782 Proceedings of the Royal Irish Academy. 



still remains surrounding the cartilage. This intervening material 

 ties the cartilages together, constituting a series of syndesmoses, or 

 more correctly (for the intervening substance is not ligament), of 

 joints of the type which Professor Hyrtl has called A gonarthrosis. 



The ends of the cartilaginous rods swell, and assume somewhat of 

 what is to be their permanent shape, and in doing so they grow 

 towards each other, in which process the disks of connective tissue 

 intervening between the contiguous ends become thinned in the middle, 

 and finally they disappear in the centre, and the cartilages come in 

 contact with each other, the fissure between the two, where the central 

 disk has disappeared, being the primary cavity of the joint. The con- 

 nective disk still remains peripherally, but in most joints gradually 

 vanishes from the region between the articular ends, remaining as a 

 continuous girdle of connective tissue around the joint in the form of 

 a capsular ligament. This becomes white fibrous tissue, as does the 

 most of the layer on the surface of the shaft of the cartilage with 

 which this is continuous, and which is now perichondrium. 



The chronology of these changes is not easily settled. In an 

 embryo of 3' 8 centimeters long the central cavity has begun to appear. 

 In one of 5"6 cm. the cavities are formed and the ligaments are 

 embryonic fibrillar tissue ; in a foetus of the thirteenth week many 

 of the permanent conditions are present. 



In such joints as are about to develop inter- articular cartilages, 

 the intervening embryonic connective disks, instead of disappearing, 

 solidify, and much later chondrify, and clefts form on both surfaces 

 between these as substantive elements and the ends of the bones. 

 This, we shall see, is the history of the sterno- clavicular joint. In 

 joints which have partial disks like the menisci of the knee, these are 

 derived from the persistence of the later stage of partial absorption. 



The hip-joint in man is developed earlier than the knee; the 

 shoulder and knee develop about the same time ; the elbow and ankle 

 are usually in the same condition in the same embryo. 



In the joints of adults we find that the ligaments are naturally 

 classifiable, according to their origin, into four groups : — 



1st. Those derived from the primitive capsule whose origin "we 

 have just traced, and which are consequently continuous with the 

 periosteum. These are the true ligaments of all joints. 



2ndly. Those derived from the tendons of muscles which surround 

 joints ; thus much of the internal lateral ligament of the knee is 

 derived from material continuous with the tendon of the adductor 

 magnus, and much of the posterior ligament from the semi-mem- 

 branosus. 



3rdly. Those derived from fasciae. In the primitive development 

 of the limb the bundles of muscle are surrounded by embryonic con- 

 nective tissue, which, as the muscles become specialized, forms a 

 fibrous sheath around each : these combined sheaths uniting, and fas- 

 tened together by the circular fibres developed in the deep subcutaneous 

 and sub-adipose layers, form the system of limb-fascise and intermus- 



