THE ATTACHMENT OF TEETH 



355 



it slides freely upon the bone, and exhibits a modified form 

 of hinge. The summit of this pedestal of bone is in the form 

 of a convex ring, slightly raised on one side, resembling the 

 socket of a ball-and-socket joint with the centre cut out, 

 leaving an elevated ring of bone. The base of the tooth is 

 accurately adapted to this surface, and slides easily upon it, 

 forming the convex side of the joint ; the opening for the 

 passage of the tissues of the pulp is large, to allow of the 



FIG. 235. Hinged tooth from lower jaw of Echineis squalipeta. e. Enamel 

 tip ; a. part of capsule ; b. capsule containing fibres forming hinge ; c. bone 

 of attachment. 



movements of the tooth taking place without injury to the 

 nerves and blood-vessels. A capsular ligament surrounds 

 the whole, giving a still greater likeness to a ball-and-socket 

 joint (fig. 235). 



The capsule is strengthened anteriorly and posteriorly 

 by a fibrous band, there being a distinct depression on the 

 bone and also on the tooth, for the attachment of these 

 fibres. It will be seen that this arrangement would allow 

 of a sliding movement of the tooth on the bone of attachment 

 in every direction, but the ring of bone being slightly more 

 elevated on the anterior aspect, the motion is limited in 

 this direction more than in the opposite, and the tooth can 

 be bent over much more in the direction of its point than 

 in any other direction. The strong elastic ligament ous band 

 being stretched, no doubt serves to draw the tooth into 



A a 2 



