202 HUMAN PHYSIOLOGY 



2. Joints, i.e. articulation without definite position of 

 equilibrium of the articulated bones. 



The two surfaces by which the two bones are jointed touch 

 each other ; they are smooth and can move over each other, 

 and this movement is aided by the synovia, a fluid found in 

 the joints which acts as a lubricant. 



Synovia is an alkaline stringy fluid which is rendered cloudy by 

 the remains of cells. It contains proteids, salts, and a nucleo- 

 albumin which is similar to but not identical with mucin. Its 

 composition varies with rest and activity. 



Frequently there is found between the two bones of the 

 joint a cartilage which serves to give greater surface to the 

 joint and aids in the movement of the bones in cases where 

 the two heads of the bones do not fit into each other. 



The joints are covered by the capsular membrane. This 

 is a connective tissue membrane fastened to the bones which, 

 by its flaccidity, allows the movements of the bones against 

 each other. 



Many joints, amphiartrosis, have a membrane so tense that no 

 movement of the bones is possible. These joints are, from a 

 mechanical standpoint, equivalent to the synchondroses. 



The surfaces of the joint are planes of rotation. A plane 

 of rotation is a plane described by a curve when it is rotated 

 around a straight line lying in the plane of the curve. 



The joints are classified according to the form of the 

 curve describing the plane of rotation and the position of the 

 axis of rotation. 



I. The curve is the arc of a circle. 



(a] The straight line passes through the centre of the 

 circle. The plane of rotation is a part of a spherical surface. 

 A joint with such planes is called a ball-and-socket joint, 

 or arthrodia. In such a joint the jointed bones turn around 

 any number of axes of rotation which all pass through the 

 centre of the sphere. We may, however^ conceive of all 

 the possible movements as movements around three lines 

 perpendicular to each other and passing through the centre 

 of the sphere. Examples: hip-joint, shoulder-joint. 



