294 THE MECHANICAL FLNCTIONS. 



being bent upon one another at a very acute angle ; 

 and the space left by these notches is filled up by 

 a pliant membrane, which performs the office of a 

 ligament. These articular tubercles and depres- 

 sions are so adjusted to one another, that the joint 

 cannot be dislocated without the fracture of some 

 of its parts. As the different axes of motion in the 

 successive joints are not coincident, but inclined at 

 different angles to one another, the extent of motion 

 in the whole limb is very greatly increased. Thus 

 in the cases where the articulation of the haunch 

 with the trunk is a hinge joint, the axes of this 

 joint, and of the next, are placed at right angles to 

 each other ; so that there results, from the combi- 

 nation of both, a capability in the thigh of executing 

 a circular motion, in a manner almost as perfect as 

 if it had revolved in a spherical socket. The prin- 

 ciple of this compound motion is the same as that 

 employed on ship-board for the mariner's compass, 

 and other instrvnuents which require to be kept 

 steady during the motion of the ship. For this 

 purpose, what are cSiWed ghnbals are used, the parts 

 of which have two axes of rotation, at right angles 

 to each other, so as to enable the compass to take 

 its proper horizontal position, whatever may be the 

 inclination of the ship. 



The tibia, or shank (s), is joined at an acute angle 

 with the femur ; and is frequently either beset with 

 spines, or else notched or serrated.* 



* In many insects, the apex of the tibia is furnished with a pair 

 of spurs, termed calcaria, which articulate with it, and support the 

 limb, projecting from it at an acute angle : in some instances, these 

 spurs are pectinated, and are employed as instruments for burrowing. 

 In the Hymcnoptera, one of these spurs, hollowed out on the inner 



