ARTHROLOGY. 101 



the ground surface). These are gin glymoid joints, and in 

 them motion may be limited by bony in'ominences sooner 

 or later in either direction ; the elbow-joint is an apposite 

 example. Axonoides or pivot-like motion is motion on the 

 longitudinal axis of the joint, as ginglymus is on_ the 

 transverse axis ; the union between atlas and axis is of 

 this nature. Rotatorial or ball-and-socket joints allow 

 flexion, extension, abduction, and adduction, axonoidal 

 motion, and circumduction (whereby the above movements 

 are manifested in combination) ; examples are the hip and 

 shoulder- joints, in which a more or less globular head fits 

 into a corresponding cavity. Joints are by no means con- 

 stant ; thus sometimes a small synovial sac may be found 

 in the centre of a symphysis ; some bones are united together 

 by two kinds of joints; thus most of the vertebrae are 

 united to their fellows both by amphiarthrosis and by 

 diarthrosis. 



The ARTICULATIONS OF THE HEAD are synarthro- 

 dial with few exceptions. They present every variety of 

 this class, while the petrous temporal bone and os hyoides 

 are united by amphiarthrosis, and the upper to the lower 

 jaw by diarthrosis. The latter only presents unusual 

 characters. 



Temporo-maxillary articulation is the apposition and 

 connection of the glenoid cavity of the scjuamous temporal 

 bone with the condyle of the inferior maxilla. In birds 

 there is an intervening bone, OS qnadratum, but in mam- 

 malia this is only represented by an inter- articular disc of 

 fihro -cartilage, which serves to separate the cavity of the 

 joint into two distinct parts, each lined by a synovial mem- 

 brane, so that both surfaces of this cartilage are lubricated 

 by synovia. It is a flattened piece of cartilage, elongated 

 from side to side, and all around its outer margin is 

 attached the capsular ligament of the joint, which extends 

 from the outer circumferent margin of the glenoid cavity 

 to that of the condyle. It is strengthened by £i fibrous hand 

 running from the outer part of the condyle to the outer 

 part of the cavity, and by another running from the pos- 

 terior part of the condyle to the mastoid process of the 

 squamous temporal bone, and continued from this up to 

 the base of the external auditory process, covering a small 

 synovial cavity. This joint internally is covered by 

 pterygoideus, externally by masseter externus. 



