ARTICULATION. 



247 



selves become more complex. In the lowest 

 grade of animals the structure is so soft and 

 pliant that nothing more is required to produce 

 motion than this contractile tissue, which acts 

 in obedience to certain stimuli. But when 

 hard parts are superaddcd to tlie structure 

 of the animal, we then find a peculiar me- 

 chanism to allow of the motion of these hard 

 parts on each other without the risk of injury. 

 It is obvious that such motion could not take 

 place were these hard parts united in one piece. 

 Hence we find that they are subdivided into 

 segments, and these segments are joined to 

 each other through the medium of some struc- 

 ture more flexible than that of the segments 

 themselves, or by an apparatus of such a con- 

 struction as to admit of the motion of one 

 segment upon the other. It is to these join- 

 ings of different segments of an animal body 

 that the term articulations or joints has been 

 applied. 



An articulation may, therefore, be defined to 

 be the union of any two segments of an animal 

 body through the intervention of a structure or 

 structures different from both. 



The most perfect and elaborate forms of 

 articulations are those which are seen in ani- 

 mals that possess a fully developed internal 

 bony skeleton, and in none may they be studied 

 with more advantage than in man. We propose 

 to treat of the forms and structure of the ar- 

 ticulations in man, and at the same time to in- 

 quire what modes of mechanism are employed 

 for analogous purposes in the lower classes. 

 In the human subject and in the vertebrated ani- 

 mals generally, we shall, indeed, have particular 

 occasion to admire the articulations, as mira- 

 biles commissuras, et ad stabilitatern aptas, et 

 ad artus finiendos accommodatas, et ad mo- 

 tum et ad omnem corporis actionem.* 



It will be observed that the definition here 

 given of articulation is of the most compre- 

 hensive nature. In most instances, in man, two 

 parts articulated together are joined by their 

 solid portions, which are never in immediate 

 apposition with each other, but have some 

 elastic structure interposed which may or may 

 not form a bond of union ; and it is obvious 

 that the fact of the intervening substance being, 

 or not being also a bond of union will greatly in- 

 fluence the extent of motion of which the joint 

 is capable. Before inquiring into the variety 

 of forms of joints, we shall first examine briefly 

 the various structures which enter into their 

 composition, and which essentially contribute 

 to the perfection of their mechanism. 



These parts may be enumerated as follows, 

 and we propose to observe the same order in 

 treating of them: 1. Bone. 2. Cartilage. 

 3. Fibro-cartilage. 4. Ligament. 5. Synovial 

 membrane. 



1. Bone. The osseous or an analogous 

 structure constitutes the fundamental portion 

 of an articulation in all the vertebrated animals, 

 in the mollusca, and in some of the articulated 

 classes. In the human subject and all ver- 

 tebrated animals we find that certain parts of 



* Ciccr. de Nat. Deor. 1. ii. c. 35. 



the bones have surfaces marked upon them 

 in correspondence with similar surfaces on 

 others with which they are connected, or 

 that, as in the long bones, the extremities 

 are expanded or enlarged, and present sur- 

 faces which are adapted to similar surfaces 

 on contiguous bones. In this way are formed 

 the articular portions of the bones, and we 

 observe that these portions present considerable 

 varieties in their characters according to the 

 nature of the articulation which they con- 

 tribute to form. In fact, in examining these 

 articular portions of the bones we cannot fail 

 to notice the diversity of their form, so that 

 some are adapted to each other in such a 

 manner as evidently to favour motion, and 

 others are so framed as to limit and restrict it. 

 The articular surfaces in dry bones are ge- 

 nerally characterised by a peculiar smooth- 

 ness, indicative of the existence on them of 

 a cartilaginous incrustation in the recent con- 

 dition. The expansion of the extremities of 

 the long bones on which the articular surfaces 

 are formed is to be attributed to the accu- 

 mulation there of a considerable quantity 

 of the reticular texture, covered by a thin 

 lamina of compact tissue, whereby a large 

 surface is obtained without the inconvenient 

 increase of weight which would necessarily 

 result did that portion of the bone contain 

 compact tissue to any extent. In the neigh- 

 bourhood of the articular portions of the bones 

 we find certain eminences, depressions, or rough- 

 nesses, which indicate the points of attach- 

 ment of those bonds of union by which the 

 joints are secured and strengthened. In ge- 

 neral it may be observed that the long bones 

 are articulated with each other by joints which 

 possess a considerable extent of motion; the flat 

 bones, again, have articulations very limited in 

 their mobility, and this is likewise the case 

 with the irregular bones. 



2. Cartilage. Pure cartilage enters into the 

 composition of almost all joints, but more 

 particularly of those which are very moveable, 

 and indeed the chief purpose for which it is 

 employed in the economy of adult animals is 

 as an important and valuable element in these 

 moveable joints. Articular cartilage, there- 

 fore, constitutes a primary subdivision of this 

 texture by systematic writers. Its hardness, 

 its elasticity, and the limited degree of or- 

 ganization which it possesses, peculiarly adapt 

 it for the purposes to which it is applied in the 

 mechanism of the articulations. 



Although cartilage is chiefly employed in 

 those joints which possess considerable mo- 

 bility, it nevertheless also exists in joints which 

 are limited in their motions, and as it possesses 

 peculiar characters according as it belongs to 

 one or other of these classes of articulations, 

 we may very conveniently subdivide it into 

 a, cartilage of moveable articulations, or ar- 

 ticular cartilage properly so called, or diar- 

 throdial cartilage ; 6, cartilage of articula- 

 tions very limited in their motions, or cartilage 

 of sutures, or synarthrodial cartilage. Under 

 these heads we propose to treat of articular 

 cartilage. 



