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, 

 may 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). ^ 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 (Fades 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 (Fades 

 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. 

 It 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 fimction 

 with regard to the embedded part of the tooth. The name pericementum has also 

 been applied to it. 



'It 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. 



