TEE ARTICULATIONS IN GENERAL. 127 



3. It may be asked of the partisans of the opinion now combated, how 

 they can believe in the existence of a serous membrane between two 

 articular surfaces, without its being exposed to bruises and destruction a 

 thousand times in the day? Do they take into account the amount of 

 pressure sustained by certain articulations, and the intense friction to which 

 their surfaces are submitted ? Have they compared the intensity of these 

 destructive influences, with the delicate texture of the serous membranes, 

 and their great inflammatory susceptibility ? It is sufficient to lightly touch 

 in this way the weak side of our adversaries' argument, and to conclude the 

 third portion of this discussion: There is friction between the cartilages 

 of the two opposed articular surfaces, therefore there must be wear , this is 

 a physical law which no body escapes, let it be as hard as the diamond, or as 

 soft as caoutchouc. And if there is wear between these rubbing surfaces, 

 there cannot be an irritable and sensitive membrane lying on the inert and 

 insensible strata which constitute them. In fine, a synovial membrane, 

 after being fixed to the margin of the articular cartilage of a diarthrodial 

 joint, is reflected in every direction to cover the inner aspect of the liga- 

 ments, and becomes attached to the periphery of the diarthrodial surface 

 corresponding to the first. 



There are generally found within articulations little masses of fat which 

 push the synovial membrane enveloping them inwards. Erroneously con- 

 sidered by Clopton Havers as glands for the secretion of synovia, these 

 accumulations of fat have been named synovial fringes. They are more par- 

 ticularly numerous in the neighbourhood of the articular margins : that is, 

 on the edges of diarthrodial surfaces. 



The synovia is a viscid, colourless, or slightly yellow fluid, in its physical 

 characters somewhat resembling oil; it does not possess them, however, 

 so far as its composition is concerned, for chemical analysis has not 

 demonstrated the presence of fatty principles. It is the albumen it contains 

 which gives it its viscidity, and which fits it for lubricating the articular 

 surfaces over which it is spread. Its use in the animal economy is 

 absolutely identical with that of the greasy substances employed to lubricate 

 the axles of carriages. 



MOVEMENTS. The movements peculiar to diarthrodial articulations are 

 divided into seven principal classes : 



1. Simple gliding, the only movement possible between two plane or 



undulating facets. 



2. Flexion, which brings two bony pieces nearer each other by closing 



more or less their angle of union. 



3. Extension, the inverse movement, during which the bones are 



straightened on each other. 



4. Adduction, which brings the inferior extremity of the movable bone 



towards the median line. 



5 Abduction, the contrary movement to the preceding 



6 Circumduction, or the sling movement, during which the bone passes 



successively through the last four positions. 

 7. Rotation, in which one bone pivots on another 



CLASSIFICATION OF THE DIARTHROSES. The basis of this classification is 

 founded on the configuration of the articular surfaces and the nature of the 

 movements they permit. This double base serves to establish five kinds of 

 diarthrodial articulation . 



1. Enarthrosis, characterised by the reception of an articular head within 



