THE ARTICULATIONS IN GENERAL. 175 



endothelium. The first sometimes adheres intimately to the inner face of the 

 funicular or membranous lit^aments of the articulation ; at other times it is loosely 

 attached to them by an abundance of connective tissue. The second layer is con- 

 stituted by a single row of flattened polygonal cells, except at the bottom of 

 certain grooves, where they are more or less crowded. Though belonging to the 

 serous membranes, the synovial membranes do not form complete sacs, as, after 

 lining the internal surface of the ligaments, they cease at the articular margins. 



Direct observation demonstrates that the cartilages have no covering, and 

 that there is no synovial membrane on their surface. The anatomists who 

 imagined that the thin pellicle, which can be rendered evident in cutting the 

 surface of cartilage obliquely and separating it by teazing, was a membrane, were 

 deceived, as this pellicle has not the texture of a serous membrane ; and it is not 

 vascular, for it has never been possible to inject vessels on the surface of such 

 cartilages, nor yet in their substance. Neither is it covered with epithelium, and, 

 if submitted to microscopical examination, it has all the characters of cartilage. 



Pathological facts prove nothing in favour of the existence of a synovial 

 membrane on cartilages. 



It may be accepted, then, that the synovial membrane never extends to the 

 surface of articular cartilage ; but, after being fixed around the margin of a 

 diarthrodial surface, it is reflected in every direction to hne the internal surface 

 of the ligaments, and become attached to the periphery of the diarthrodial surface 

 in contact wrth the other, so that it entirely isolates the interior of the joint from 

 the peri-articular connective tissue. 



There are generally found within articulations, little masses of fat which push 

 the synovial membrane enveloping them inwards. Erroneously considered by 

 Clopton Havers as glands for the secretion of synovia, these accumulations of fat 

 have been named synovial fringps, or viUi. They are more particularly numerous 

 in the neighbourhood of the articular margins — that is, on the border of diarthro- 

 dial surfaces. They are formed by a prolongation of the synovial membrane, 

 which covers some connective tissue fibres associated with adipose cells, or an 

 amorphous substance provided with nuclei. 



The synovial membrane, after lining the inner surface of ligaments, sometimes 

 escapes between these, forming a hernia. The term sijnovinl culs-de-sac has been 

 given to these external prolongations of the articular serous membrane. This 

 membrane often covers the inner surface of a tendon or ligament, in order to 

 facilitate their gliding over a bony eminence. 



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 to it its 

 viscidity, and which fits it for lubricating the articular surfaces over which it is 

 spread. Its use in the anima] economy is absolutely identical with that of the 

 greasy substances employed to lubricate the axles of carriages. 



To the means of union described under the names of ligaments and synovial 

 membranes, may be added atmospheric pressure, the influence of which is 

 relatively considerable, as the experiments of Weber have demonstrated. 



Movements. — The movements peculiar to diarthrodial articulations are 

 divided into seven principal classes : 



1. Simple yJidiny, the only movement possible between two plane or undu- 

 lating facets. 



