THE FORMS, ORIGIN, AND DEVELOPMENT OF THE TEETH. 89 
became gradually modified from dermal scales into dermal cones, 
and then, starting from that simple form, became further modified 
in the way I have pointed out, so as to become adapted to the 
various kinds of work they have to perform. And in this way I 
think we can account for the different varieties of the teeth, while 
they retain some of their primitive characteristics. 
The teeth are fixed in their position in the mouth by various 
methods, which pass by gradational forms into one another ; so 
that a simple as well as absolutely correct classification of these 
arrangements is impossible. But we may reduce the modes to 
four general forms, and say it is either by means of attachment 
by fibrous membrane, or by a hinge-like arrangement, or by direct 
adhesion (or anchylosis) to the jaw, or by implantation i in the bony 
sockets which arise from the maxillary bones for that express 
purpose. Most fishes, particularly the sharks and rays, afford an 
excellent example of the first-named method, whereby teeth are 
held solely by the tough mucous membrane which covers their 
more or less calcified jaws. In fishes of predatory habits there are 
found tecth which yield to pressure, and then, by means of a dense 
fibrous elastic ligament, radiating from the side of their base on 
the subjacent bone, return into their position with a snap. The 
use of these hinged teeth is to catch the prey, and also, as in the 
pike, for swallowing it when caught. Again, a tooth is so com- 
pletely in unison with the adjacent bone that it is difficult to see 
with the unaided eye the line of juncture. Lastly, there is a 
special development of bony sockets, which arise from the max- 
illary arches and envelope the roots of the teeth, as in the human 
jaw. These sockets are perfectly subservient to and dependent 
upon the teeth, and soon become absorbed when the teeth are 
lost. In some cases the sockets are absorbed before the teeth are 
lost. There is no anchylosis, or fusion of the roots of the teeth 
to the bone; but between the roots and the “‘alveoli,” as the teeth 
sockets are called, there is a highly organised membrane known as 
the “alveolar dental periosteum.” This is the membrane which is 
severed when the tooth is extracted, and it also plays an important 
part in producing the pain from a devitalized or so-called “dead” 
tooth. The notion so much promulgated by the uninformed den- 
tist, that a “dead” tooth will not be the cause of pain, is founded 
ona misapprehension of the facts of the case. This membrane 
serves as a cushion to the tooth, and is also its attachment to the 
socket by means of the prolongations of its vessels into the walls 
of the socket on the one side and the cementum on the other. 
This accounts for the necessity for a covering to the roots of the 
teeth which is not so dense as the dentine. 
( To be continued.) 
