ARTICULATIONS OR JOINTS 25 



We may find a type of this articulation between the temporal 

 and inferior maxillary bones, or between the femur and tibia. 

 Articulations of this kind are formed by oval heads fitting in 

 oval sockets. 



Gliding joints permit only simple gliding movement between 

 the articular surfaces. The articular surfaces in joints of this 

 kind are more or less nearly fiat. 



Immovable articulations. — At immovable articulations the 

 bones are united by cartilage and practically continuous. The 

 only movement permitted is by cartilage elasticity, as between 

 the cranial and the pelvic bones. 



Slightly movable articulations. — At slightly movable articu- 

 lations, bones are united by a cartilage which is elastic enough 

 to permit of slight movement, e.g. the common intervertebral 

 articulations. 



At freely movable articulations the articulating bony surfaces 

 are each covered by a thin layer of smooth, glistening, and elas- 

 tic articular cartilage. This cartilage gives a smooth gliding 

 surface. It also lessens jar by its elasticity and protects the 

 bony surface beneath. 



Structures at joints. — These structures are bones, cartilage, 

 ligaments, tendons, muscles, synovial membrane, and synovial 

 fluid. Any or all of these may be involved in injury and disease. 



Ligaments are either bands or sheets of white or yellow con- 

 nective tissue. The white ligaments are very strong and in- 

 elastic. Their sole function is to hold bones together at articu- 

 lations. The yellow ligaments are composed of yellow elastic 

 connective tissue, and are especially useful as assistants to 

 muscles, particularly in supporting parts of the body, like the 

 head of the horse, which constantly tend to fall. 



Synovial membranes are sometimes called capsules. These 

 are serous membranes, quite thin, but composed of two layers, 

 deep and superficial. This membrane affords at each joint a 

 closed sack and is for the purpose of secreting synovia. The 

 synovial membrane does not cover the surface of the articular 

 cartilage, as is frequently supposed, but incloses the articula- 

 tion like a sack, being attached at one side near the edge of 

 the articular surface of one bone, and at the other side near the 

 articular edge of the other bone. The synovial membrane is 

 usually attached also to the inner surface of the capsular or 

 other ligaments of the joint. Disease and injury of a synovial 



