ARTICULATIONS OR JOINTS 263 



convexities or prominences on one bone fit into depressions or grooves 

 in its fellow. In this form of articulation the movement of the joint, 

 like that of a door, is only to and fro or in the direction of flexion and 

 extension. A good example of this class of articulation will be found in 

 the tibio-tarsal or true hock joint (fig. 361), the elbow-joint, &c. 



Arthrodia or Gliding Joints. — This variety of diarthrodial joint is 

 found in the knee (fig. 354) and the hock joints where the small flat bones 

 are closely united together, one upon the other, so as to allow simply a 

 limited gliding motion in various <lireetioiis. 



Amphiarthrosis or Mixed Joints. — In this variety the bones are 

 connected by a disc of fibro-cartilage, and possess just as much movement 

 as the flexibility and compressibility of the joining substance allows. 

 Mixed joints have no smooth surfaces or synovial capsules. Examples 

 of this description of joint are found in the union of the bodies of the 

 vertebrae (fig. 346), where a pad of elastic fiVtro-cartilage exists and gives 

 to the spine as a whole its flexibility and springiness without permitting 

 undue mobility between each pair of vertebrae. 



Pivot Joints. — Here a pointed extremity of one bone furnishes a 

 pivot on which another bone turns. This is the case with the odontoid 

 process of the dentata or second vertebra (fig. 350), which, as has l)een 

 previously explained, passes into the ring of the atlas or first neck-bone, 

 and permits a rotary movement of the latter upon the former. 



Synarthrosis or Immovable Joints. — This form of articulation 



prevails where flat bones are united together by their l)orders to form 

 cavities, as in the case of the cranium and the cavities of the face. In 

 some of these a joining is effected by the overlapping of thin plates of 

 bone. In others, small tooth-like processes from one bone project into 

 the other, while a third is united by the dovetailing of small serrations, &c. 



ARTICULATIONS OF THE TRUNK 



Intervertebral Articulations. — All the vertebree beyond the second 

 and as far backward as the first sacral are united together by their bodies 

 and processes. The bodies are connected one to another by circular discs 

 of fibro-cartilage which intervene between them, and by the superior and 

 inferior vertebral ligaments, the former runnino- alone; the floor of the 

 vertebral canal, to which it is attached in small festoons. The latter is 

 situated along the under part of the bodies from the sixth dorsal vertebra 

 to the sacrum. 



Union of the Processes. — The superior processes are connected by 

 the supra-spinous ligament and the inter-spinous ligament (fig. 346). 



