CHAP. VI.] THE HINGE-JOINT. 137 



generally whose extremities are developed. In birds and reptiles 

 the bodies of the vertebrae are articulated by enarthrosis. 



This species of joint is capable of motion of all kinds, opposition 

 and circumduction being the most perfect, but rotation limited. 

 Indeed, what is called rotation at the hip-joint, is effected by a glid- 

 ing of the head of the femur from before backwards, and vice versa, 

 in the acetabulum ; it is not a rotation of the head and neck, but of 

 the shaft, of the femur. 



c. Ginglymus (7677X1^09, cardo). The articular surfaces in the 

 hinge-joint are marked with elevations and depressions which 

 exactly fit into each other, so as to restrict motion in all but one 

 direction. They are always provided with strong lateral ligaments, 

 which are the chief bonds of union of the articular surfaces. 



The elbow and ankle joints in man are perfect ginglymi; the 

 knee also belongs to this class, but is by no means a perfect speci- 

 men, for, in a certain position of the bones of this joint, the liga- 

 ments are so relaxed as to allow a slight rotation to take place. 

 The phalangeal articulations, both of the fingers and toes, are 

 ginglymi. This form of joint is most extensively employed among 

 the lower animals. In quadrupeds, most of the joints of the extre- 

 mities come under this head. In amphibia and reptiles, too, there 

 are many examples of the hinge-joint. The bivalve shells of conchi- 

 ferous mollusca are united by a very perfect hinge, and a great 

 number of the joints of Crustacea and insects are of this form. 



The true ginglymus is only susceptible of limited opposition : 

 hence the knee-joint cannot be regarded as a perfect example ; in 

 fact, in the perfect ginglymus there is every possible provision 

 against lateral motion. 



d. Diarthrosis rotatorius. A pivot and a ring constitute the me- 

 chanism of this form of joint. The ring is generally formed partly 

 of bone and partly of ligament, and sometimes moves on the pivot, 

 sometimes the pivot moves in it. The motion is evidently confined 

 to rotation, the axis of which is the axis of the pivot. 



In the human subject the best example of this articulation is that 

 between the atlas and odontoid process of the axis or vertebra 

 dentata. The ring is formed by a portion of the anterior arch of 

 the atlas, completed behind by a transverse ligament. Here the 

 atlas rotates round the odontoid process, which is the axis of 

 motion. Another example is the superior radio-ulnar articulation ; 

 here one-fourth of the ring is formed by bone, namely, the lesser 

 sigmoid cavity of the ulna, and the remaining three-fourths by the 

 round ligament called the coronary ligament of the radius. In this 



