The enamel of teeth and of placoid scales is also epiblastic in origin[2] , and it may be well at this point to give some account of the structure of teeth, though they are partly mesoblastic in origin. The simplest teeth are those met with in sharks and dogfish, where they are merely the slightly modified scales developed in the integument of the mouth. They pass by quite insensible gradations into normal placoid scales, such as cover the general surface of the body. A placoid scale[3] is developed on a papilla of the dermis which projects outwards and backwards, and is covered by the columnar Malpighian layer of the epidermis. The outer layer of the dermal papilla then gradually becomes converted into dentine and bone, while enamel is developed on the inner side of the Malpighian layer, forming a cap to the scale. The Malpighian and horny layers of the epidermis get rubbed off the enamel cap, so that it comes to project freely on the surface of the body.
As regards their attachment teeth may be (1) attached to the fibrous integument of the mouth, or (2) fixed to the jaws or other bones of the mouth, or (3) planted in grooves, or (4) in definite sockets in the jaw-bones (see p. 107).
Teeth in general consist of three tissues, enamel, dentine and cement, enclosing a central pulp-cavity containing blood-vessels and nerves. Enamel is, however, often absent, as in all living Edentates.
Enamel generally forms the outermost layer of the crown or visible part of the tooth; it is the hardest tissue occurring in the animal body and consists of prismatic fibres arranged at right angles to the surface of the tooth. It is characterised by its bluish-white translucent appearance.
II. Mesoblastic (mesodermal).
Dentine or ivory generally forms the main mass of a tooth. It is a hard, white substance allied to bone. When examined microscopically dentine is seen to be traversed by great numbers of nearly parallel branching tubules which radiate outwards from the pulp-cavity. In fishes as a rule, and sometimes in other animals, a variety of dentine containing blood-vessels occurs, this is called vasodentine.
Cement or crusta petrosa forms the outermost layer of the root of the tooth. In composition and structure it is practically identical with bone. In the more complicated mammalian teeth, besides enveloping the root, it fills up the spaces between the folds of the enamel.
The hard parts of a tooth commonly enclose a central pulp-cavity into which projects the pulp, a papilla of the dermis including blood-vessels and nerves. As long as growth continues the outer layers of this pulp become successively calcified and added to the substance of the dentine. In young growing teeth the pulp-cavity remains widely open, but in mammals the general rule is that as a tooth gets older and the crown becomes fully formed, the remainder of the pulp becomes converted into one or more tapering roots which are imbedded in the alveolar cavities of the jaws. The opening of the pulp-cavity is then reduced to a minute perforation at the base of each root. A tooth of this kind is called a rooted tooth.
But it is not only in young teeth that the pulp-cavity sometimes remains widely open; for some teeth, such as the tusks of Elephants and the incisor teeth of Rodents, form no roots and continue to grow throughout the animal's life. Such teeth are said to be rootless or to have persistent pulps.
An intermediate condition is seen in some teeth, such as the grinding teeth of Horses. These teeth grow for a very long time, their crowns wearing away as fast as their bases are produced; finally however definite roots are formed and growth ceases.