SUTURES VARIETY OF FORM IN JOINTS. 31 



skull are attached by the notches on their edges; these are 

 termed the sutures of the skull. They ossify with age, and 

 may be considered temporary joints, or rather a transition 

 between the separation of the bones and their unification. 

 The other joints, on the contrary, are permanent, and de- 

 signed to leave to the bones which they unite a mobility 

 which continues during life. 



In some of the joints the surfaces are nearly plane or flat; 

 others present projections with corresponding depressions; 

 sometimes there is a segment of a spheroid upon which the 

 cavity which receives it moulds itself; sometimes a cylinder 

 which turns upon its axis in a ring, or a pulley-groove around 

 which slides an apophysis, or a mortise in which a tenon 

 is set. 



Here, as in all the works of nature, we admire the inex- 

 haustible variety of form and of mechanism. Doubtless there 

 exists between certain articulations resemblances which per- 

 mit them to be classed together; but all are as distinct as 

 the bones which they unite, and like them present diversity 

 of character. Separately considered they astonish us no 

 less by the multiplicity of detail in their mechanism, whether 

 we examine the most complex or those whose surfaces pre- 

 sent the least irregularity. In fact we nowhere mid a uniform 

 plan, and the projections as well as the depressions are 

 curved in the most capricious manner. 



These details of the general outline belong to no precise 

 geometrical form ; they are neither cubes nor spheres, neither 

 cylinders, cones, nor pyramids, although in anatomical 

 language these terms are applied to them. There is an assem- 

 blage in the same apophysis, or in the same cavity, of curved 

 surfaces borrowed from the most widely differing solids, 

 united under angles the most varied, and modelled on sinu- 

 osities which defy geometrical description. 



In addition to these distinctive characters of the joints, 

 we may mention others which are common to them all. All 

 the joints have cartilages covering the bones which form 

 them; all are kept together by special ligaments, and are 

 lined with a sy no vial membrane, whose functions we shall 

 subsequently describe. 



