THE TEETH IN GENERAL 395 
The individual teeth of each group are designated numerically, the starting- 
point being the middle line; thus the incisor on either side of the middle line is the 
first incisor, and may be conveniently indicated by the notation I’. The deciduous 
teeth may be designated in a similar manner, prefixing D (for deciduous) to the 
letter indicating the kind of tooth. In addition to the above systematic method of 
notation other terms have received the sanction of popular usage. Thus the first 
incisors are commonly called middle incisors, “ pinchers,”’ or ‘‘nippers’’; the second, 
intermediate; and the third, corner teeth. The canines, when highly developed, 
thay be termed tusks or fangs. The vestigial and inconstant first premolar of the 
horse is popularly termed the “wolf tooth.” 
Each tooth presents for description a part coated with enamel, termed the 
crown (Corona dentis), and a part covered with cement, termed the root (Radix 
dentis). The line of union of these parts is the neck (Collum dentis).1_ In many 
teeth the neck is distinct and is embraced by the gum, e. g., the teeth of the dog 
and the temporary incisors of the horse. In other teeth no constriction is present, 
e. g., the permanent incisors of the horse. Between these extremes are the molars 
of the horse, in which the neck is seen only in advanced age. 
The surface of a tooth directed toward the lips is termed labial; that toward 
the cheek, buccal; and that toward the tongue, lingual (Facies labialis, buccalis, 
lingualis). The surface opposed to a neighboring tooth of the same dental arch 
is termed the contact surface (Facies contactus). The masticatory surface (Facies 
masticatoria) is that which comes in contact with a tooth or teeth of the opposite 
jaw.” 
Structure.—Teeth are composed of four tissues, which are considered here 
from within outward. The pulp of the tooth (Pulpa dentis) is a soft, gelatinous 
tissue, which occupies a space in the central part of the tooth termed the pulp 
cavity (Cavum dentis). The pulp is well supplied with blood-vessels and nerves. 
Tt occupies a relatively large space in young growing teeth, but later the dentine 
deposited on its surface gradually encroaches on it until, in advanced age, the cavity 
is much reduced or obliterated. The dentine (Substantia eburnea) forms the bulk 
of most teeth, covering the surface of the pulp. It is hard, and is yellowish-white 
in color. The enamel (Substantia adamantina),the hardest tissue of the body, 
constitutes a layer of varying thickness covering the dentine of the crown of the 
tooth. It is easily distinguished by its clear, bluish-white appearance and its ex- 
treme density. The cement (Substantia ossea) is the outermost tooth substance. 
In simple teeth it forms usually a thin layer on the surface of the root only, but 
in complex teeth it exists in considerable quantity, tending to fill in the spaces 
between the enamel folds of the crown also. Its structure is practically the same 
as that of bone without Haversian canals, and even these occur where the cement 
forms a thick layer. The embedded part of the tooth is attached to the alveolus 
by a vascular layer of connective tissue, the alveolar periosteum (Periosteum 
alveolare); this is the periosteum of the alveolus and performs a like function 
with regard to the embedded part of the tooth. The name pericementum has also 
been applied to it. 
1Tt will be noted that this definition of crown and root does not agree exactly with the 
popular view that the crown is the free part and the root the embedded part. The objection to 
the latter statement lies in the fact that it is not capable of general application. Thus the mor- 
phological crown of the permanent molars in the horse is extremely long, and is, for the most 
part, embedded in the bone in the young animal. The root proper begins to form at four or five 
years of age, and continues its growth for about eight years. As the exposed part of the crown 
wears down, the embedded part erupts, thus preventing deficiency of length. On the old basis 
we should have to say that successive portions of the root become crown, while in point of fact it 
is only in very extreme age that the true root comes into wear. This difficulty does not arise in 
cases In which the eruption of the teeth is completed rapidly, e.g.,man,dog. In such brachydont 
forms the short crown is clearly marked off from the root or roots by a neck, which is embraced 
by the gum. The opposite extreme is seen in the incisors of typical rodents, which grow con- 
tinuously and have no roots. 
* This is popularly termed the grinding surface or “table” of the tooth. 
