have been equally unavailing. 



The design of articular cartilages, as means 

 to break the violence of shocks, is well illus- 

 trated by comparing the different arrangement 

 of the cartilaginous incrustation on convex arti- 

 cular surfaces from that on concave. In the 

 former, we observe the layer of cartilage to be 

 very thin at the circumference of the articular 

 surface, its thickest portion being in the centre, 

 while the opposite arrangement obtains on con- 

 cave surfaces : there the thinnest portion of the 

 cartilage is in the centre, and the layer increases 

 in thickness as it approaches the circumference. 



" The articulating cartilages are most hap- 

 pily contrived to all purposes of motion in 

 those parts. By their uniform surface they 

 move upon one another with ease : by their 

 soft, smooth, and slippery surface mutual abra- 

 sion is prevented : by their flexibility, the con- 

 tiguous surfaces are constantly adapted to each 

 other, and the friction diffused equally over the 

 whole : by their elasticity, the violence of any 

 shock, which may happen in running, jumping, 

 &c. is broken and gradually spent; which 

 must have been extremely pernicious, if the 

 hard surfaces of bones had been immediately 

 contiguous. As the course of the cartilaginous 

 fibres appears calculated chiefly for this last 

 advantage, to illustrate it, we need only reflect 

 on the soft undulatory motion of coaches, which 

 mechanics want to procure by springs, or upon 

 the difference betwixt riding a chamber-horse 

 and a real one. v f 



* Loc. cit. p. 516. 



t Hunter, in loco citato. Hunter's account of 

 articular cartilage is completely confirmed by M. 

 De Lasone in a paper in the Mem, de 1'Academie 

 Royale des Sciences, An 1752. He describes the 

 cartilage as " une multitude des petits filets adosses 

 et lies les uns aux autres tous perpendiculaires an 

 plan de 1'os, en un mot parfaitement semblables 

 par leur structure, ou par leur position a la substance 

 emaillee des dents, laquelle n'est composee, comme 

 on sait, que de filets osseux, poses perpendiculaire- 

 ment sur le corps de la dent : la comparaison est 

 des plus exactes." 



