LIGAMENTS OF THE RIBS. 127 



by its outer border with the external lateral ligament, and in front re- 

 ceives some fibres of insertion of the external pterygoid muscle. Oc- 

 casionally it is incomplete in the centre. It divides the joint into two 

 distinct cavities, the one being above and the other below the carti- 

 lage. 



The synovial membranes are situated the one above, the other below 

 the fibrous-cartilage, the former being the larger of the two. When 

 the fibrous-cartilage is perforate, the synovial membranes communi- 

 cate. 



Besides the lower jaw, there are several other joints provided with 

 a complete interarticular cartilage, and, consequently, with two syno- 

 vial membranes ; they are, the sterno-clavicular articulation, the acro- 

 mio-clavicular and the articulation of the ulna with the cuneiform 

 bone. 



The interarticular fibrous-cartilages of the knee-joint are partial, and 

 there is but one synovial membrane. 



The articulations of the heads of the ribs with the vertebrae have 

 two synovial membranes, separated by an interarticular ligament with- 

 out fibrous-cartilage. 



Fig. 59.* 



Actions. The movements of the lower jaw are depression, by which 

 the mouth is opened; elevation, by which it is closed ; a forward and 

 backward movement, and a movement from side to side. 



In the movement of depression the interarticular cartilage glides for- 



* In this sketch a section has been carried through the joint, in order to 

 shew the natural position of the interarticular nbro-cartilage, and the manner 

 in which it is adapted to the difference of form of the articulating surfaces. 

 1. The glenoid fossa. 2. The eminentia articularis. 3. The interarticular nbro- 

 cartilage. 4. The superior synovial cavity. 5. The inferior synovial cavity. 

 6. An interarticular nbro-cartilage, removed from the joint, in order to shew its 

 oval and concave form ; it is seen from below. 



