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They retained the evidence of their close relationship with the Devonian fishes in their cold blood, their gills and aquatic habit during their larval stage, their teeth with dentine infolded like those of the Devonian ganoids but still more intricately, and their biconcave vertebrae which never completely ossified.
Eight times out of ten, if you take care of the gums the teeth will take care of themselves. Structure of the Teeth. The upper half of the tooth, which pushes through and stands up above the jaw and the gum, we call the crown; and this is the portion that is covered with enamel, or "living glass." The body of the tooth under the enamel is formed of a hard kind of bone called dentine.
The mouth by its construction is well adapted to carrying on the processes of mastication and insalivation. By the first process the solid food is reduced, by the cutting and grinding action of the teeth, to a finely divided condition. By the second, the saliva becomes mixed with the food and is made to act upon it. Pulp. 2. Dentine. 3. Enamel. 4. Crown. 5. Neck. 6. Root.
CUVIER committed himself to the statement that the tusks of the elephant have no attachments to connect them with the pulp lodged in the cavity at their base, from which the peculiar modification of dentine, known as "ivory," is secreted ; and hence, by inference, that they would be devoid of sensation.
DENTAL DISEASES. Inflammation of the alveolar periosteum is a common dental disease in domestic animals. This is an inflammation of the alveolar dental membrane that fixes the tooth in the tooth cavity. Injuries to the gums and cracks or fissures in the tooth are the common causes. Caries or tooth decay is not uncommon. The predisposing factor is a poor quality of enamel and dentine.
This is a convenient term to apply to certain hard dental tumours which are met with in the jaws, and consist of enamel, dentine, and cement. The tumour is to be regarded as being derived from an abnormal growth of all the elements of a tooth germ, or of two or more tooth germs, indiscriminately fused with one another.
The back teeth are broad and irregular, having surfaces that are adapted to crushing and grinding. Each tooth is composed mainly of a bone-like substance, called dentine, which surrounds a central space, containing blood vessels and nerves, known as the pulp cavity. It is set in a depression in the jaw where it is held firmly in place by a bony substance, known as cement.
The acid thus generated attacks the enamel of the teeth, causing decay of the dentine. Decayed teeth are often the cause of an offensive breath and a foul stomach. To keep the teeth clean and wholesome, they should be thoroughly cleansed at bedtime and in the morning with a soft brush and warm water.
Many examples of this variety of odontoma, growing in the nasal cavity or in the maxillary sinus, have been erroneously regarded as osteomas even after removal. On section, the tumour is usually laminated, and is seen to consist mainly of dentine with a partial covering of enamel and cement. Diagnosis. Odontomas are often only diagnosed after removal.
The front teeth are now chopping incisors, and the grinding teeth approach those of the modern horse in the distribution of the enamel, dentine, and cement. They are now about the size of a donkey, and must have had a distinctly horsy appearance, with their long necks and heads and tapering limbs. One of them, Merychippus, was probably in the direct line of the evolution of the horse.
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