320 ELEMENTARY PHYSIOLOGY 



While these co-adapted surfaces and synovial mem- 

 branes provide for the free mobility of the bones entering 

 into a joint, the nature and extent of their motion is 

 defined, partly by the forms of the articular surfaces, 

 and partly by the disposition of the ligaments, 

 or firm, fibrous cords wliich pass from one bone to the 

 other. 



As respects the nature of the articular surfaces, joints 

 may be what are called ball and socket joints, when 

 the spheroidal surface furnished by one bone plays in a 

 cup furnished by another. In this case the motion of the 

 former bone may take place in any direction, but the 

 extent of the motion depends upon the shape of the cup 

 — being very great when the cup is shallow, and small in 

 proportion as it is deep. The shoulder is an example of 

 a ball and socket joint with a shallow cup (Fig. 5, B) ; the 

 hip, of such a joint with a deep cup (Fig. 98). 



Hinge-joints are single or double. In the former 

 case, the nearly cylindrical head of one bone fits into a 

 corresponding socket of the other. In this form of hinge- 

 joint the only motion possible is in the direction of a plane 

 perpendicular to the axis of the cylinder, just as a door 

 can only be made to move round an axis passing through 

 its hinges. The elbow is the best example of this joint in 

 the human body, but the movement here is limited, be- 

 cause the olecranon, or part of the ulna which rises up 

 behind the humerus, prevents the arm being carried back 

 behind the straight line ; the arm can thus be bent to, or 

 straightened, but not bent back (Fig. 09). The knee 

 (Fig. 97) and ankle present less perfect specimens of a 

 single hinge-joint. 



A double hinge-joint is one in which the articular sur- 

 face of each bone is concave in one direction, and convex 

 in another, at right angles to the former. A man seated 

 in a saddle is "articulated" with the saddle by such a 

 joint. For the saddle is concave from before backwards, 

 and convex from side to side, while the man presents to it 



